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Spinks, Geoffrey M., Nicolas D. Martino, Sina Naficy, David J. Shepherd y Javad Foroughi. "Dual high-stroke and high–work capacity artificial muscles inspired by DNA supercoiling". Science Robotics 6, n.º 53 (28 de abril de 2021): eabf4788. http://dx.doi.org/10.1126/scirobotics.abf4788.
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Powering miniature robots using actuating materials that mimic skeletal muscle is attractive because conventional mechanical drive systems cannot be readily downsized. However, muscle is not the only mechanically active system in nature, and the thousandfold contraction of eukaryotic DNA into the cell nucleus suggests an alternative mechanism for high-stroke artificial muscles. Our analysis reveals that the compaction of DNA generates a mass-normalized mechanical work output exceeding that of skeletal muscle, and this result inspired the development of composite double-helix fibers that reversibly convert twist to DNA-like plectonemic or solenoidal supercoils by simple swelling and deswelling. Our modeling-optimized twisted fibers give contraction strokes as high as 90% with a maximum gravimetric work 36 times higher than skeletal muscle. We found that our supercoiling coiled fibers simultaneously provide high stroke and high work capacity, which is rare in other artificial muscles.
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Ross, Stephanie A., Barbora Rimkus, Nicolai Konow, Andrew A. Biewener y James M. Wakeling. "Added mass in rat plantaris muscle causes a reduction in mechanical work". Journal of Experimental Biology 223, n.º 19 (31 de julio de 2020): jeb224410. http://dx.doi.org/10.1242/jeb.224410.
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ABSTRACTMost of what we know about whole muscle behaviour comes from experiments on single fibres or small muscles that are scaled up in size without considering the effects of the additional muscle mass. Previous modelling studies have shown that tissue inertia acts to slow the rate of force development and maximum velocity of muscle during shortening contractions and decreases the work and power per cycle during cyclic contractions; however, these results have not yet been confirmed by experiments on living tissue. Therefore, in this study we conducted in situ work-loop experiments on rat plantaris muscle to determine the effects of increasing the mass of muscle on mechanical work during cyclic contractions. We additionally simulated these experimental contractions using a mass-enhanced Hill-type model to validate our previous modelling work. We found that greater added mass resulted in lower mechanical work per cycle relative to the unloaded trials in which no mass was added to the muscle (P=0.041 for both 85 and 123% increases in muscle mass). We additionally found that greater strain resulted in lower work per cycle relative to unloaded trials at the same strain to control for length change and velocity effects on the work output, possibly due to greater accelerations of the muscle mass at higher strains. These results confirm that tissue mass reduces muscle mechanical work at larger muscle sizes, and that this effect is likely amplified for lower activations.
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Caiozzo, V. J. y K. M. Baldwin. "Determinants of work produced by skeletal muscle: potential limitations of activation and relaxation". American Journal of Physiology-Cell Physiology 273, n.º 3 (1 de septiembre de 1997): C1049—C1056. http://dx.doi.org/10.1152/ajpcell.1997.273.3.c1049.
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The objective of this study was to estimate the limitations imposed by the kinetics of activation and relaxation on the ability of slow skeletal muscle to produce mechanical work. These estimates were made by the following methods: 1) using the work loop technique and measuring the actual mechanical work (WA) produced by rat soleus muscles (n = 6) at four different frequencies (0.5, 1, 2, and 4 Hz) and seven different amplitudes of length change (1, 2, 3, 4, 5, 6, and 7 mm); 2) determining the force-velocity relationships of the soleus muscles and using this data to quantify the theoretical mechanical work (WT) that could be produced under the work loop conditions described above; and 3) subtracting WA from WT. The difference between WT and WA was interpreted to represent limitations imposed by activation and relaxation. Under certain conditions (high frequency, small strain), factors controlling the kinetics of activation and relaxation reduced the mechanical work of the soleus muscle by approximately 60%. Hence, activation and relaxation collectively represent important factors limiting the production of mechanical work by slow skeletal muscle.
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BOUTILIER, R. G., M. G. EMILIO y G. SHELTON. "The Effects of Mechanical Work on Electrolyte and Water Distribution in Amphibian Skeletal Muscle". Journal of Experimental Biology 120, n.º 1 (1 de enero de 1986): 333–50. http://dx.doi.org/10.1242/jeb.120.1.333.
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The present experiments were undertaken to confirm whether the increase in haematocrit that consistently accompanies the build-up of lactate in amphibian muscle cells during exercise can be explained in terms of a movement of water from the blood into the active muscles. Electrically stimulated sartorius and gastrocnemius muscles isolated from Rana ridibunda and Xenopus laevis had consistently higher total water contents than their paired control muscles. In both instances, it was the intracellular water volume which gave rise to the increase in total muscle water. These results were corroborated in vivo by sampling gastrocnemius muscles from exercising and resting Xenopus laevis. Analyses of tissue electrolyte levels in the working muscles of each experimental series showed an increase in intracellular [lactate−] and [Na+]. A corresponding decline in cellular [K+] occurred in concert with increases in extracellular [K+. In saline-perfused gastrocnemii of Xenopus, the uptake of vascular water was proportional to the total mechanical work performed. Saline leaving the femoral vein of isotonically contracting gastrocnemius muscles had a greater osmotic pressure than that of the arterial perfusate, whereas arterio-venous osmolality differences of control muscles were negligible. Calculations show that the haemoconcentration during exercise in vivo can be attributed at least in part to a net flow of plasma water to osmotically enriched muscle cells.
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Olberding, Jeffrey P., Stephen M. Deban, Michael V. Rosario y Emanuel Azizi. "Modeling the Determinants of Mechanical Advantage During Jumping: Consequences for Spring- and Muscle-Driven Movement". Integrative and Comparative Biology 59, n.º 6 (9 de agosto de 2019): 1515–24. http://dx.doi.org/10.1093/icb/icz139.
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Abstract Systems powered by elastic recoil need a latch to prevent motion while a spring is loaded but allow motion during spring recoil. Some jumping animals that rely on elastic recoil use the increasing mechanical advantage of limb extensor muscles to accomplish latching. We examined the ways in which limb morphology affects latching and the resulting performance of an elastic-recoil mechanism. Additionally, because increasing mechanical advantage is a consequence of limb extension that may be found in many systems, we examined the mechanical consequences for muscle in the absence of elastic elements. By simulating muscle contractions against a simplified model of an extending limb, we found that increasing mechanical advantage can limit the work done by muscle by accelerating muscle shortening during limb extension. The inclusion of a series elastic element dramatically improves mechanical output by allowing for additional muscle work that is stored and released from the spring. This suggests that elastic recoil may be beneficial for more animals than expected when assuming peak isotonic power output from muscle during jumping. The mechanical output of elastic recoil depends on limb morphology; long limbs moving small loads maximize total work, but it is done at a low power, whereas shorter limbs moving larger loads do less work at a higher power. This work-power trade-off of limb morphology is true with or without an elastic element. Systems with relatively short limbs may have performance that is robust to variable conditions such as body mass or muscle activation, while long-limbed systems risk complete failure with relatively minor perturbations. Finally, a changing mechanical advantage latch allows for muscle work to be done simultaneously with spring recoil, changing the predictions for spring mechanical properties. Overall, the design constraints revealed by considering the mechanics of this particular latch will inform our understanding of the evolution of elastic-recoil mechanisms and our attempts to engineer similar systems.
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Sponberg, Simon, Thomas Libby, Chris H. Mullens y Robert J. Full. "Shifts in a single muscle's control potential of body dynamics are determined by mechanical feedback". Philosophical Transactions of the Royal Society B: Biological Sciences 366, n.º 1570 (27 de mayo de 2011): 1606–20. http://dx.doi.org/10.1098/rstb.2010.0368.
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Muscles are multi-functional structures that interface neural and mechanical systems. Muscle work depends on a large multi-dimensional space of stimulus (neural) and strain (mechanical) parameters. In our companion paper, we rewrote activation to individual muscles in intact, behaving cockroaches ( Blaberus discoidalis L.), revealing a specific muscle's potential to control body dynamics in different behaviours. Here, we use those results to provide the biologically relevant parameters for in situ work measurements. We test four hypotheses about how muscle function changes to provide mechanisms for the observed control responses. Under isometric conditions, a graded increase in muscle stress underlies its linear actuation during standing behaviours. Despite typically absorbing energy, this muscle can recruit two separate periods of positive work when controlling running. This functional change arises from mechanical feedback filtering a linear increase in neural activation into nonlinear work output. Changing activation phase again led to positive work recruitment, but at different times, consistent with the muscle's ability to also produce a turn. Changes in muscle work required considering the natural sequence of strides and separating swing and stance contributions of work. Both in vivo control potentials and in situ work loops were necessary to discover the neuromechanical coupling enabling control.
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Milic-Emili, Joseph y Marcello M. Orzalesi. "Mechanical work of breathing during maximal voluntary ventilation". Journal of Applied Physiology 85, n.º 1 (1 de julio de 1998): 254–58. http://dx.doi.org/10.1152/jappl.1998.85.1.254.
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With the use of the esophageal balloon technique, the working capacity of the respiratory muscles was assessed in four normal subjects by measuring the work per breath (W) and respiratory power (W˙) during maximal voluntary ventilation with imposed respiratory frequencies (f) ranging from 20 to 273 cycles/min. Measurements were made in a body plethysmograph to assess the work wasted as a result of alveolar gas compressibility (Wg′). In line with other types of human voluntary muscle activity, W decreased with increasing f, whereasW˙ exhibited a maximum at f of ∼100 cycles/min. Up to this f value, Wg′ was small relative to W. With further increase in f, the Wg′/W ratio increased progressively, amounting to 8–22% of W˙ at f of 200 cycles/min.
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Farris, Dominic James, Benjamin D. Robertson y Gregory S. Sawicki. "Elastic ankle exoskeletons reduce soleus muscle force but not work in human hopping". Journal of Applied Physiology 115, n.º 5 (1 de septiembre de 2013): 579–85. http://dx.doi.org/10.1152/japplphysiol.00253.2013.
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Inspired by elastic energy storage and return in tendons of human leg muscle-tendon units (MTU), exoskeletons often place a spring in parallel with an MTU to assist the MTU. However, this might perturb the normally efficient MTU mechanics and actually increase active muscle mechanical work. This study tested the effects of elastic parallel assistance on MTU mechanics. Participants hopped with and without spring-loaded ankle exoskeletons that assisted plantar flexion. An inverse dynamics analysis, combined with in vivo ultrasound imaging of soleus fascicles and surface electromyography, was used to determine muscle-tendon mechanics and activations. Whole body net metabolic power was obtained from indirect calorimetry. When hopping with spring-loaded exoskeletons, soleus activation was reduced (30–70%) and so was the magnitude of soleus force (peak force reduced by 30%) and the average rate of soleus force generation (by 50%). Although forces were lower, average positive fascicle power remained unchanged, owing to increased fascicle excursion (+4–5 mm). Net metabolic power was reduced with exoskeleton assistance (19%). These findings highlighted that parallel assistance to a muscle with appreciable series elasticity may have some negative consequences, and that the metabolic cost associated with generating force may be more pronounced than the cost of doing work for these muscles.
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James, R. S., V. M. Cox, I. S. Young, J. D. Altringham y D. F. Goldspink. "Mechanical properties of rabbit latissimus dorsi muscle after stretch and/or electrical stimulation". Journal of Applied Physiology 83, n.º 2 (1 de agosto de 1997): 398–406. http://dx.doi.org/10.1152/jappl.1997.83.2.398.
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James, R. S., V. M. Cox, I. S. Young, J. D. Altringham, and D. F. Goldspink Mechanical properties of rabbit latissimus dorsi muscle after stretch and/or electrical stimulation. J. Appl. Physiol. 83(2): 398–406, 1997.—The work loop technique was used to measure the mechanical performance in situ of the latissimus dorsi (LD) muscles of rabbits maintained under fentanyl anesthesia. After 3 wk of incrementally applied stretch the LD muscles were 36% heavier, but absolute power output (195 mW/muscle) was not significantly changed relative to that of external control muscle (206 mW). In contrast, continuous 10-Hz electrical stimulation reduced power output per kilogram of muscle >75% after 3 or 6 wk and muscle mass by 32% after 6 wk. When combined, stretch and 10-Hz electrical stimulation preserved or increased the mass of the treated muscles but failed to prevent an 80% loss in maximum muscle power. However, this combined treatment increased fatigue resistance to a greater degree than electrical stimulation alone. These stretched/stimulated muscles, therefore, are more suitable for cardiomyoplasty. Nonetheless, further work will be necessary to find an ideal training program for this surgical procedure.
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Takarada, Yudai, Hiroyuki Iwamoto, Haruo Sugi, Yuichi Hirano y Naokata Ishii. "Stretch-induced enhancement of mechanical work production in frog single fibers and human muscle". Journal of Applied Physiology 83, n.º 5 (1 de noviembre de 1997): 1741–48. http://dx.doi.org/10.1152/jappl.1997.83.5.1741.
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Takarada, Yudai, Hiroyuki Iwamoto, Haruo Sugi, Yuichi Hirano, and Naokata Ishii. Stretch-induced enhancement of mechanical work production in frog single fibers and human muscle. J. Appl. Physiol. 83(5): 1741–1748, 1997.—The relations between the velocity of prestretch and the mechanical energy liberated during the subsequent isovelocity release were studied in contractions of frog single fibers and human muscles. During isometric contractions of frog single fibers, a ramp stretch of varied velocity (amplitude, 0.02 fiber length; velocity, 0.08–1.0 fiber length/s) followed by a release (amplitude, 0.02 fiber length; velocity, 1.0 fiber length/s) was given, and the amount of work liberated during the release was measured. For human muscles, elbow flexions were performed with a prestretch of varied velocity (range, 40°; velocity, 30–180°/s) followed by an isokinetic shortening (velocity, 90°/s). In both frog single fibers and human muscles, the work production increased with both the velocity of stretch and the peak of force attained before the release up to a certain level; thereafter it declined with the further increases of these variables. In human muscles, the enhancement of work production was not associated with a significant increase in integrated electromyogram. This suggests that changes in intrinsic mechanical properties of muscle fibers play an important role in the stretch-induced enhancement of work production.
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Josephson, R. K., J. G. Malamud y D. R. Stokes. "Asynchronous muscle: a primer". Journal of Experimental Biology 203, n.º 18 (15 de septiembre de 2000): 2713–22. http://dx.doi.org/10.1242/jeb.203.18.2713.
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The asynchronous muscles of insects are characterized by asynchrony between muscle electrical and mechanical activity, a fibrillar organization with poorly developed sarcoplasmic reticulum, a slow time course of isometric contraction, low isometric force, high passive stiffness and delayed stretch activation and shortening deactivation. These properties are illustrated by comparing an asynchronous muscle, the basalar flight muscle of the beetle Cotinus mutabilis, with synchronous wing muscles from the locust, Schistocerca americana. Because of delayed stretch activation and shortening deactivation, a tetanically stimulated beetle muscle can do work when subjected to repetitive lengthening and shortening. The synchronous locust muscle, subjected to similar stimulation and length change, absorbs rather than produces work.
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Mckay, William Paul, Philip D. Chilibeck, Brian L. F. Daku y Brendan Lett. "Quantifying the mechanical work of resting quadriceps muscle tone". European Journal of Applied Physiology 108, n.º 4 (3 de noviembre de 2009): 641–48. http://dx.doi.org/10.1007/s00421-009-1261-9.
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Song, Weihua, Petr G. Vikhorev, Mavin N. Kashyap, Christina Rowlands, Michael A. Ferenczi, Roger C. Woledge, Kenneth MacLeod, Steven Marston y Nancy A. Curtin. "Mechanical and energetic properties of papillary muscle from ACTC E99K transgenic mouse models of hypertrophic cardiomyopathy". American Journal of Physiology-Heart and Circulatory Physiology 304, n.º 11 (1 de junio de 2013): H1513—H1524. http://dx.doi.org/10.1152/ajpheart.00951.2012.
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We compared the contractile performance of papillary muscle from a mouse model of hypertrophic cardiomyopathy [α-cardiac actin ( ACTC) E99K mutation] with nontransgenic (non-TG) littermates. In isometric twitches, ACTC E99K papillary muscle produced three to four times greater force than non-TG muscle under the same conditions independent of stimulation frequency and temperature, whereas maximum isometric force in myofibrils from these muscles was not significantly different. ACTC E99K muscle relaxed slower than non-TG muscle in both papillary muscle (1.4×) and myofibrils (1.7×), whereas the rate of force development after stimulation was the same as non-TG muscle for both electrical stimulation in intact muscle and after a Ca2+ jump in myofibrils. The EC50 for Ca2+ activation of force in myofibrils was 0.39 ± 0.33 μmol/l in ACTC E99K myofibrils and 0.80 ± 0.11 μmol/l in non-TG myofibrils. There were no significant differences in the amplitude and time course of the Ca2+ transient in myocytes from ACTC E99K and non-TG mice. We conclude that hypercontractility is caused by higher myofibrillar Ca2+ sensitivity in ACTC E99K muscles. Measurement of the energy (work + heat) released in actively cycling heart muscle showed that for both genotypes, the amount of energy turnover increased with work done but with decreasing efficiency as energy turnover increased. Thus, ACTC E99K mouse heart muscle produced on average 3.3-fold more work than non-TG muscle, and the cost in terms of energy turnover was disproportionately higher than in non-TG muscles. Efficiency for ACTC E99K muscle was in the range of 11–16% and for non-TG muscle was 15–18%.
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Kiriazis, H. y C. L. Gibbs. "Papillary muscles split in the presence of 2,3-butanedione monoxime have normal energetic and mechanical properties". American Journal of Physiology-Heart and Circulatory Physiology 269, n.º 5 (1 de noviembre de 1995): H1685—H1694. http://dx.doi.org/10.1152/ajpheart.1995.269.5.h1685.
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A number of studies have used 2,3-butanedione monoxime (BDM) to avoid myocardial damage when small muscle preparations were cut from large hearts. The present study investigates the mechanical and energetic effects of varying muscle cross-sectional area (CSA) by dissection in physiological saline containing BDM. By use of adult rat hearts, three muscle groups were obtained: whole left ventricular papillary muscles (Whole) and left ventricular papillary muscles split longitudinally in the presence of 30 mM BDM, with removal of approximately 10% (BDMSP1) or 40-50% (BDMSP2) of the muscle (5 animals in each group). The isolated muscle preparations were studied at 27 degrees C and stimulated at 0.167 Hz. The Whole and BDMSP1 preparations had comparable CSAs; in isotonically contracting muscles working against a range of afterloads, work, enthalpy (energy use), and mechanical efficiency (work/enthalpy x 100%) were similar for the two groups. In addition, isometric performance [e.g., developed stress (force/CSA), length-tension relationship, and contraction time course] was also similar for the two groups. The thinner BDMSP2 preparations showed an enhanced mechanical performance compared with the Whole and BDMSP1 groups. This outcome was in accordance with data in the literature documenting a negative correlation between stress and CSA. The results suggest that BDM-split and intact papillary muscles of similar CSA have comparable energetic and mechanical properties.
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Luciani, Bhillie D., David M. Desmet, Amani A. Alkayyali, Joshua M. Leonardis y David B. Lipps. "Identifying the mechanical and neural properties of the sternocleidomastoid muscles". Journal of Applied Physiology 124, n.º 5 (1 de mayo de 2018): 1297–303. http://dx.doi.org/10.1152/japplphysiol.00892.2017.
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Neck muscles are preferentially activated in specific force directions, but the constraints that the central nervous system considers when programming these preferred directions of muscle activity are unknown. The current study used ultrasound shear wave elastography (SWE) to investigate whether the material properties of the sternocleidomastoid (SCM) muscles exhibit preferred directions similar to their preferred direction of muscle activity during an isometric task. Twenty-four healthy participants matched isometric forces in 16 axial directions. All force targets were scaled to 20% of a maximum voluntary contraction. Muscle activity was recorded with surface electromyography (EMG) from six muscles (the bilateral SCMs, upper trapezius, and splenius capitis muscles), and shear wave velocities (SWVs) were recorded with SWE from both SCM muscles. We observed statistically significant differences between the preferred directions of muscle activity and SWVs for both the left SCM ( P = 0.002) and the right SCM ( P < 0.001), with the SWE data exhibiting a more lateral preferred direction. Significant differences in the spatial focus ( P < 0.001) were also observed, with the dispersion of SWV data covering a greater angular range than the EMG data during isometric tasks. The preferred directions of muscle activity and material properties for the SCM muscles were closer than previous comparisons of muscle activity and moment arms, suggesting muscle mechanics could play a more important role than anatomy in how the central nervous system spatially tunes muscle activation. NEW & NOTEWORTHY Our study used a novel combination of surface electromyography and ultrasound shear wave elastography to investigate the neuromuscular control of the neck. Our work highlights differences in how the activation and material properties of the sternocleidomastoid muscles are modulated as the central nervous system stabilizes the neck during isometric force production. These findings provide normative data for future studies to investigate pathologic changes to both the activation and material properties of the sternocleidomastoid muscles.
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Schenau, Gerrit Jan van Ingen, Maarten F. Bobbert y Arnold de Haan. "Does Elastic Energy Enhance Work and Efficiency in the Stretch-Shortening Cycle?" Journal of Applied Biomechanics 13, n.º 4 (noviembre de 1997): 389–415. http://dx.doi.org/10.1123/jab.13.4.389.
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This target article addresses the role of storage and reutilization of elastic energy in stretch-shortening cycles. It is argued that for discrete movements such as the vertical jump, elastic energy does not explain the work enhancement due to the prestretch. This enhancement seems to occur because the prestretch allows muscles to develop a high level of active state and force before starting to shorten. For cyclic movements in which stretch-shortening cycles occur repetitively, some authors have claimed that elastic energy enhances mechanical efficiency. In the current article it is demonstrated that this claim is often based on disputable concepts such as the efficiency of positive work or absolute work, and it is argued that elastic energy cannot affect mechanical efficiency simply because this energy is not related to the conversion of metabolic energy into mechanical energy. A comparison of work and efficiency measures obtained at different levels of organization reveals that there is in fact no decisive evidence to either support or reject the claim that the stretch-shortening cycle enhances muscle efficiency. These explorations lead to the conclusion that the body of knowledge about the mechanics and energetics of the stretch-shortening cycle is in fact quite lean. A major challenge is to bridge the gap between knowledge obtained at different levels of organization, with the ultimate purpose of understanding how the intrinsic properties of muscles manifest themselves underin-vivo-like conditions and how they are exploited in whole-body activities such as running. To achieve this purpose, a close cooperation is required between muscle physiologists and human movement scientists performing inverse and forward dynamic simulation studies of whole-body exercises.
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Syme, Douglas A. y Robert E. Shadwick. "Effects of longitudinal body position and swimming speed on mechanical power of deep red muscle from skipjack tuna (Katsuwonus pelamis)". Journal of Experimental Biology 205, n.º 2 (15 de enero de 2002): 189–200. http://dx.doi.org/10.1242/jeb.205.2.189.
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SUMMARY The mechanical power output of deep, red muscle from skipjack tuna (Katsuwonus pelamis) was studied to investigate (i) whether this muscle generates maximum power during cruise swimming, (ii) how the differences in strain experienced by red muscle at different axial body locations affect its performance and (iii) how swimming speed affects muscle work and power output. Red muscle was isolated from approximately mid-way through the deep wedge that lies next to the backbone; anterior (0.44 fork lengths, ANT) and posterior (0.70 fork lengths, POST) samples were studied. Work and power were measured at 25°C using the work loop technique. Stimulus phases and durations and muscle strains (±5.5 % in ANT and ±8 % in POST locations) experienced during cruise swimming at different speeds were obtained from previous studies and used during work loop recordings. In addition, stimulus conditions that maximized work were determined. The stimulus durations and phases yielding maximum work decreased with increasing cycle frequency (analogous to tail-beat frequency), were the same at both axial locations and were almost identical to those used by the fish during swimming, indicating that the muscle produces near-maximal work under most conditions in swimming fish. While muscle in the posterior region undergoes larger strain and thus produces more mass-specific power than muscle in the anterior region, when the longitudinal distribution of red muscle mass is considered, the anterior muscles appear to contribute approximately 40 % more total power. Mechanical work per length cycle was maximal at a cycle frequency of 2–3 Hz, dropping to near zero at 15 Hz and by 20–50 % at 1 Hz. Mechanical power was maximal at a cycle frequency of 5 Hz, dropping to near zero at 15 Hz. These fish typically cruise with tail-beat frequencies of 2.8–5.2 Hz, frequencies at which power from cyclic contractions of deep red muscles was 75–100 % maximal. At any given frequency over this range, power using stimulation conditions recorded from swimming fish averaged 93.4±1.65 % at ANT locations and 88.6±2.08 % at POST locations (means ± s.e.m., N=3–6) of the maximum using optimized conditions. When cycle frequency was held constant (4 Hz) and strain amplitude was increased, work and power increased similarly in muscles from both sample sites; work and power increased 2.5-fold when strain was elevated from ±2 to ±5.5 %, but increased by only approximately 12 % when strain was raised further from ±5.5 to ±8 %. Taken together, these data suggest that red muscle fibres along the entire body are used in a similar fashion to produce near-maximal mechanical power for propulsion during normal cruise swimming. Modelling suggests that the tail-beat frequency at which power is maximal (5 Hz) is very close to that used at the predicted maximum aerobic swimming speed (5.8 Hz) in these fish.
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Konow, Nicolai y Thomas J. Roberts. "The series elastic shock absorber: tendon elasticity modulates energy dissipation by muscle during burst deceleration". Proceedings of the Royal Society B: Biological Sciences 282, n.º 1804 (7 de abril de 2015): 20142800. http://dx.doi.org/10.1098/rspb.2014.2800.
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During downhill running, manoeuvring, negotiation of obstacles and landings from a jump, mechanical energy is dissipated via active lengthening of limb muscles. Tendon compliance provides a ‘shock-absorber’ mechanism that rapidly absorbs mechanical energy and releases it more slowly as the recoil of the tendon does work to stretch muscle fascicles. By lowering the rate of muscular energy dissipation, tendon compliance likely reduces the risk of muscle injury that can result from rapid and forceful muscle lengthening. Here, we examine how muscle–tendon mechanics are modulated in response to changes in demand for energy dissipation. We measured lateral gastrocnemius (LG) muscle activity, force and fascicle length, as well as leg joint kinematics and ground-reaction force, as turkeys performed drop-landings from three heights (0.5–1.5 m centre-of-mass elevation). Negative work by the LG muscle–tendon unit during landing increased with drop height, mainly owing to greater muscle recruitment and force as drop height increased. Although muscle strain did not increase with landing height, ankle flexion increased owing to increased tendon strain at higher muscle forces. Measurements of the length–tension relationship of the muscle indicated that the muscle reached peak force at shorter and likely safer operating lengths as drop height increased. Our results indicate that tendon compliance is important to the modulation of energy dissipation by active muscle with changes in demand and may provide a mechanism for rapid adjustment of function during deceleration tasks of unpredictable intensity.
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Rubenson, Jonas y Richard L. Marsh. "Mechanical efficiency of limb swing during walking and running in guinea fowl (Numida meleagris)". Journal of Applied Physiology 106, n.º 5 (mayo de 2009): 1618–30. http://dx.doi.org/10.1152/japplphysiol.91115.2008.
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Understanding the mechanical determinants of the energy cost of limb swing is crucial for refining our models of locomotor energetics, as well as improving treatments for those suffering from impaired limb-swing mechanics. In this study, we use guinea fowl ( Numida meleagris) as a model to explore whether mechanical work at the joints explains limb-swing energy use by combining inverse dynamic modeling and muscle-specific energetics from blood flow measurements. We found that the overall efficiencies of the limb swing increased markedly from walking (3%) to fast running (17%) and are well below the usually accepted maximum efficiency of muscle, except at the fastest speeds recorded. The estimated efficiency of a single muscle used during ankle flexion (tibialis cranialis) parallels that of the total limb-swing efficiency (3% walking, 15% fast running). Taken together, these findings do not support the hypothesis that joint work is the major determinant of limb-swing energy use across the animal's speed range and warn against making simple predictions of energy use based on joint mechanical work. To understand limb-swing energy use, mechanical functions other than accelerating the limb segments need to be explored, including isometric force production and muscle work arising from active and passive antagonist muscle forces.
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Layland, J., I. S. Young y J. D. Altringham. "The length dependence of work production in rat papillary muscles in vitro." Journal of Experimental Biology 198, n.º 12 (1 de diciembre de 1995): 2491–99. http://dx.doi.org/10.1242/jeb.198.12.2491.
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The influence of length on work production was investigated for rat papillary muscles using the work loop technique. Active and passive length-force relationships were first determined under isometric conditions and the length for maximum force production (Lmax) was derived. Starting from different lengths within the physiological range, a series of work loops was generated using the stimulation phase shift, strain amplitude and cycle frequency previously found to be optimal for power output at 37 degrees C. The relationship between muscle length and net work was used to determine the length at which work output was maximal (Lopt). In order to examine the dynamic passive properties of the muscles, unstimulated muscles were subjected to the same regime of sinusoidal oscillation as used for the active loops. From the hysteresis loops, lengthening work (work done to extend the passive muscle), passive shortening work (work returned during shortening) and net energy loss (hysteresis) could be measured. The decline in net work production at lengths greater than 95% Lmax could largely be attributed to the rapid and non-linear increase in muscle stiffness and the increase in net energy loss over this range of lengths. The physiological significance of the length-work relationship is considered and the mechanical properties of active and passive papillary muscles are discussed with reference to sarcomere length and cardiac muscle ultrastructure.
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Peplowski, M. M. y R. L. Marsh. "Work and power output in the hindlimb muscles of Cuban tree frogs Osteopilus septentrionalis during jumping." Journal of Experimental Biology 200, n.º 22 (1 de noviembre de 1997): 2861–70. http://dx.doi.org/10.1242/jeb.200.22.2861.
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It has been suggested that small frogs use a catapult mechanism to amplify muscle power production during the takeoff phase of jumping. This conclusion was based on an apparent discrepancy between the power available from the hindlimb muscles and that required during takeoff. The present study provides integrated data on muscle contractile properties, morphology and jumping performance that support this conclusion. We show here that the predicted power output during takeoff in Cuban tree frogs Osteopilus septentrionalis exceeds that available from the muscles by at least sevenfold. We consider the sartorius muscle as representative of the bulk of the hindlimb muscles of these animals, because this muscle has properties typical of other hindlimb muscles of small frogs. At 25 degrees C, this muscle has a maximum shortening velocity (Vmax) of 8.77 +/- 0.62 L0 s-1 (where L0 is the muscle length yielding maximum isometric force), a maximum isometric force (P0) of 24.1 +/- 2.3 N cm-2 and a maximum isotonic power output of 230 +/- 9.2 W kg-1 of muscle (mean +/- S.E.M.). In contrast, the power required to accelerate the animal in the longest jumps measured (approximately 1.4 m) is more than 800 W kg-1 of total hindlimb muscle. The peak instantaneous power is expected to be twice this value. These estimates are probably conservative because the muscles that probably power jumping make up only 85% of the total hindlimb muscle mass. The total mechanical work required of the muscles is high (up to 60 J kg-1), but is within the work capacities predicted for vertebrate skeletal muscle. Clearly, a substantial portion of this work must be performed and stored prior to takeoff to account for the high power output during jumping. Interestingly, muscle work output during jumping is temperature-dependent, with greater work being produced at higher temperatures. The thermal dependence of work does not follow from simple muscle properties and instead must reflect the interaction between these properties and the other components of the skeletomuscular system during the propulsive phase of the jump.
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Josephson, R. K., J. G. Malamud y D. R. Stokes. "Power output by an asynchronous flight muscle from a beetle". Journal of Experimental Biology 203, n.º 17 (1 de septiembre de 2000): 2667–89. http://dx.doi.org/10.1242/jeb.203.17.2667.
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The basalar muscle of the beetle Cotinus mutabilis is a large, fibrillar flight muscle composed of approximately 90 fibers. The paired basalars together make up approximately one-third of the mass of the power muscles of flight. Changes in twitch force with changing stimulus intensity indicated that a basalar muscle is innervated by at least five excitatory axons and at least one inhibitory axon. The muscle is an asynchronous muscle; during normal oscillatory operation there is not a 1:1 relationship between muscle action potentials and contractions. During tethered flight, the wing-stroke frequency was approximately 80 Hz, and the action potential frequency in individual motor units was approximately 20 Hz. As in other asynchronous muscles that have been examined, the basalar is characterized by high passive tension, low tetanic force and long twitch duration. Mechanical power output from the basalar muscle during imposed, sinusoidal strain was measured by the work-loop technique. Work output varied with strain amplitude, strain frequency, the muscle length upon which the strain was superimposed, muscle temperature and stimulation frequency. When other variables were at optimal values, the optimal strain for work per cycle was approximately 5%, the optimal frequency for work per cycle approximately 50 Hz and the optimal frequency for mechanical power output 60–80 Hz. Optimal strain decreased with increasing cycle frequency and increased with muscle temperature. The curve relating work output and strain was narrow. At frequencies approximating those of flight, the width of the work versus strain curve, measured at half-maximal work, was 5% of the resting muscle length. The optimal muscle length for work output was shorter than that at which twitch and tetanic tension were maximal. Optimal muscle length decreased with increasing strain. The curve relating work output and muscle length, like that for work versus strain, was narrow, with a half-width of approximately 3 % at the normal flight frequency. Increasing the frequency with which the muscle was stimulated increased power output up to a plateau, reached at approximately 100 Hz stimulation frequency (at 35 degrees C). The low lift generated by animals during tethered flight is consistent with the low frequency of muscle action potentials in motor units of the wing muscles. The optimal oscillatory frequency for work per cycle increased with muscle temperature over the temperature range tested (25–40 degrees C). When cycle frequency was held constant, the work per cycle rose to an optimum with increasing temperature and then declined. We propose that there is a temperature optimum for work output because increasing temperature increases the shortening velocity of the muscle, which increases the rate of positive work output during shortening, but also decreases the durations of the stretch activation and shortening deactivation that underlie positive work output, the effect of temperature on shortening velocity being dominant at lower temperatures and the effect of temperature on the time course of activation and deactivation being dominant at higher temperatures. The average wing-stroke frequency during free flight was 94 Hz, and the thoracic temperature was 35 degrees C. The mechanical power output at the measured values of wing-stroke frequency and thoracic temperature during flight, and at optimal muscle length and strain, averaged 127 W kg(−1)muscle, with a maximum value of 200 W kg(−1). The power output from this asynchronous flight muscle was approximately twice that measured with similar techniques from synchronous flight muscle of insects, supporting the hypothesis that asynchronous operation has been favored by evolution in flight systems of different insect groups because it allows greater power output at the high contraction frequencies of flight.
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Brown, David A. y Steven A. Kautz. "Speed-Dependent Reductions of Force Output in People With Poststroke Hemiparesis". Physical Therapy 79, n.º 10 (1 de octubre de 1999): 919–30. http://dx.doi.org/10.1093/ptj/79.10.919.
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Abstract Background and Purpose. Movement is slow in people with poststroke hemiparesis. Moving at faster speeds is thought by some researchers to exacerbate of abnormal or unwanted muscle activity. The purpose of this study was to quantify the effects of increased speed on motor performance during pedaling exercise in people with poststroke hemiparesis. Subjects. Twelve elderly subjects with no known neurological impairment and 15 subjects with poststroke hemiparesis of greater than 6 months' duration were tested. Methods. Subjects pedaled at 12 randomly ordered workload and cadence combinations (45-, 90-, 135-, and 180-J workloads at 25, 40, and 55 rpm). Pedal reaction forces were used to calculate work done by each lower extremity. Electromyographic activity was recorded from 7 lower-extremity muscles. Results. The main finding was that net mechanical work done by the paretic lower extremity decreased as speed increased in all subjects. The occurrence of inappropriate muscle activity on the paretic side, however, was not exacerbated in that the vastus medialis muscle on the paretic side did not show a consistent further increase in its prolonged activity at higher speeds. The mechanics of faster pedaling resulted in greater net negative mechanical work because, at higher pedaling rates, the prolonged vastus medialis muscle activity is present during a greater portion of the cycle. Conclusion and Discussion. The lessened force output by the paretic limb is mainly the result of the inherent mechanical demands of higher-speed pedaling and not due to exacerbation of impaired neural control.
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Heglund, N. C. y G. A. Cavagna. "Mechanical work, oxygen consumption, and efficiency in isolated frog and rat muscle". American Journal of Physiology-Cell Physiology 253, n.º 1 (1 de julio de 1987): C22—C29. http://dx.doi.org/10.1152/ajpcell.1987.253.1.c22.
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The total work done during shortening, in repeated stretch-shortening cycles and the subsequent recovery oxygen consumption were measured in isolated frog (Rana esculenta) sartorius at 12 degrees C and rat (Wistar strain) extensor digitorum longus (EDL) and soleus at 20 degrees C. Two procedures were followed. In the first, the muscles were lengthened in the relaxed state and stimulated isometrically just before and during the first part of shortening. The peak efficiency (positive work done divided by the energetic equivalent of the oxygen consumed) was approximately 25% at 0.75–1.5 muscle lengths/s (Lo/s) in sartorius, 19% at 1.0 Lo/s in EDL, and 15% at 0.5 Lo/s in the soleus. In contrast to the measured efficiency values, the ratio between the tension-time integral and the oxygen consumption (the economy) is greater in soleus than in EDL. In the second procedure, stimulation began before stretching and continued during the first part of shortening. In this case, the efficiency attained values of approximately 35% in sartorius, 50% in EDL, and 40% in soleus. These values are in rough agreement with those measured in vivo during running.
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Mellors, L. J. y C. J. Barclay. "The energetics of rat papillary muscles undergoing realistic strain patterns". Journal of Experimental Biology 204, n.º 21 (1 de noviembre de 2001): 3765–77. http://dx.doi.org/10.1242/jeb.204.21.3765.
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SUMMARYStudies of cardiac muscle energetics have traditionally used contraction protocols with strain patterns that bear little resemblance to those observed in vivo. This study aimed to develop a realistic strain protocol, based on published in situ measurements of contracting papillary muscles, for use with isolated preparations. The protocol included the three phases observed in intact papillary muscles: an initial isometric phase followed by isovelocity shortening and re-lengthening phases. Realistic papillary muscle dynamics were simulated in vitro (27°C) using preparations isolated from the left ventricle of adult male rats. The standard contraction protocol consisted of 40 twitches at a contraction rate of 2 Hz. Force, changes in muscle length and changes in muscle temperature were measured simultaneously. To quantify the energetic costs of contraction, work output and enthalpy output were determined, from which the maximum net mechanical efficiency could be calculated. The most notable result from these experiments was the constancy of enthalpy output per twitch, or energy cost, despite the various alterations made to the protocol. Changes in mechanical efficiency, therefore, generally reflected changes in work output per twitch. The variable that affected work output per twitch to the greatest extent was the amplitude of shortening, while changes in the duration of the initial isometric phase had little effect. Decreasing the duration of the shortening phase increased work output per twitch without altering enthalpy output per twitch. Increasing the contraction frequency from 2 to 3 Hz resulted in slight decreases in the work output per twitch and in efficiency. Using this realistic strain protocol, the maximum net mechanical efficiency of rat papillary muscles was approximately 15 %. The protocol was modified to incorporate an isometric relaxation period, thus allowing the model to simulate the main mechanical features of ventricular function.
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Tu, M. y M. Dickinson. "MODULATION OF NEGATIVE WORK OUTPUT FROM A STEERING MUSCLE OF THE BLOWFLY CALLIPHORA VICINA". Journal of Experimental Biology 192, n.º 1 (1 de julio de 1994): 207–24. http://dx.doi.org/10.1242/jeb.192.1.207.
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Of the 17 muscles responsible for flight control in flies, only the first basalar muscle (b1) is known to fire an action potential each and every wing beat at a precise phase of the wing-beat period. The phase of action potentials in the b1 is shifted during turns, implicating the b1 in the control of aerodynamic yaw torque. We used the work loop technique to quantify the effects of phase modulation on the mechanical output of the b1 of the blowfly Calliphora vicina. During cyclic length oscillations at 10 and 50 Hz, the magnitude of positive work output by the b1 was similar to that measured previously from other insect muscles. However, when tested at wing-beat frequency (150 Hz), the net work performed in each cycle was negative. The twitch kinetics of the b1 suggest that negative work output reflects intrinsic specializations of the b1 muscle. Our results suggest that, in addition to a possible role as a passive elastic element, the phase-sensitivity of its mechanical properties may endow the b1 with the capacity to modulate wing-beat kinematics during turning maneuvers.
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Rankin, Jeffery W., Jonas Rubenson y John R. Hutchinson. "Inferring muscle functional roles of the ostrich pelvic limb during walking and running using computer optimization". Journal of The Royal Society Interface 13, n.º 118 (mayo de 2016): 20160035. http://dx.doi.org/10.1098/rsif.2016.0035.
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Owing to their cursorial background, ostriches ( Struthio camelus ) walk and run with high metabolic economy, can reach very fast running speeds and quickly execute cutting manoeuvres. These capabilities are believed to be a result of their ability to coordinate muscles to take advantage of specialized passive limb structures. This study aimed to infer the functional roles of ostrich pelvic limb muscles during gait. Existing gait data were combined with a newly developed musculoskeletal model to generate simulations of ostrich walking and running that predict muscle excitations, force and mechanical work. Consistent with previous avian electromyography studies, predicted excitation patterns showed that individual muscles tended to be excited primarily during only stance or swing. Work and force estimates show that ostrich gaits are partially hip-driven with the bi-articular hip–knee muscles driving stance mechanics. Conversely, the knee extensors acted as brakes, absorbing energy. The digital extensors generated large amounts of both negative and positive mechanical work, with increased magnitudes during running, providing further evidence that ostriches make extensive use of tendinous elastic energy storage to improve economy. The simulations also highlight the need to carefully consider non-muscular soft tissues that may play a role in ostrich gait.
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Josephson, Robert K., Jean G. Malamud y Darrell R. Stokes. "The efficiency of an asynchronous flight muscle from a beetle". Journal of Experimental Biology 204, n.º 23 (1 de diciembre de 2001): 4125–39. http://dx.doi.org/10.1242/jeb.204.23.4125.
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SUMMARYMechanical power output and metabolic power input were measured from an asynchronous flight muscle, the basalar muscle of the beetle Cotinus mutabilis. Mechanical power output was determined using the work loop technique and metabolic power input by monitoring CO2 production or both CO2 production and O2 consumption. At 35°C, and with conditions that maximized power output (60 Hz sinusoidal strain, optimal muscle length and strain amplitude, 60 Hz stimulation frequency), the peak mechanical power output during a 10 s burst was approximately 140 W kg–1, the respiratory coefficient 0.83 and the muscle efficiency 14–16 %. The stimulus intensity used was the minimal required to achieve a maximal isometric tetanus. Increasing or decreasing the stimulus intensity from this level changed mechanical power output but not efficiency, indicating that the efficiency measurements were not contaminated by excitation of muscles adjacent to that from which the mechanical recordings were made. The CO2 produced during an isometric tetanus was approximately half that during a bout of similar stimulation but with imposed sinusoidal strain and work output, suggesting that up to 50 % of the energy input may go to muscle activation costs. Reducing the stimulus frequency to 30 Hz from its usual value of 60 Hz reduced mechanical power output but had no significant effect on efficiency. Increasing the frequency of the sinusoidal strain from 60 to 90 Hz reduced power output but not CO2 consumption; hence, there was a decline in efficiency. The respiratory coefficient was the same for 10 s and 30 s bursts of activity, suggesting that there was no major change in the fuel used over this time range.The mass-specific mechanical power output and the efficiency of the beetle muscle were each 2–3 times greater than values measured in previous studies, using similar techniques, from locust flight muscles, which are synchronous muscles. These results support the hypothesis that asynchronous flight muscles have evolved in several major insect taxa because they can provide greater power output and are more efficient than are synchronous muscles for operation at the high frequencies of insect flight.
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Ríos-Castro, Francisco, Felipe González-Seguel y Jorge Molina. "Respiratory drive, inspiratory effort, and work of breathing: review of definitions and non-invasive monitoring tools for intensive care ventilators during pandemic times". Medwave 22, n.º 03 (29 de abril de 2022): e002550-e002550. http://dx.doi.org/10.5867/medwave.2022.03.002550.
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Technological advances in mechanical ventilation have been essential to increasing the survival rate in intensive care units. Usually, patients needing mechanical ventilation use controlled ventilation to override the patient’s respiratory muscles and favor lung protection. Weaning from mechanical ventilation implies a transition towards spontaneous breathing, mainly using assisted mechanical ventilation. In this transition, the challenge for clinicians is to avoid under and over assistance and minimize excessive respiratory effort and iatrogenic diaphragmatic and lung damage. Esophageal balloon monitoring allows objective measurements of respiratory muscle activity in real time, but there are still limitations to its routine application in intensive care unit patients using mechanical ventilation. Like the esophageal balloon, respiratory muscle electromyography and diaphragmatic ultrasound are minimally invasive tools requiring specific training that monitor respiratory muscle activity. Particularly during the coronavirus disease pandemic, non invasive tools available on mechanical ventilators to monitor respiratory drive, inspiratory effort, and work of breathing have been extended to individualize mechanical ventilation based on patient’s needs. This review aims to identify the conceptual definitions of respiratory drive, inspiratory effort, and work of breathing and to identify non invasive maneuvers available on intensive care ventilators to measure these parameters. The literature highlights that although respiratory drive, inspiratory effort, and work of breathing are intuitive concepts, even distinguished authors disagree on their definitions.
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STOKES, DARRELL R. y ROBERT K. JOSEPHSON. "The Mechanical Power Output of a Crab Respiratory Muscle". Journal of Experimental Biology 140, n.º 1 (1 de noviembre de 1988): 287–99. http://dx.doi.org/10.1242/jeb.140.1.287.
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The mechanical power output was measured from scaphognathite (SG = gill bailer) muscle L2B of the crab Carcinus maenas (L.). The work was determined from the area of the loop formed by plotting muscle length against force when the muscle was subjected to sinusoidal length change (strain) and phasic stimulation in the length cycle. The stimulation pattern (10 stimuli per burst, burst length = 20% of cycle length) mimicked that which has been recorded from muscle L2B in intact animals. Work output was measured at cycle frequencies ranging from 0.5 to 5 Hz. The work output at optimum strain and stimulus phase increased with increasing cycle frequency to a maximum at 2–3 Hz and declined thereafter. The maximum work per cycle was 2.7 J kg−1 (15 °C). The power output reached a maximum (8.8 W kg−1) at 4 Hz. Both optimum strain and optimum stimulus phase were relatively constant over the range of burst frequencies examined. Based on the fraction of the total SG musculature represented by muscle L2B (18%) and literature values for the oxygen consumption associated with ventilation in C. maenas and for the hydraulic power output from an SG, we estimate that at a beat frequency of 2 Hz the SG muscle is about 10% efficient in converting metabolic energy to muscle power, and about 19% efficient in converting muscle power to hydraulic power.
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Trumble, D. R. y J. A. Magovern. "Ergometric studies of untrained skeletal muscle demonstrate feasibility of muscle-powered cardiac assistance". Journal of Applied Physiology 77, n.º 4 (1 de octubre de 1994): 2036–41. http://dx.doi.org/10.1152/jappl.1994.77.4.2036.
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The feasibility of biomechanical circulatory assistance hinges on the capacity of skeletal muscle to generate significant hemodynamic work. This study quantifies linear contractile energetics via a customized hydraulic ergometer. Six normal canine latissimus dorsi (LD) muscles (200 +/- 25 g) were evaluated. The muscles were not mobilized; thereby their collateral circulation was preserved. The humeral insertion of the LD muscle was transected and connected to the ergometer. Preload was adjusted to return the LD muscle to its in situ length, and one pulse train was delivered every second. The resulting contractions generated peak pressures of 134 +/- 17 mmHg with mean pressures during shortening of 102 +/- 12 mmHg. Flow rates averaged 5.45 +/- 0.26 l/min. Mechanical work output was calculated at 1.14 +/- 0.18 J/contraction, yielding an average power production of 4.57 +/- 0.72 W during shortening. Continuous LD output power, measured at 5.76 +/- 0.90 mW/g, compares favorably with the 3.48 mW/g typically generated by a 350-g human heart. We therefore conclude that skeletal muscle of sufficient mass can sustain work rates suitable for cardiac assistance despite the 50% power losses typically experienced after muscle training.
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Tallis, Jason, Rob S. James, Alexander G. Little, Val M. Cox, Michael J. Duncan y Frank Seebacher. "Early effects of ageing on the mechanical performance of isolated locomotory (EDL) and respiratory (diaphragm) skeletal muscle using the work-loop technique". American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 307, n.º 6 (15 de septiembre de 2014): R670—R684. http://dx.doi.org/10.1152/ajpregu.00115.2014.
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Previous isolated muscle studies examining the effects of ageing on contractility have used isometric protocols, which have been shown to have poor relevance to dynamic muscle performance in vivo. The present study uniquely uses the work-loop technique for a more realistic estimation of in vivo muscle function to examine changes in mammalian skeletal muscle mechanical properties with age. Measurements of maximal isometric stress, activation and relaxation time, maximal power output, and sustained power output during repetitive activation and recovery are compared in locomotory extensor digitorum longus (EDL) and core diaphragm muscle isolated from 3-, 10-, 30-, and 50-wk-old female mice to examine the early onset of ageing. A progressive age-related reduction in maximal isometric stress that was of greater magnitude than the decrease in maximal power output occurred in both muscles. Maximal force and power developed earlier in diaphragm than EDL muscle but demonstrated a greater age-related decline. The present study indicates that ability to sustain skeletal muscle power output through repetitive contraction is age- and muscle-dependent, which may help rationalize previously reported equivocal results from examination of the effect of age on muscular endurance. The age-related decline in EDL muscle performance is prevalent without a significant reduction in muscle mass, and biochemical analysis of key marker enzymes suggests that although there is some evidence of a more oxidative fiber type, this is not the primary contributor to the early age-related reduction in muscle contractility.
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Usherwood, James Richard (Jim). "The muscle-mechanical compromise framework: Implications for the scaling of gait and posture". Journal of Human Kinetics 52, n.º 1 (1 de septiembre de 2016): 107–14. http://dx.doi.org/10.1515/hukin-2015-0198.
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Abstract Many aspects of animal and human gait and posture cannot be predicted from purely mechanical work minimization or entirely based on optimizing muscle efficiency. Here, the Muscle-Mechanical Compromise Framework is introduced as a conceptual paradigm for considering the interactions and compromises between these two objectives. Current assumptions in implementing the Framework are presented. Implications of the compromise are discussed and related to the scaling of running mechanics and animal posture.
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Biewener, A. A. y G. B. Gillis. "Dynamics of muscle function during locomotion: accommodating variable conditions". Journal of Experimental Biology 202, n.º 23 (1 de diciembre de 1999): 3387–96. http://dx.doi.org/10.1242/jeb.202.23.3387.
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Much of what we know about animal locomotion is derived from studies examining animals moving within a single, homogeneous environment, at a steady speed and along a flat grade. As a result, the issue of how musculoskeletal function might shift to accommodate variability within the external environment has remained relatively unexplored. One possibility is that locomotor muscles are differentially recruited depending upon the environment in which the animal is moving. A second possibility is that the same muscles are recruited, but that they are activated in a different manner so that their contractile function differs according to environment. Finally, it is also possible that, in some cases, animals may not need to alter their musculoskeletal function to move under different external conditions. In this case, however, the mechanical behavior appropriate for one environmental condition may constrain locomotor performance in another. To begin to explore the means by which animals accommodate variable conditions in their environment, we present three case studies examining how musculoskeletal systems function to allow locomotion under variable conditions: (1) eels undulating through water and across land, (2) turkeys running on level and inclined surfaces, and (3) ducks using their limbs to walk and to paddle. In all three of these examples, the mechanical behavior of some muscle(s) involved in locomotion are altered, although to different degrees and in different ways. In the running turkeys, the mechanical function of a major ankle extensor muscle shifts from contracting isometrically on a flat surface (producing little work and power), to shortening actively during contraction on an uphill gradient (increasing the amount of work and power generated). In the ducks, the major ankle extensor undergoes the same general pattern of activation and shortening in water and on land, except that the absolute levels of muscle stress and strain and work output are greater during terrestrial locomotion. In eels, a transition to land elicits changes in electromyographic duty cycles and the relative timing of muscle activation, suggesting some alteration in the functional mechanics of the underlying musculature. Only by studying muscle function in animals moving under more variable conditions can we begin to characterize the functional breadth of the vertebrate musculoskeletal system and understand more fully its evolutionary design.
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Danos, Nicole, Natalie C. Holt, Gregory S. Sawicki y Emanuel Azizi. "Modeling age-related changes in muscle-tendon dynamics during cyclical contractions in the rat gastrocnemius". Journal of Applied Physiology 121, n.º 4 (1 de octubre de 2016): 1004–12. http://dx.doi.org/10.1152/japplphysiol.00396.2016.
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Efficient muscle-tendon performance during cyclical tasks is dependent on both active and passive mechanical tissue properties. Here we examine whether age-related changes in the properties of muscle-tendon units (MTUs) compromise their ability to do work and utilize elastic energy storage. We empirically quantified passive and active properties of the medial gastrocnemius muscle and material properties of the Achilles tendon in young (∼6 mo) and old (∼32 mo) rats. We then used these properties in computer simulations of a Hill-type muscle model operating in series with a Hookean spring. The modeled MTU was driven through sinusoidal length changes and activated at a phase that optimized muscle-tendon tuning to assess the relative contributions of active and passive elements to the force and work in each cycle. In physiologically realistic simulations where young and old MTUs started at similar passive forces and developed similar active forces, the capacity of old MTUs to store elastic energy and produce positive work was compromised. These results suggest that the observed increase in the metabolic cost of locomotion with aging may be in part due to the recruitment of additional muscles to compensate for the reduced work at the primary MTU. Furthermore, the age-related increases in passive stiffness coupled with a reduced active force capacity in the muscle can lead to shifts in the force-length and force-velocity operating range that may significantly impact mechanical and metabolic performance. Our study emphasizes the importance of the interplay between muscle and tendon mechanical properties in shaping MTU performance during cyclical contractions.
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Josephson, Robert K. "Mechanical Power output from Striated Muscle during Cyclic Contraction". Journal of Experimental Biology 114, n.º 1 (1 de enero de 1985): 493–512. http://dx.doi.org/10.1242/jeb.114.1.493.
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1. The mechanical power output of a synchronous insect muscle was determined by measuring tension as the muscle was subjected to sinusoidal length change and stimuli which occurred at selected phases of the length cycle. The area of the loop formed by plotting muscle tension against length over a full cycle is the work done on that cycle; the work done times the cycle frequency is the mechanical power output. The muscle was a flight muscle of the tettigoniid Neoconocephalus triops. The measurements were made at the normal wing-stroke frequency for flight (25 Hz) and operating temperature (30°C). 2. The power output with a single stimulus per cycle, optimal excursion amplitude, and optimal stimulus phase was 1.52 J kg−1 cycle−1 or 37W kg−1. The maximum power output occurs at a phase such that the onset of the twitch coincides with the onset of the shortening half of the length cycle. The optimum excursion amplitude was 5.5% rest length; with greater excursion, work output declined because of decreasing muscle force associated with the more rapid shortening velocity. 3. Multiple stimulation per cycle increases the power output above that available with twitch contractions. In this muscle, the maximum mechanical power output at 25 Hz was 76 W kg−1 which was achieved with three stimuli per cycle separated by 4-ms intervals and an excursion amplitude of 6.0% rest length. 4. The maximum work output during the shortening of an isotonic twitch contraction was about the same as the work done over a full sinusoidal shortening-lengthening cycle with a single stimulus per cycle and optimum excursion amplitude and phase.
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Heglund, N. C. y G. A. Cavagna. "Efficiency of vertebrate locomotory muscles". Journal of Experimental Biology 115, n.º 1 (1 de marzo de 1985): 283–92. http://dx.doi.org/10.1242/jeb.115.1.283.
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We have examined the efficiency of vertebrate striated muscle at two different organizational levels: whole animals and isolated muscles. Terrestrial locomotion is used as a model of ‘normal’ muscular contraction; animal size and running speed are used as independent variables in order to change either the metabolic requirements of the muscles or the mechanical power production by the muscles over a wide range of values. The weight-specific metabolic power input to an animal increases nearly linearly with speed and increases with decreasing body size, while the weight-specific mechanical power output increases curvilinearly with speed and is independent of size. Consequently, the efficiency of the muscles in producing positive work increases with speed and the peak efficiency increases with increasing body size, attaining values of over 70% in large animals, but only 7% in small ones. The isolated muscle experiments were performed on frog muscle, and rat ‘fast’ and ‘slow’ muscles. We measured the work done, the oxygen consumed during recovery from the stimulation, and calculated the efficiency and the ‘economy’ (the cost of maintaining tension). The muscles were made to: (i) emulate the contractions seen during locomotion, i.e. shorten after a pre-stretch; or (ii) shorten at the same velocity and from the same muscle length as in (i), but without the pre-stretch. It was found that in mammalian muscles the peak efficiency with a pre-stretch attained high values, approaching the peak efficiencies in large animals. The maximum efficiency (attained at 1 length s-1 in fast muscle and at 0.5 lengths s-1 in slow muscle) did not differ much in the two muscles, whereas economy was greater in the slow muscle than in the fast muscle.
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Roberts, Thomas J. y Emanuel Azizi. "The series-elastic shock absorber: tendons attenuate muscle power during eccentric actions". Journal of Applied Physiology 109, n.º 2 (agosto de 2010): 396–404. http://dx.doi.org/10.1152/japplphysiol.01272.2009.
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Elastic tendons can act as muscle power amplifiers or energy-conserving springs during locomotion. We used an in situ muscle-tendon preparation to examine the mechanical function of tendons during lengthening contractions, when muscles absorb energy. Force, length, and power were measured in the lateral gastrocnemius muscle of wild turkeys. Sonomicrometry was used to measure muscle fascicle length independently from muscle-tendon unit (MTU) length, as measured by a muscle lever system (servomotor). A series of ramp stretches of varying velocities was applied to the MTU in fully activated muscles. Fascicle length changes were decoupled from length changes imposed on the MTU by the servomotor. Under most conditions, muscle fascicles shortened on average, while the MTU lengthened. Energy input to the MTU during the fastest lengthenings was −54.4 J/kg, while estimated work input to the muscle fascicles during this period was only −11.24 J/kg. This discrepancy indicates that energy was first absorbed by elastic elements, then released to do work on muscle fascicles after the lengthening phase of the contraction. The temporary storage of energy by elastic elements also resulted in a significant attenuation of power input to the muscle fascicles. At the fastest lengthening rates, peak instantaneous power input to the MTU reached −2,143.9 W/kg, while peak power input to the fascicles was only −557.6 W/kg. These results demonstrate that tendons may act as mechanical buffers by limiting peak muscle forces, lengthening rates, and power inputs during energy-absorbing contractions.
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Askew, Graham N., Valerie M. Cox, John D. Altringham y David F. Goldspink. "Mechanical properties of the latissimus dorsi muscle after cyclic training". Journal of Applied Physiology 93, n.º 2 (1 de agosto de 2002): 649–59. http://dx.doi.org/10.1152/japplphysiol.00218.2002.
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Cardiomyoplasty is a procedure developed to improve heart performance in patients suffering from congestive heart failure. The latissimus dorsi (LD) muscle is surgically wrapped around the failing ventricles and stimulated to contract in synchrony with the heart. The LD muscle is easily fatigued and as a result is unsuitable for cardiomyoplasty. For useful operation as a cardiac-assist device, the fatigue resistance of the LD muscle must be improved while retaining a high power output. The LD muscle of rabbits was subjected to a training regime in which cyclic work was performed. Training transformed the fiber-type composition from approximately equal proportions of fast oxidative glycolytic (FOG) and fast glycolytic (FG) fibers to one composed of almost entirely of FOG with no FG, which increased fatigue resistance while retaining rapid contraction kinetics. Muscle mass and cross-sectional area increased but power output decreased, relative to control muscles. This training regime represents a significant improvement in terms of preserving muscle mass and power compared with other training regimes, while enhancing fatigue resistance, although some fiber damage occurred. The power output of the trained LD muscle was calculated to be sufficient to deliver a significant level of assistance to a failing heart during cardiomyoplasty.
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Roberts, Thomas J. y Jeffrey A. Scales. "Mechanical power output during running accelerations in wild turkeys". Journal of Experimental Biology 205, n.º 10 (15 de mayo de 2002): 1485–94. http://dx.doi.org/10.1242/jeb.205.10.1485.
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SUMMARYWe tested the hypothesis that the hindlimb muscles of wild turkeys(Meleagris gallopavo) can produce maximal power during running accelerations. The mechanical power developed during single running steps was calculated from force-plate and high-speed video measurements as turkeys accelerated over a trackway. Steady-speed running steps and accelerations were compared to determine how turkeys alter their running mechanics from a low-power to a high-power gait. During maximal accelerations, turkeys eliminated two features of running mechanics that are characteristic of steady-speed running: (i) they produced purely propulsive horizontal ground reaction forces, with no braking forces, and (ii) they produced purely positive work during stance, with no decrease in the mechanical energy of the body during the step. The braking and propulsive forces ordinarily developed during steady-speed running are important for balance because they align the ground reaction force vector with the center of mass. Increases in acceleration in turkeys correlated with decreases in the angle of limb protraction at toe-down and increases in the angle of limb retraction at toe-off. These kinematic changes allow turkeys to maintain the alignment of the center of mass and ground reaction force vector during accelerations when large propulsive forces result in a forward-directed ground reaction force. During the highest accelerations, turkeys produced exclusively positive mechanical power. The measured power output during acceleration divided by the total hindlimb muscle mass yielded estimates of peak instantaneous power output in excess of 400 W kg-1 hindlimb muscle mass. This value exceeds estimates of peak instantaneous power output of turkey muscle fibers. The mean power developed during the entire stance phase increased from approximately zero during steady-speed runs to more than 150 W kg-1muscle during the highest accelerations. The high power outputs observed during accelerations suggest that elastic energy storage and recovery may redistribute muscle power during acceleration. Elastic mechanisms may expand the functional range of muscle contractile elements in running animals by allowing muscles to vary their mechanical function from force-producing struts during steady-speed running to power-producing motors during acceleration.
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Russell, Brenda, Delara Motlagh y William W. Ashley. "Form follows function: how muscle shape is regulated by work". Journal of Applied Physiology 88, n.º 3 (1 de marzo de 2000): 1127–32. http://dx.doi.org/10.1152/jappl.2000.88.3.1127.
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What determines the shape, size, and force output of cardiac and skeletal muscle? Chicago architect Louis Sullivan (1856–1924), father of the skyscraper, observed that “form follows function.” This is as true for the structural elements of a striated muscle cell as it is for the architectural features of a building. Function is a critical evolutionary determinant, not form. To survive, the animal has evolved muscles with the capacity for dynamic responses to altered functional demand. For example, work against an increased load leads to increased mass and cross-sectional area (hypertrophy), which is directly proportional to an increased potential for force production. Thus a cell has the capacity to alter its shape as well as its volume in response to a need for altered force production. Muscle function relies primarily on an organized assembly of contractile and other sarcomeric proteins. From analysis of homogenized cells and molecular and biochemical assays, we have learned about transcription, translation, and posttranslational processes that underlie protein synthesis but still have done little in addressing the important questions of shape or regional cell growth. Skeletal muscles only grow in length as the bones grow; therefore, most studies of adult hypertrophy really only involve increased cross-sectional area. The heart chamber, however, can extend in both longitudinal and transverse directions, and cardiac cells can grow in length and width. We know little about the regulation of these directional processes that appear as a cell gets larger with hypertrophy or smaller with atrophy. This review gives a brief overview of the regulation of cell shape and the composition and aggregation of contractile proteins into filaments, the sarcomere, and myofibrils. We examine how mechanical activity regulates the turnover and exchange of contraction proteins. Finally, we suggest what kinds of experiments are needed to answer these fundamental questions about the regulation of muscle cell shape.
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Barclay, C. J. "Efficiency of fast- and slow-twitch muscles of the mouse performing cyclic contractions." Journal of Experimental Biology 193, n.º 1 (1 de agosto de 1994): 65–78. http://dx.doi.org/10.1242/jeb.193.1.65.
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The mechanical efficiency of mouse fast- and slow-twitch muscle was determined during contractions involving sinusoidal length changes. Measurements were made of muscle length, force production and initial heat output from bundles of muscle fibres in vitro at 31 degrees C. Power output was calculated as the product of the net work output per sinusoidal length cycle and the cycle frequency. The initial mechanical efficiency was defined as power output/(rate of initial heat production+power output). Both power output and rate of initial heat production were averaged over a full cycle of length change. The amplitude of length changes was +/- 5% of muscle length. Stimulus phase and duration were adjusted to maximise net work output at each cycle frequency used. The maximum initial mechanical efficiency of slow-twitch soleus muscle was 0.52 +/- 0.01 (mean +/- 1 S.E.M. N = 4) and occurred at a cycle frequency of 3 Hz. Efficiency was not significantly different from this at cycle frequencies of 1.5-4 Hz, but was significantly lower at cycle frequencies of 0.5 and 1 Hz. The maximum efficiency of fast-twitch extensor digitorum longus muscle was 0.34 +/- 0.03 (N = 4) and was relatively constant (0.32-0.34) over a broad range of frequencies (4-12 Hz). A comparison of these results with those from previous studies of the mechanical efficiency of mammalian muscles indicates that efficiency depends markedly on contraction protocol.
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Baxi, J., C. J. Barclay y C. L. Gibbs. "Energetics of rat papillary muscle during contractions with sinusoidal length changes". American Journal of Physiology-Heart and Circulatory Physiology 278, n.º 5 (1 de mayo de 2000): H1545—H1554. http://dx.doi.org/10.1152/ajpheart.2000.278.5.h1545.
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The mechanical efficiency of rat cardiac muscle was determined using a contraction protocol involving cyclical, sinusoidal length changes and phasic stimulation at physiological frequencies (1–4 Hz). Experiments were performed in vitro (27°C) using rat left ventricular papillary muscles. Efficiency was determined from measurements of the net work performed and enthalpy produced by muscles during a series of 40 contractions. Net mechanical efficiency was defined as the percentage of the total, suprabasal enthalpy output that appeared as mechanical work. Maximum efficiency was ∼15% at contraction frequencies between 2 and 2.5 Hz. At lower and higher frequencies, efficiency was ∼10%. Enthalpy output per cycle was independent of cycle frequency at all but the highest frequency used. The basis of the high efficiency between 2 and 2.5 Hz was that work output was also greatest at these frequencies. At these frequencies, the duration of the applied length change was well matched to the kinetics of force generation, and active force generation occurred throughout the shortening period.
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Prilutsky, B. I., W. Herzog y T. L. Allinger. "Mechanical power and work of cat soleus, gastrocnemius and plantaris muscles during locomotion: possible functional significance of muscle design and force patterns." Journal of Experimental Biology 199, n.º 4 (1 de abril de 1996): 801–14. http://dx.doi.org/10.1242/jeb.199.4.801.
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Electrical activity, forces, power and work of the soleus (SO), the gastrocnemius (GA) and the plantaris (PL) muscles were measured during locomotion in the cat in order to study the functional role of these ankle extensor muscles. Forces and electrical activity (EMG) of the three muscles were measured using home-made force transducers and bipolar, indwelling wire electrodes, respectively, for walking and trotting at speeds of 0.4 to 1.8 m s-1 on a motor-driven treadmill. Video records and a geometrical model of the cat hindlimb were used for calculating the rates of change in lengths of the SO, GA and PL muscles. The instantaneous maximum possible force that can be produced by a muscle at a given fibre length and the rate of change in fibre length (termed contractile abilities) were estimated for each muscle throughout the step cycle. Fibre lengths of the SO, GA and PL were calculated using a planar, geometrical muscle model, measured muscle forces and kinematics, and morphological measurements from the animal after it had been killed. Mechanical power and work of SO, GA and PL were calculated for 144 step cycles. The contribution of the positive work done by the ankle extensor muscles of one hindlimb to the increase of the total mechanical energy of the body (estimated from values in the literature) increased from 4-11% at speeds of locomotion of 0.4 and 0.8 m s-1 to 7-16% at speeds of 1.2 m s-1 and above. The relative contributions of the negative and positive work to the total negative and positive work done by the three ankle extensor muscles increased for GA, decreased for SO and remained about the same for PL, with increasing speeds of locomotion. At speeds of 0.4-0.8 m s-1, the positive work normalized to muscle mass was 7.5-11.0 J kg-1, 1.9-3.0 J kg-1 and 5.3-8.4 J kg-1 for SO, GA and PL, respectively. At speeds of 1.2-1.8 m s-1, the corresponding values were 9.8-16.7 J kg-1, 6.0-10.7 J kg-1 and 13.4-25.0 J kg-1. Peak forces of GA and PL increased and peak forces of SO did not change substantially with increasing speeds of locomotion. The time of decrease of force and the time of decrease of power after peak values had been achieved were much shorter for SO than the corresponding times for GA and PL at fast speeds of locomotion. The faster decrease in the force and power of SO compared with GA and PL was caused by the fast decrease of the contractile abilities and the activation of SO. The results of this study suggest that the ankle extensor muscles play a significant role in the generation of mechanical energy for locomotion.
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Ross, Stephanie A. y James M. Wakeling. "Muscle shortening velocity depends on tissue inertia and level of activation during submaximal contractions". Biology Letters 12, n.º 6 (junio de 2016): 20151041. http://dx.doi.org/10.1098/rsbl.2015.1041.
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In order to perform external work, muscles must do additional internal work to deform their tissue, and in particular, to overcome the inertia due to their internal mass. However, the contribution of the internal mass within a muscle to the mechanical output of that muscle has only rarely been studied. Here, we use a dynamic, multi-element Hill-type muscle model to examine the effects of the inertial mass within muscle on its contractile performance. We find that the maximum strain-rate of muscle is slower for lower activations and larger muscle sizes. As muscle size increases, the ability of the muscle to overcome its inertial load will decrease, as muscle tension is proportional to cross-sectional area and inertial load is proportional to mass. Thus, muscles that are larger in size will have a higher inertial cost to contraction. Similarly, when muscle size and inertial load are held constant, decreasing muscle activation will increase inertial cost to contraction by reducing muscle tension. These results show that inertial loads within muscle contribute to a slowing of muscle contractile velocities (strain-rates), particularly at the submaximal activations that are typical during animal locomotion.
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Willems, P. A., G. A. Cavagna y N. C. Heglund. "External, internal and total work in human locomotion." Journal of Experimental Biology 198, n.º 2 (1 de febrero de 1995): 379–93. http://dx.doi.org/10.1242/jeb.198.2.379.
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The muscle-tendon work performed during locomotion can, in principle, be measured from the mechanical energy of the centre of mass of the whole body and the kinetic energy due to the movements of the body segments relative to the centre of mass of the body. Problems arise when calculating the muscle-tendon work from increases in mechanical energy, largely in correctly attributing these increases either to energy transfer or to muscle-tendon work. In this study, the kinetic and gravitational potential energy of the centre of mass of the whole human body was measured (using a force platform) simultaneously with calculation of the kinetic and potential energy of the body segments due to their movements relative to the body centre of mass (using cinematography) at different speeds of walking and running. Upper and lower boundaries to the total work were determined by including or excluding possible energy transfers between the segments of each limb, between the limbs and between the centre of mass of the body and the limbs. It appears that the muscle-tendon work of locomotion is most accurately measured when energy transfers are only included between segments of the same limb, but not among the limbs or between the limbs and the centre of mass of the whole body.
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Ettema, G. J. "Mechanical efficiency and efficiency of storage and release of series elastic energy in skeletal muscle during stretch-shorten cycles." Journal of Experimental Biology 199, n.º 9 (1 de septiembre de 1996): 1983–97. http://dx.doi.org/10.1242/jeb.199.9.1983.
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The mechanical energy exchanges between components of a muscle-tendon complex, i.e. the contractile element (CE) and the series elastic element (SEE), and the environment during stretch-shorten cycles were examined. The efficiency of the storage and release of series elastic energy (SEE efficiency) and the overall mechanical efficiency of the rat gastrocnemius muscle (N = 5) were determined for a range of stretch-shorten contractions. SEE efficiency was defined as elastic energy released to the environment divided by external work done upon the muscle-tendon complex plus internal work exchange from the CE to the SEE. Mechanical efficiency is external work done by the muscle-tendon complex divided by the external work done upon the muscle-tendon complex plus work done by the CE. All stretch-shorten cycles were performed with a movement amplitude of 3mm (6.7% strain). Cycle frequency, duty factor and the onset of stimulation were altered for the different cycles. SEE efficiency varied from 0.02 to 0.85, mechanical efficiency from 0.43 t 0.92. SEE efficiency depended on the timing of stimulation and net muscle power in different ways. Mechanical efficiency was much more closely correlated with net power. The timing of muscle relaxation was crucial for the effective release of elastic energy. Simulated in vivo contractions indicated that during rat locomotion the gastrocnemius may have a role other than that of effectively storing elastic energy and generating work. Computer simulations showed that the amount of series elastic compliance can affect the internal energetics of a muscle contraction strongly without changing the muscle force generation dramatically.
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Usherwood, J. R. y N. W. Gladman. "Why are the fastest runners of intermediate size? Contrasting scaling of mechanical demands and muscle supply of work and power". Biology Letters 16, n.º 10 (octubre de 2020): 20200579. http://dx.doi.org/10.1098/rsbl.2020.0579.
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The fastest land animals are of intermediate size. Cheetah, antelope, greyhounds and racehorses have been measured running much faster than reported for elephants or elephant shrews. Can this be attributed to scaling of physical demands and explicit physiological constraints to supply? Here, we describe the scaling of mechanical work demand each stride, and the mechanical power demand each stance. Unlike muscle stress, strain and strain rate, these mechanical demands cannot be circumvented by changing the muscle gearing with minor adaptations in bone geometry or trivial adjustments to limb posture. Constraints to the capacity of muscle to supply work and power impose fundamental limitations to maximum speed. Given an upper limit to muscle work capacity each contraction, maximum speeds in big animals are constrained by the mechanical work demand each step. With an upper limit to instantaneous muscle power production, maximal speeds in small animals are limited by the high power demands during brief stance periods. The high maximum speed of the cheetah may therefore be attributed as much to its size as to its other anatomical and physiological adaptations.
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Brechue, W. F., K. E. Gropp, B. T. Ameredes, D. M. O'Drobinak, W. N. Stainsby y J. W. Harvey. "Metabolic and work capacity of skeletal muscle of PFK-deficient dogs studied in situ". Journal of Applied Physiology 77, n.º 5 (1 de noviembre de 1994): 2456–67. http://dx.doi.org/10.1152/jappl.1994.77.5.2456.
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Mechanical and metabolic relationships of muscle lacking phosphofructokinase (PFKD) activity were compared with muscle having normal phosphofructokinase (NORM) activity by using the gastrocnemius-plantaris muscle group with isolated circulation in situ. Muscle contractile properties were similar in both groups. Initial power output (W) during repetitive tetanic (200 ms, 50 impulses/s) isotonic contractions was similar in both groups; however, W declined significantly more (30–80%) in PFKD than in NORM muscle over time, with a constant O2 uptake (VO2)/W. Despite similar O2 and substrate delivery, PFKD muscle had a lower VO2 (42–55%), less glucose uptake, similar free fatty acid uptake, and lactic acid uptake rather than output, during contractions. Muscle venous H+ concentration, strong ion difference, and PCO2 increased during contractions, the magnitude of change being smaller in PFKD muscle. Elevating arterial lactate concentration before contractions in PFKD muscle resulted in significant improvements in W and VO2 without altering the acid-base exchange at the muscle. Increasing O2 delivery by increasing arterial O2 concentration in PFKD dogs did not improve W or VO2. We conclude that, despite no inherent mechanical or contractile differences, PFKD muscle has a severely limited oxidative capacity and exaggerated fatigue and blood flow responses to contractions due to limited substrate metabolism resulting from the inability to utilize glycogen and/or glucose.
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Gerry, Shannon P. y David J. Ellerby. "Serotonin modulates muscle function in the medicinal leech Hirudo verbana". Biology Letters 7, n.º 6 (11 de mayo de 2011): 885–88. http://dx.doi.org/10.1098/rsbl.2011.0303.
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The body wall muscles of sanguivorous leeches power mechanically diverse behaviours: suction feeding, crawling and swimming. These require longitudinal muscle to exert force over an extremely large length range, from 145 to 46 per cent of the mean segmental swimming length. Previous data, however, suggest that leech body wall muscle has limited capacity for force production when elongated. Serotonin (5-HT) alters the passive properties of the body wall and stimulates feeding. We hypothesized that 5-HT may also have a role in allowing force production in elongated muscle by changing the shape of the length–tension relationship (LTR). LTRs were measured from longitudinal muscle strips in vitro in physiological saline with and without the presence of 10 µM 5-HT. The LTR was much broader than previously measured for leech muscle. Rather than shifting the LTR, 5-HT reduced passive muscle tonus and increased active stress at all lengths. In addition to modulating leech behaviour and passive mechanical properties, 5-HT probably enhances muscle force and work production during locomotion and feeding.