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1
Siegle, M. L., S. Buhner, M. Schemann, H. R. Schmid, and H. J. Ehrlein. "Propagation velocities and frequencies of contractions along canine small intestine." American Journal of Physiology-Gastrointestinal and Liver Physiology 258, no. 5 (May 1, 1990): G738—G744. http://dx.doi.org/10.1152/ajpgi.1990.258.5.g738.
Повний текст джерелаАнотація:
This study was performed to clarify in detail the behavior of the propagation velocities and frequencies of contractions along the canine small intestine. In conscious dogs, duodenal, jejunal, and ileal contractions were recorded by multiple, closely spaced strain gauges and analyzed by a computerized method. During both the interdigestive and postprandial states, the propagation velocity increased from the duodenal bulb to the distal duodenum and declined aborally within the jejunum, reaching rather constant values in the ileum. The decrease was steepest in the proximal part of the jejunum. In contrast to the propagation velocities, the contraction frequencies were almost constant in the upper small intestine. In the ileum, the contraction frequencies were markedly lower than in the upper small intestine, indicating that the aboral decrease in frequency occurred in the distal parts of the jejunum. We conclude that both the propagation velocities and the frequencies of contractions decline aborally in a nonlinear fashion. However, the nonlinear patterns of the frequency and the propagation velocity gradients are different.
2
Yapıcı, Ayşegül. "Factors effecting hamstrings to quadriceps peak torque ratio in volleyball players." Journal of Human Sciences 13, no. 3 (December 8, 2016): 5282. http://dx.doi.org/10.14687/jhs.v13i3.4149.
Повний текст джерелаАнотація:
The aim of this study was to analyze of hamstring to quadriceps peak torque ratio (H:Q) measured in isokinetic testing with respect to different angular velocities (60, 180, 300°/s), gender, dominant side and mode of contraction (concentric, eccentric) in volleyball players. Twenty male and ten female healthy volleyball players participated in this study. An independent t-test was used to compare the differences between gender. One-way analysis of variance test was conducted to test for differences by effecting factors. There was a statistically significant difference between dominant and non-dominant side in H:Q ratio at 300°/s in males (p<0.05). There was no statistically significant difference between dominant and non-dominant side in H:Q ratio between female and male volleyball players (p>0.05). There was no statistically significant difference between at 60-180-300°/s velocities by Hconc:Qconc contractions and at 60°/s by Hconc:Qecc contractions for male and female’s peak torques in dominant side (p>0.05). There was a statistically significant difference between at 60°/s by Hconc:Qconc and Hconc:Qecc contractions for male and female’s peak torques in dominant side (p<0.05). There was a statistically significant difference H:Q ratio obtained by concentric contraction between at 60-300°/s and 180-300°/s velocities (p<0.05), whereas there was no statistically significant difference between 60-180°/s in male and female’s peak torques in dominant side (p>0.05). It was found in our study that H:Q ratio increases with increasing angular velocity. The findings of the present study indicated that angular velocity, type of contraction and leg dominance influence isokinetic strength profiles of male and female, consequently, muscular balance that is H:Q at the knee. This implies that isokinetic concentric knee strength plays more role in high intensity contractions and has more effect at high velocities of contraction in maximal performance.
3
Reeves, Neil D., and Marco V. Narici. "Behavior of human muscle fascicles during shortening and lengthening contractions in vivo." Journal of Applied Physiology 95, no. 3 (September 2003): 1090–96. http://dx.doi.org/10.1152/japplphysiol.01046.2002.
Повний текст джерелаАнотація:
The aim of the present study was to investigate the behavior of human muscle fascicles during dynamic contractions. Eight subjects performed maximal isometric dorsiflexion contractions at six ankle joint angles and maximal isokinetic concentric and eccentric contractions at five angular velocities. Tibialis anterior muscle architecture was measured in vivo by use of B-mode ultrasonography. During maximal isometric contraction, fascicle length was shorter and pennation angle larger compared with values at rest ( P < 0.01). During isokinetic concentric contractions from 0 to 4.36 rad/s, fascicle length measured at a constant ankle joint angle increased curvilinearly from 49.5 to 69.7 mm (41%; P < 0.01), whereas pennation angle decreased curvilinearly from 14.8 to 9.8° (34%; P < 0.01). During eccentric muscle actions, fascicles contracted quasi-isometrically, independent of angular velocity. The behavior of muscle fascicles during shortening contractions was believed to reflect the degree of stretch applied to the series elastic component, which decreases with increasing contraction velocity. The quasi-isometric behavior of fascicles during eccentric muscle actions suggests that the series elastic component acts as a mechanical buffer during active lengthening.
4
Ducros, Laurent, Philippe Bonnin, Bernard P. Cholley, Eric Vicaut, Moncef Benayed, Denis Jacob, and Didier Payen. "Increasing Maternal Blood Pressure with Ephedrine Increases Uterine Artery Blood Flow Velocity during Uterine Contraction." Anesthesiology 96, no. 3 (March 1, 2002): 612–16. http://dx.doi.org/10.1097/00000542-200203000-00017.
Повний текст джерелаАнотація:
Background During labor, ephedrine is widely used to prevent or to treat maternal arterial hypotension and restore uterine perfusion pressure to avoid intrapartum fetal asphyxia. However, the effects of ephedrine on uterine blood flow have not been studied during uterine contractions. The purpose of the study was to assess the effects of ephedrine on uterine artery velocities and resistance index using the Doppler technique during the active phase of labor. Methods Ten normotensive, healthy parturients with uncomplicated pregnancies at term received intravenous ephedrine during labor to increase mean arterial pressure up to a maximum of 20% above their baseline pressure. Peak systolic and end-diastolic Doppler flow velocities and resistance indices were measured in the uterine artery before and immediately after administration of bolus intravenous ephedrine and after ephedrine washout. Umbilical and fetal middle cerebral arterial resistance indices and fetal heart rate were also calculated. Results After ephedrine administration, mean arterial pressure increased by 17 +/- 4%. End-diastolic flow velocity in the uterine artery at peak amplitude of uterine contraction was restored to 74% of the value observed in the absence of contraction. The systolic velocity was totally restored, and the uterine resistance index was significantly decreased, compared with the values in the absence of contraction. Between uterine contractions, ephedrine induced similar but less marked effects. Fetal hemodynamic parameters were not altered by ephedrine administration. Conclusions Bolus administration of intravenous ephedrine reversed the dramatic decrease in diastolic uteroplacental blood flow velocity and the increase in resistance index during uterine contraction, without altering fetal hemodynamic parameters. This suggests that the increase in uterine perfusion pressure during labor could in part restore uterine blood flow to the placenta during uterine contraction.
5
Chaouachi, Anis, Monoem Haddad, Carlo Castagna, Del P. Wong, Fathi Kaouech, Karim Chamari, and David G. Behm. "Potentiation and Recovery Following Low- and High-Speed Isokinetic Contractions in Boys." Pediatric Exercise Science 23, no. 1 (February 2011): 136–50. http://dx.doi.org/10.1123/pes.23.1.136.
Повний текст джерелаАнотація:
The objective of this study was to examine the response and recovery to a single set of maximal, low and high angular velocity isokinetic leg extension-flexion contractions with boys. Sixteen boys (11–14 yrs) performed 10 isokinetic contractions at 60°.s−1 (Isok60) and 300°.s−1 (Isok300). Three contractions at both velocities, blood lactate and ratings of perceived exertion were monitored pretest and at 2, 3, 4, and 5 min of recovery (RI). Participants were tested in a random counterbalanced order for each velocity and recovery period. Only a single contraction velocity (300°.s−1 or 60°.s−1) was tested during recovery at each session to remove confounding influences between the recovery intervals. Recovery results showed no change in quadriceps’ power at 300°.s−1, quadriceps’ power, work and torque at 60°.s−1 and hamstrings’ power and work with 60°.s−1. There was an increase during the 2 min RI in hamstrings’ power, work and torque and quadriceps’
6
Livingston, E. H., T. J. Howard, T. R. Garrick, E. P. Passaro, and P. H. Guth. "Strong gastric contractions cause mucosal ischemia." American Journal of Physiology-Gastrointestinal and Liver Physiology 260, no. 3 (March 1, 1991): G524—G530. http://dx.doi.org/10.1152/ajpgi.1991.260.3.g524.
Повний текст джерелаАнотація:
Contractions of a segment of bowel result in alterations of its blood flow. However, the precise temporal and spacial relationships between contractions and mucosal blood flow are unknown. Rats were fitted with strain gauge force transducers and implanted with silver wire electrodes into the muscularis externa of the stomach. In vivo microscopic observation of motility and of the gastric mucosal blood flow was performed during electrical field-stimulated contractions. Contractions originated in the midcorpus, were 0.237 +/- 0.018 cm wide, traveled along the corpus at 0.133 +/- 0.024 cm/s, and had a duration of 5.9 +/- 0.1 s. Antral contractions were 0.174 +/- 0.032 cm wide, traveled at 0.070 +/- 0.009 cm/s, and had a duration of 5.6 +/- 0.7 s. During the contraction, capillary flow velocity in the corpus decreased from a basal value of 410 +/- 105 to 206 +/- 104 microns/s at the peak of a contraction. Five seconds after the contraction was released hyperemia was observed with the flow velocity increasing to 570 +/- 102 microns/s. In the antrum, flow stopped completely during the contraction irrespective of the initial flow velocity and no hyperemia occurred with release of the contraction; rather, flow velocity slowly returned to baseline values. In both regions the flow reductions were in phase with the contractions as measured by the force transducers. These studies provide direct evidence that strong gastric contractions can effectively reduce or stop gastric mucosal blood flow.
7
Christou, Evangelos A., Minoru Shinohara, and Roger M. Enoka. "Fluctuations in acceleration during voluntary contractions lead to greater impairment of movement accuracy in old adults." Journal of Applied Physiology 95, no. 1 (July 2003): 373–84. http://dx.doi.org/10.1152/japplphysiol.00060.2003.
Повний текст джерелаАнотація:
The purpose of the study was to assess the effect of movement velocity on the relation between fluctuations in acceleration and the ability to achieve a target velocity during voluntary contractions performed by young (29.5 ± 4.3 yr) and old (74.9 ± 6.2 yr) adults. Subjects performed concentric and eccentric contractions with the first dorsal interosseus muscle while lifting a submaximal load (15% of maximum) at six movement velocities (0.03–1.16 rad/s). Fluctuations in acceleration, the accuracy of matching the target velocity, and electromyographic (EMG) activity were determined from three trials for each contraction type and movement velocity. The fluctuations in acceleration increased with movement velocity for both concentric and eccentric contractions, but they were greatest during fast eccentric contractions (∼135%) when there was stronger modulation of acceleration in the 5- to 10-Hz bandwidth. Nonetheless, EMG amplitude for first dorsal interosseus increased with movement velocity only for concentric and not eccentric contractions. Consistent with the minimum variance theory, movement accuracy was related to the fluctuations in acceleration for both types of contractions in all subjects. For a given level of fluctuations in acceleration, however, old subjects were three times less accurate than young subjects. Although the EMG amplitude at each speed was similar for young and old adults, only the young adults modulated the power in the EMG spectrum with speed. Thus the fluctuations in acceleration during voluntary contractions had a more pronounced effect on movement accuracy for old adults compared with young adults, probably due to factors that influenced the frequency-domain characteristics of the EMG.
8
Siegle, M. L., and H. J. Ehrlein. "Interdigestive contractile patterns of the ileum in dogs." American Journal of Physiology-Gastrointestinal and Liver Physiology 253, no. 4 (October 1, 1987): G452—G460. http://dx.doi.org/10.1152/ajpgi.1987.253.4.g452.
Повний текст джерелаАнотація:
The aim of this study is to elucidate the nature of ileal interdigestive contractile patterns by the computerized analysis of the contraction spread and by videofluoroscopy. Conscious dogs equipped with closely spaced strain-gauge force transducers were used. Two patterns of repetitive, phasic contractions were recorded, migrating clusters and phase IIIs; both patterns consisted of repetitive, propagated contractions. Both patterns migrated aborad by sequential movement of contraction waves down the bowel. Consequently, the rate of migration of either of the entire patterns was slower than the propagation velocity of constituent, individual contraction waves. Both patterns differed in several parameters, especially the propagated contractions of the clusters spread over shorter distances (1.47 +/- 0.4 cm) than those of phase III (4.65 +/- 0.99 cm). Compared with these complex patterns, propagating power contractions represented single contractions that propagated aborad at the same velocity as the contraction waves of the complex patterns. All three patterns propelled luminal contents distally.
9
Forrester, Stephanie E., and Matthew T. G. Pain. "A Combined Muscle Model and Wavelet Approach to Interpreting the Surface EMG Signals from Maximal Dynamic Knee Extensions." Journal of Applied Biomechanics 26, no. 1 (February 2010): 62–72. http://dx.doi.org/10.1123/jab.26.1.62.
Повний текст джерелаАнотація:
This study aimed to identify areas of reduced surface EMG amplitude and changed frequency across the phase space of a maximal dynamic knee extension task. The hypotheses were that (1) amplitude would be lower for eccentric contractions compared with concentric contractions and unaffected by fiber length and (2) mean frequency would also be lower for eccentric contractions and unaffected by fiber length. Joint torque and EMG signals from the vasti and rectus femoris were recorded for eight athletic subjects performing maximum knee extensions at 13 preset crank velocities spanning ±300°⋅s−1. The instantaneous amplitude and mean frequency were calculated using the continuous wavelet transform time–frequency method, and the fiber dynamics were determined using a muscle model of the knee extensions. The results indicated that (1) only for the rectus femoris were amplitudes significantly lower for eccentric contractions (p= .019) and, for the vasti, amplitudes during eccentric contractions were less than maximal but this was also the case for concentric contractions due to a significant reduction in amplitude toward knee extension (p= .023), and (2) mean frequency increased significantly with decreasing fiber length for all knee extensors and contraction velocities (p= .029). Using time–frequency processing of the EMG signals and a muscle model allowed the simultaneous assessment of fiber length, velocity, and EMG.
10
Harwood, B., and C. L. Rice. "Short interspike intervals and double discharges of anconeus motor unit action potentials for the production of dynamic elbow extensions." Journal of Neurophysiology 111, no. 10 (May 15, 2014): 2039–46. http://dx.doi.org/10.1152/jn.00412.2013.
Повний текст джерелаАнотація:
Incidence of double discharges (DDs; >100 Hz) and short interspike intervals (ISIs; >50 to <100 Hz) is reported to vary widely among different muscles and tasks, with a higher incidence in motor unit (MU) trains of fast muscles and for the production of fast contractions in humans. However, it is unclear whether human muscles with a large composition of slower motor units exhibit DDs or short ISIs when activated with maximal synaptic drive, such as those required for maximal velocity dynamic contractions. Thus the purpose of this study was to determine the effect of increasing peak contraction velocity on the incidence of DDs and short ISIs in the anconeus muscle. Seventeen anconeus MUs in 10 young males were recorded across dynamic elbow extensions ranging from low submaximal velocities (16% of maximal velocity) up to maximal velocities. A low incidence of DDs (4%) and short ISIs (29%) was observed among the 583 MU trains recorded. Despite the low incidence in individual MU trains, a majority (71% and 94%, respectively) of MUs exhibited at least one DD or short ISI. The number of short ISIs shared no variance with MU recruitment threshold ( R2 = 0.02), but their distribution was skewed toward higher peak velocities ( G = −1.26) and a main effect of peak elbow extension velocity was observed ( P < 0.05). Although a greater number of short ISIs was observed with increasing velocity, the low incidence of DDs and short ISIs in the anconeus muscle is likely related to the function of the anconeus as a stabilizer rather than voluntary elbow extensor torque and velocity production.
11
Baudry, Stéphane, and Jacques Duchateau. "Postactivation potentiation in a human muscle: effect on the load-velocity relation of tetanic and voluntary shortening contractions." Journal of Applied Physiology 103, no. 4 (October 2007): 1318–25. http://dx.doi.org/10.1152/japplphysiol.00403.2007.
Повний текст джерелаАнотація:
Recently it was demonstrated that postactivation potentiation (PAP), which refers to the enhancement of the muscle twitch torque as a result of a prior conditioning contraction, increased the maximal rate of torque development of tetanic and voluntary isometric contractions ( 3 ). In this study, we investigated the effects of PAP and its decay over time on the load-velocity relation. To that purpose, angular velocity of thumb adduction in response to a single electrical stimulus (twitch), a high-frequency train of 15 pulses at 250 Hz (HFT250), and during ballistic voluntary shortening contractions, performed against loads ranging from 10 to 50% of the maximum torque, were recorded before and after a conditioning 6-s maximal voluntary contraction (MVC). The results showed an increase of the peak angular velocity for the different loads tested after the conditioning MVC ( P < 0.001), but the effect was greatest for the twitch (∼182%) compared with the HFT250 or voluntary contractions (∼14% for both contraction types). The maximal potentiation occurred immediately following the conditioning MVC for the twitch, whereas it was reached 1 min later for the tetanic and ballistic voluntary contractions. At that time, the load-velocity relation was significantly shifted upward, and the maximal power of the muscle was increased (∼13%; P < 0.001). Furthermore, the results also indicated that the effect of PAP on shortening contractions was not related to the modality of muscle activation. In conclusion, the findings suggest a functional significance of PAP in human movements by improving muscle performance of voluntary dynamic contractions.
12
Behm, D. G., and D. G. Sale. "Intended rather than actual movement velocity determines velocity-specific training response." Journal of Applied Physiology 74, no. 1 (January 1, 1993): 359–68. http://dx.doi.org/10.1152/jappl.1993.74.1.359.
Повний текст джерелаАнотація:
Eight men and eight women trained 3 days/wk for 16 wk by doing attempted ballistic unilateral ankle dorsiflexions against resistance that either rendered the resultant contractions isometric (one limb) or allowed a relatively high-velocity (5.23 rad/s on an isokinetic dynamometer) movement (other limb). Training sessions consisted of five sets of 10 contractions of each type. Training produced the same high-velocity-specific training response in both limbs (P < 0.001). Peak torque increased most at 5.23 rad/s (38%) in comparison to lower velocities (0, 0.26, 0.52, 1.04, 1.55, 3.02, and 4.19 rad/s). Both limbs also showed similar increases in voluntary isometric rate of torque development (26%) and relaxation (47%) and in evoked tetanus rate of torque development (14%). A similar decrease in evoked twitch time to peak torque (6%) and half-relaxation time (11%) was also observed. Thus, all of these training responses, previously associated specifically with high-velocity resistance training, were produced by a training regimen that prevented an actual rapid movement through a range of movement. The results suggest that the principal stimuli for the high-velocity training response are the repeated attempts to perform ballistic contractions and the high rate of force development of the ensuing contraction. The type of muscle action (isometric or concentric) appears to be of lesser importance.
13
Boels, P. J., V. A. Claes, and D. L. Brutsaert. "Mechanics of K(+)-induced isotonic and isometric contractions in isolated canine coronary microarteries." American Journal of Physiology-Cell Physiology 258, no. 3 (March 1, 1990): C512—C523. http://dx.doi.org/10.1152/ajpcell.1990.258.3.c512.
Повний текст джерелаАнотація:
The effects of shortening in isotonic contractions on the mechanics of microvascular smooth muscle were investigated. Intramyocardial canine coronary microarteries (in situ diameter 60 +/- 3 microns) were mounted as rings, connected to a newly developed photoelectromagnetic force-length transducer, and activated with 125 mM K+. Shortening during isotonic contractions depressed the length-force relation (shortening deactivation) compared with the length-force relation obtained from isometric contractions; the effect was present at the earliest moments after activation, suggesting that a fundamental mechanism associated with the actual sliding of contractile filaments delayed onset of contractile activity in isotonic contractions compared with isometric contractions. Force-velocity relations were obtained by isotonic quick releases from isotonic and isometric contractions at various times. Isotonic shortening before the quick releases reduced the constants of the apparent hyperbolic force-velocity relations and maximal velocity of shortening (Vmax) compared with isometric contractions released at the same time. Increasing contraction duration reduced Vmax but more so in isotonic than in isometric contractions. Vmax also decreased with decreasing instantaneous length. A possible effect of force development on Vmax before the isotonic quick release was also described. Quick increments of load during isotonic contractions were sustained during active shortening in the phasic part, but during the tonic part loading resulted in a pronounced transient relaxation. Thus, in microvascular preparations, active isotonic shortening altered the length-force, force-velocity, and velocity-time relations and uncovered a time-dependent sensitivity to loading conditions. These experiments suggested that the mechanics of smooth muscle contraction may contribute significantly to the mechanisms of the physiological control of coronary microvascular diameter.
14
Lange, Fiete, Tiemen W. Van Weerden, and Johannes H. Van Der Hoeven. "A new surface electromyography analysis method to determine spread of muscle fiber conduction velocities." Journal of Applied Physiology 93, no. 2 (August 1, 2002): 759–64. http://dx.doi.org/10.1152/japplphysiol.00594.2001.
Повний текст джерелаАнотація:
Muscle fiber conduction velocity (MFCV) estimation from surface signals is widely used to study muscle function, e.g., in neuromuscular disease and in fatigue studies. However, most analysis methods do not yield information about the velocity distribution of the various motor unit action potentials. We have developed a new method–the interpeak latency method (IPL)–to calculate both the mean MFCV and the spread of conduction velocities in vivo, from bipolar surface electromyogram (sEMG) during isometric contractions. sEMG was analyzed in the biceps brachii muscle in 15 young male volunteers. The motor unit action potential peaks are automatically detected with a computer program. Associated peaks are used to calculate a mean MFCV and the SD. The SD is taken as a measure of the MFCV spread. The main finding is that the IPL method can derive a measure of MFCV spread at different contraction levels. In conclusion, the IPL method provides accurate values for the MFCV and additionally gives information about the scatter of conduction velocities.
15
Gazzoni, Marco, Federico Camelia, and Dario Farina. "Conduction Velocity of Quiescent Muscle Fibers Decreases During Sustained Contraction." Journal of Neurophysiology 94, no. 1 (July 2005): 387–94. http://dx.doi.org/10.1152/jn.01182.2004.
Повний текст джерелаАнотація:
We tested the hypothesis that conduction velocity of quiescent muscle fibers decreases during sustained contraction due to the activity of the active motor units in the muscle. Ten subjects trained for the identification of a target motor unit in the abductor pollicis brevis with feedback on surface EMG signals detected with a two-dimensional array of 61 electrodes. The subjects activated the target motor unit in two 10-s long contractions, before (contraction C1) and after (C3) a 3-min contraction (C2), all in ischemic condition. The target motor unit was not activated during C2. Eight of the 10 subjects (control group) performed a second experimental session identical to the first but with a resting period of 3 min instead of the contraction C2. Exerted force and target motor unit discharge rate were not different between the two subject groups and between C1 and C3 (mean ± SD, over C1 and C3; C2 group: 15.8 ± 10.4% maximal voluntary contractions and 13.1 ± 1.9 pps; control group: 15.6 ± 22.1% maximal voluntary contractions and 14.5 ± 1.9 pps, respectively). Muscle fiber conduction velocity of the target motor unit decreased in C3 with respect to C1 in the C2 group (3.59 ± 0.57 and 3.34 ± 0.47 m/s for C1 and C3, respectively; P < 0.05) but not in the control group (3.47 ± 0.68 and 3.46 ± 0.73 m/s). In the C2 group, the percent decrease in conduction velocity of the target motor unit between C1 and C3 (6.4 ± 7.1%) was not significantly different from the percent decrease in the average conduction velocity of the motor units active during C2 (9.6 ± 5.4%). In conclusion, the contraction-induced modifications in electrophysiological membrane properties of muscle fibers are partly independent on fiber activation.
16
Jager, Franc, Ksenija Geršak, Paula Vouk, Žiga Pirnar, Andreja Trojner-Bregar, Miha Lučovnik, and Ana Borovac. "Assessing Velocity and Directionality of Uterine Electrical Activity for Preterm Birth Prediction Using EHG Surface Records." Sensors 20, no. 24 (December 20, 2020): 7328. http://dx.doi.org/10.3390/s20247328.
Повний текст джерелаАнотація:
The aim of the present study was to assess the capability of conduction velocity amplitudes and directions of propagation of electrohysterogram (EHG) waves to better distinguish between preterm and term EHG surface records. Using short-time cross-correlation between pairs of bipolar EHG signals (upper and lower, left and right), the conduction velocities and their directions were estimated using preterm and term EHG records of the publicly available Term–Preterm EHG DataSet with Tocogram (TPEHGT DS) and for different frequency bands below and above 1.0 Hz, where contractions and the influence of the maternal heart rate on the uterus, respectively, are expected. No significant or preferred continuous direction of propagation was found in any of the non-contraction (dummy) or contraction intervals; however, on average, a significantly lower percentage of velocity vectors was found in the vertical direction, and significantly higher in the horizontal direction, for preterm dummy intervals above 1.0 Hz. The newly defined features—the percentages of velocities in the vertical and horizontal directions, in combination with the sample entropy of the EHG signal recorded in the vertical direction, obtained from dummy intervals above 1.0 Hz—showed the highest classification accuracy of 86.8% (AUC=90.3%) in distinguishing between preterm and term EHG records of the TPEHGT DS.
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Brooks, S. V., J. A. Faulkner, and D. A. McCubbrey. "Power outputs of slow and fast skeletal muscles of mice." Journal of Applied Physiology 68, no. 3 (March 1, 1990): 1282–85. http://dx.doi.org/10.1152/jappl.1990.68.3.1282.
Повний текст джерелаАнотація:
The purpose of this study was to contrast the frequency-power relationship of slow soleus and fast extensor digitorum longus (EDL) muscles to their frequency-force relationships and to investigate factors involved in the development of maximum power during a single contraction. Stimulation frequency-force and stimulation frequency-power relationships were determined for soleus and EDL muscles of the mouse for single contractions in situ at 35 degrees C. Power was measured during isovelocity shortening contractions with displacement through 10% of fiber length at the optimum velocity. Optimum velocity was defined as the shortening velocity for the generation of maximum power for a given stimulation frequency. Both force (N/cm2) and power (watts/kg) increased with stimulation frequency until a plateau was reached. For the frequency-force relationship, the curve for soleus muscles was merely shifted to the left of that for EDL muscles. In contrast, the power developed by EDL muscles was greater than that of soleus muscles (P less than 0.05) at each stimulation frequency. The higher power was a direct consequence of higher optimum velocities for EDL muscles compared with soleus muscles.
18
Cheng, Arthur J., and Charles L. Rice. "Isometric torque and shortening velocity following fatigue and recovery of different voluntary tasks in the dorsiflexors." Applied Physiology, Nutrition, and Metabolism 34, no. 5 (October 2009): 866–74. http://dx.doi.org/10.1139/h09-085.
Повний текст джерелаАнотація:
The present study was designed to compare the relative influence of various fatigue-related factors involved in isometric and dynamic task failure following an equivalent decrease in isometric maximum voluntary contraction (MVC) torque. Using a similar duty cycle (∼1-s contraction per 2 s) and contraction load (50% of MVC), 9 young males performed in the dorsiflexors a dynamic task, and on a separate occasion an intermittent isometric task, to an equal decrease in isometric MVC torque. Dynamic contractions had greater motor unit activation and maximum rate of torque development, and required fewer contractions to task failure than the isometric task, indicating a faster development of fatigue during the velocity-dependent dynamic contractions. Peripheral factors, rather than impairments in voluntary drive, were responsible for the equivalent decrease in MVC torque at task failure and its subsequent incomplete recovery. These included, for both tasks, similar changes during fatigue and recovery in voluntary isometric MVC torque, shortening velocity, stimulated twitch and 50 Hz torque, and 50 Hz maximum rate of relaxation. Irrespective of the task, however, the greater reduction in shortening velocity at task failure and its subsequent faster recovery than MVC torque suggest that changes in metabolites affect velocity to a greater extent than isometric torque.
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Farina, Dario, Lars Arendt-Nielsen, and Thomas Graven-Nielsen. "Experimental muscle pain reduces initial motor unit discharge rates during sustained submaximal contractions." Journal of Applied Physiology 98, no. 3 (March 2005): 999–1005. http://dx.doi.org/10.1152/japplphysiol.01059.2004.
Повний текст джерелаАнотація:
The aim of this human study was to investigate the effect of experimentally induced muscle pain on the modifications of motor unit discharge rate during sustained, constant-force contractions. Intramuscular and multichannel surface electromyographic (EMG) signals were collected from the right and left tibialis anterior muscle of 11 volunteers. The subjects performed two 4-min-long isometric contractions at 25% of the maximal dorsiflexion torque, separated by a 20-min rest. Before the beginning of the second contraction, hypertonic (painful; right leg) or isotonic (nonpainful; left leg) saline was injected into the tibialis anterior. Pain intensity scores did not change significantly in the first 150 s of the painful contraction. Exerted torque and its coefficient of variation were the same for the painful and nonpainful contractions. Motor unit discharge rate was higher in the beginning of the nonpainful contraction than the painful contraction on the right side [means ± SE, 11.3 ± 0.2 vs. 10.6 ± 0.2 pulses/s (pps); P < 0.01] whereas it was the same for the two contractions on the left side (11.6 ± 0.2 vs. 11.5 ± 0.2 pps). The decrease in discharge rate in 4 min was smaller for the painful (0.4 ± 0.1 pps) than for the control contractions (1.3 ± 0.1 pps). Initial value and decrease in motor unit conduction velocity were not different in the four contractions (right leg, 4.0 ± 0.1 m/s with decrease of 0.6 ± 0.1 m/s in 4 min; left leg, 4.1 ± 0.1 m/s with 0.7 ± 0.1 m/s decrease). In conclusion, stimulation of nociceptive afferents by injection of hypertonic saline did not alter motor unit conduction velocity but reduced the initial motor unit discharge rates and the difference between initial and final discharge rates during sustained contraction.
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Ryschon, T. W., M. D. Fowler, A. A. Arai, R. E. Wysong, S. B. Leighton, Sr Clem TR, and R. S. Balaban. "A multimode dynamometer for in vivo MRS studies of human skeletal muscle." Journal of Applied Physiology 79, no. 6 (December 1, 1995): 2139–47. http://dx.doi.org/10.1152/jappl.1995.79.6.2139.
Повний текст джерелаАнотація:
The implementation of muscle ergometry during magnetic resonance spectroscopy and imaging is complicated by the restrictive dimensions of the magnet bore and the presence of a strong static magnetic field. We have developed a dynamometer that is compatible with these constraints. This device can provide resistance to voluntary muscle contraction during isometric, isokinetic concentric, and isokinetic eccentric muscle contractions. While controlling muscle contraction speed, the dynamometer simultaneously records muscle torque production at a 10-Hz sampling frequency to allow assessment of compliance and retrospective normalization of power output for the mass of active muscle. All parameters relevant to muscle contraction are selectable, including percentage of maximal voluntary contraction, velocity of muscle contraction, duty cycle, and range of motion for the contraction. This paper provides examples of 31P-magnetic resonance spectroscopic measurements during isokinetic concentric contractions of the ankle dorsiflexors, isokinetic eccentric contractions of the soleus, and isometric contractions of the soleus. Operation of the dynamometer has no adverse effects on the integrity of the 31P-magnetic resonance spectra at 4 T, permitting temporal resolution of the phosphocreatine resynthesis rate of approximately 1 spectrum/s. The unique capabilities of this dynamometer will facilitate studies into the metabolic response of working muscle in healthy and diseased populations.
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Harwood, B., and C. L. Rice. "Changes in motor unit recruitment thresholds of the human anconeus muscle during torque development preceding shortening elbow extensions." Journal of Neurophysiology 107, no. 10 (May 15, 2012): 2876–84. http://dx.doi.org/10.1152/jn.00902.2011.
Повний текст джерелаАнотація:
Rate of torque development and the subsequent movement velocity are modulated by motor unit (MU) properties, primarily MU discharge rate and MU recruitment threshold (MURT). In isometric conditions, MURTs have been shown to decrease with increased rates of torque development. It is unclear whether this relationship is similar in the production of dynamic shortening contractions. Using fast joint velocities to drive the system, we aimed to determine how anconeus MURTs recorded during the torque production phase preceding movement were affected in relation to the resultant peak elbow extension velocity. Recruitment thresholds of 17 MUs from 9 young men were tracked throughout non-isokinetic dynamic elbow extensions with velocities ranging from 64°/s to 500°/s at a constant resistance of 25% of maximal voluntary isometric contraction and during isometric elbow extensions (0°/s). Relative MURTs decreased ∼50% from the slowest (<25% of maximal velocity) to the fastest (>75% of maximal velocity) resultant velocity ranges ( P < 0.05). Although a significant ( P < 0.001) but weak ( r = −0.27, R2 = 0.08) relationship was observed between MURT and resultant peak elbow extension velocity for the group, only 7 of the 17 MUs displayed significant moderate ( r = −0.40, R2 = 0.17) to strong ( r = −0.85, R2 = 0.73) negative MURT-velocity relationships. These data indicate variable responses of MURTs with increasing resultant peak velocity, which may be related to the intrinsic properties of individual MUs.
22
Ren, J., and K. Schulze-Delrieu. "Movement of wax particles by contractions in the isolated opossum esophagus." American Journal of Physiology-Gastrointestinal and Liver Physiology 258, no. 1 (January 1, 1990): G164—G170. http://dx.doi.org/10.1152/ajpgi.1990.258.1.g164.
Повний текст джерелаАнотація:
We investigated whether the contractions of the isolated smooth muscle opossum esophagus can propel luminal contents. Wax particles were placed into the esophagus in vitro, and esophageal contractions were evoked by stimulating either primarily the intrinsic esophageal nerves (using electrical pulses of 0.5 ms) or the esophageal muscle directly (using pulses of 0.5 s). Direct muscle stimulation and neural stimulation produced circular muscle contractions of similar amplitude, but only neural stimulation was associated with a propagating ring contraction and longitudinal muscle contraction. Movement of the wax particle occurred after 21% of all stimulus responses. Aborad movement of the wax particle was 10 times as common as was its orad movement. Wax movement occurred less commonly and over shorter distance with muscle as compared with neural stimulation. The distance the wax moved was enhanced when the esophagus was allowed to shorten in its longitudinal axis. Movement occurred late during the contraction response and at velocities less than that of the ring contraction. Also, the ring contraction passed over the particle. The amplitude of circular muscle contractions had no predictive value for the occurrence of propulsion. The finding that the isolated esophagus can propel luminal contents in the aborad direction supports the thesis that peristalsis is primarily a function of the intrinsic neuromuscular organization of the smooth muscle esophagus.
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De Haan, A., G. J. Van Ingen Schenau, G. J. Ettema, P. A. Huijing, and M. A. Lodder. "Efficiency of rat medial gastrocnemius muscle in contractions with and without an active prestretch." Journal of Experimental Biology 141, no. 1 (January 1, 1989): 327–41. http://dx.doi.org/10.1242/jeb.141.1.327.
Повний текст джерелаАнотація:
The efficiency of positive work was measured for rat medial gastrocnemius muscle at 25 degrees C during repeated contractions. Six muscles were stimulated to perform concentric contractions preceded by an active prestretch (PS contractions) and six muscles made to give concentric contractions from an isometric state (PI contractions). Both lengthening and shortening of the muscles (distance: 6 mm) occurred at a constant velocity of 20 mm s-1 (1.5 fibre lengths s-1). Stimulation was started 150 ms prior to the onset of concentric contraction for both types of contraction. For the PS contractions this meant that the active state was developed during the last 2.4 mm of the lengthening. Energy consumption (calculated from high-energy phosphate consumption) appeared to be equal for both types of contraction, although positive work output was 39.4% higher in the PS contractions than in the PI contractions. The efficiency of positive work was 36.8 +/− 3.5% in the PS contractions and 26 +/− 2.0% in the PI contractions. In contrast to results of previous studies, the positive work done by the muscle in the PS contractions was much larger than the negative work done on the muscle during stretch owing to the applied stimulation protocol which was intended to simulate in vivo conditions during running. The efficiency of positive work in the PS contractions is too low to explain the efficiencies of 40–70% reported for human and animal running.
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Grabiner, M. D. "Maximum rate of force development is increased by antagonist conditioning contraction." Journal of Applied Physiology 77, no. 2 (August 1, 1994): 807–11. http://dx.doi.org/10.1152/jappl.1994.77.2.807.
Повний текст джерелаАнотація:
Nine subjects performed maximum contractions with the knee extensor muscles on an isokinetic device set at 4.36 rad/s. The knee extensions were preceded by isometric conditioning contractions of the antagonist knee flexor muscles. The conditioning contraction forces were 0, 25, 50, 75, and 100% of the maximum isometric force. The purpose of the study was to determine 1) the effects of graded conditioning contractions with the knee flexor muscles on the maximum performance of the quadriceps femoris and 2) whether the effects of the conditioning contractions are similarly distributed to components of the quadriceps femoris. The increased initial quadriceps femoris activation levels associated with the conditioning contractions were similarly distributed to quadriceps femoris components. In contrast with previously published research on exercise performed at substantially lower knee extension velocities, the maximum knee extension force and the work performed were not affected by the conditioning contractions. However, the maximum rate of force development increased significantly (P < 0.05). The intensity of the knee flexor conditioning contractions likely influences agonist motor unit activation variables, but this influence may be related to the speed of the agonist contraction.
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Devrome, Andrea N., and Brian R. MacIntosh. "Force-velocity relationship during isometric and isotonic fatiguing contractions." Journal of Applied Physiology 125, no. 3 (September 1, 2018): 706–14. http://dx.doi.org/10.1152/japplphysiol.01119.2017.
Повний текст джерелаАнотація:
Fatiguing contractions change the force-velocity relationship, but assessment of this relationship in fatigue has usually been obtained after isometric contractions. We studied fatigue caused by isometric or isotonic contractions, by assessment of the force-velocity relationship while the contractions maintaining fatigue were continued. This approach allowed determination of the force-velocity relationship during a steady condition of fatigue. We used the in situ rat medial gastrocnemius muscle, a physiologically relevant preparation. Intermittent (1/s) stimulation at 170 Hz for 100 ms resulted in decreased isometric force to ~35% of initial or decreased peak velocity of shortening in dynamic contractions to ~45% of initial. Dynamic contractions resulted in a transient initial increase in velocity, followed by a rapid decline until a reasonably steady level was maintained. Data were fit to the classic Hill equation for determination of the force-velocity relationship. Isometric and dynamic contractions resulted in similar decreases in maximal isometric force and peak power. Only Vmax was different between the types of contraction ( P < 0.005) with greater decrease in Vmax during isotonic contractions to 171.7 ± 7.3 mm/s than during isometric contractions to 208.8 mm/s. Curvature indicated by a/Po (constants from fit to Hill equation) changed from 0.45 ± 0.04 to 0.71 ± 0.11 during isometric contractions and from 0.51 ± 0.04 to 0.85 ± 0.18 during isotonic contractions. Recovery was incomplete 45 min after stopping the intermittent contractions. At this time, recovery of low-frequency isometric force was substantially less after isometric contractions, implicating force during intermittent contractions as a determining factor with this measure of fatigue. NEW & NOTEWORTHY The force-velocity relationship was captured while fatigue was maintained at a constant level during isometric and dynamic contractions. The curvature of the force-velocity relationship was less curved during fatigue than prefatigued, but within 45 min this recovered. Low-frequency fatigue persisted with greater depression of low-frequency force after isometric contractions, possibly because of higher force contractions during intermittent contractions.
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Miyagawa, Taimei, Yohsuke Imai, Shunichi Ishida, and Takuji Ishikawa. "Relationship between gastric motility and liquid mixing in the stomach." American Journal of Physiology-Gastrointestinal and Liver Physiology 311, no. 6 (December 1, 2016): G1114—G1121. http://dx.doi.org/10.1152/ajpgi.00346.2016.
Повний текст джерелаАнотація:
The relationship between gastric motility and the mixing of liquid food in the stomach was investigated with a numerical analysis. Three parameters of gastric motility were considered: the propagation velocity, frequency, and terminal acceleration of peristaltic contractions. We simulated gastric flow with an anatomically realistic geometric model of the stomach, considering free surface flow and moving boundaries. When a peristaltic contraction approaches the pylorus, retropulsive flow is generated in the antrum. Flow separation then occurs behind the contraction. The extent of flow separation depends on the Reynolds number (Re), which quantifies the inertial forces due to the peristaltic contractions relative to the viscous forces of the gastric contents; no separation is observed at low Re, while an increase in reattachment length is observed at high Re. While mixing efficiency is nearly constant for low Re, it increases with Re for high Re because of flow separation. Hence, the effect of the propagation velocity, frequency, or terminal acceleration of peristaltic contractions on mixing efficiency increases with Re.
27
Hellstrand, P., and I. Nordstrom. "Cross-bridge kinetics during shortening in early and sustained contraction of intestinal smooth muscle." American Journal of Physiology-Cell Physiology 265, no. 3 (September 1, 1993): C695—C703. http://dx.doi.org/10.1152/ajpcell.1993.265.3.c695.
Повний текст джерелаАнотація:
Mechanisms responsible for the decrease in shortening velocity after prolonged contraction ("latch" state) were investigated at identical force during early (20 s, "phasic") and sustained (5 min, "tonic") phases of high-K+ (25-30 mM) contractions in smooth muscle of guinea pig taenia coli. Cytoplasmic Ca2+ concentration, myosin light-chain phosphorylation, and maximum shortening velocity all declined from 20 s to 5 min of contraction. The time course of shortening following isotonic quick release was biexponential, with a fastest rate constant of approximately 80 s-1 in both phasic and tonic contractions. Stiffness was identical in phasic and tonic contraction; however, after a release to slack length and unloaded shortening, stiffness during restretch was greater in tonic contraction (51 vs. 43% of isometric stiffness after 16 ms of unloaded shortening). Stiffness decreased after release with a rate constant of approximately 200 s-1, slightly greater in phasic than in tonic contraction. The results indicate that the number of attached cross bridges during unloaded shortening, while substantially reduced relative to the isometric value, is higher in latch than in nonlatch, consistent with a lower detachment relative to attachment rate.
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Munkvik, Morten, Per Kristian Lunde, and Ole M. Sejersted. "Causes of fatigue in slow-twitch rat skeletal muscle during dynamic activity." American Journal of Physiology-Regulatory, Integrative and Comparative Physiology 297, no. 3 (September 2009): R900—R910. http://dx.doi.org/10.1152/ajpregu.91043.2008.
Повний текст джерелаАнотація:
Skeletal muscle fatigue is most often studied in vitro at room temperature and is classically defined as a decline in maximum force production or power output, exclusively linked to repeated isometric contractions. However, most muscles shorten during normal use, and we propose that both the functional correlate of fatigue, as well as the fatigue mechanism, will be different during dynamic contractions compared with static contractions. Under isoflurane anesthesia, fatigue was induced in rat soleus muscles in situ by isotonic shortening contractions at 37°C. Muscles were stimulated repeatedly for 1 s at 30 Hz every 2 s for a total of 15 min. The muscles were allowed to shorten isotonically against a load corresponding to one-third of maximal isometric force. Maximal unloaded shortening velocity (V0), maximum force production (Fmax), and isometric relaxation rate (−dF/d t) was reduced after 100 s but returned to almost initial values at the end of the stimulation protocol. Likewise, ATP and creatine phosphate (CrP) were reduced after 100 s, but the level of CrP was partially restored to initial values after 15 min. The rate of isometric force development, the velocity of shortening, and isotonic shortening were also reduced at 100 s, but in striking contrast, did not recover during the remainder of the stimulation protocol. The regulatory myosin light chain (MLC2s) was dephosphorylated after 100 s and did not recover. Although metabolic changes may account for the changes of Fmax, −dF/d t, and V0, dephosphorylation of MLC2s may be involved in the fatigue seen as sustained slower contraction velocities and decreased muscle shortening.
29
Khezzar, L., and J. H. Whitelaw. "Flows Through Round Sudden Contractions." Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science 202, no. 4 (July 1988): 295–300. http://dx.doi.org/10.1243/pime_proc_1988_202_122_02.
Повний текст джерелаАнотація:
Measurements of the mean axial and radial components of velocity, the r.m.s. of the corresponding fluctuations and the wall static pressure are reported for flows through round sudden axi- and asymmetric contractions with an area ratio of 2.5 and were obtained at a Reynolds number of 40 000 based on the bulk velocity in the upstream pipe. In the axisymmetric case, the main changes in the approach velocity profiles occurred within a distance of one-half diameter upstream of the contraction, and the velocity profiles suggested that the flow separates at the entrance to the smaller diameter pipe. The wall static pressures revealed the presence of a vena-contracta downstream of the contraction plane. In the asymmetric case, the velocity profiles were very three-dimensional but regained symmetry by 9.5 diameters from the contraction plane.
30
Abbate, F., A. J. Sargeant, P. W. L. Verdijk, and A. de Haan. "Effects of high-frequency initial pulses and posttetanic potentiation on power output of skeletal muscle." Journal of Applied Physiology 88, no. 1 (January 1, 2000): 35–40. http://dx.doi.org/10.1152/jappl.2000.88.1.35.
Повний текст джерелаАнотація:
The effects of high-frequency initial pulses (HFIP) and posttetanic potentiation on mechanical power output during concentric contractions were examined in the in situ medial gastrocnemius of the rat with an intact origin on the femur and blood supply. Stimulation of the muscle was performed via the severed sciatic nerve. In the experiments, HFIP or the potentiating tetanus was followed by a stimulation of 80, 120, or 200 Hz. The results showed that both HFIP and the tetanus increased power output at high contraction velocities (>75 mm/s) when followed by a train of 80 or 120 Hz (200 Hz resulted in no effects). Mechanical power output was increased maximally by HFIP to 120 and 168% by the tetanus. Furthermore, when HFIP or the tetanus were followed by a train of 80 Hz, the peak power in the power-velocity curve tended to be shifted to a higher velocity.
31
Farina, Dario, Marco Gazzoni, and Federico Camelia. "Conduction velocity of low-threshold motor units during ischemic contractions performed with surface EMG feedback." Journal of Applied Physiology 98, no. 4 (April 2005): 1487–94. http://dx.doi.org/10.1152/japplphysiol.01032.2004.
Повний текст джерелаАнотація:
The aim of this study was to analyze the effect of ischemia on low-threshold motor unit conduction velocity. Nine subjects were trained to isolate the activity of a single motor unit (target motor unit) in the abductor pollicis brevis muscle with feedback on surface EMG signals recorded with a 16-electrode linear array. After training, the subjects activated the target motor unit at ∼8 pulses per second (pps) for five 3-min-long contractions. During the third and fourth contractions, a cuff inflated at 180 mmHg around the forearm induced ischemia of the hand. The exerted force (mean ± SE, 4.6 ± 2.1% of the maximal voluntary contraction force), discharge rate (8.6 ± 0.4 pps), interpulse interval variability (34.8 ± 2.5%), and peak-to-peak amplitude of the target motor unit action potentials (176.6 ± 18.2 μV) were not different among the five contractions. Conduction velocity, mean power spectral frequency, and action potential duration were the same in the beginning of the five contractions (2.8 ± 0.2 m/s, 195.2 ± 10.5 Hz, and 5.4 ± 0.3 ms, respectively) and changed over the 3 min of sustained activation only during the fourth contraction. Conduction velocity and mean power spectral frequency decreased (10.05 ± 1.8% and 8.50 ± 2.18% during the 3 min, respectively) and action potential duration increased (8.2 ± 4.6% in the 3 min) during the fourth contraction. In conclusion, 1) subjects were able to isolate the activity of a single motor unit with surface EMG visual feedback in ischemic conditions maintained for 16 min, and 2) the activation-induced decrease in single motor unit conduction velocity was significantly larger with ischemia than with normal circulation, probably due to the alteration of mechanisms of ion exchange across the fiber membrane.
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Broman, H., G. Bilotto, and C. J. De Luca. "Myoelectric signal conduction velocity and spectral parameters: influence of force and time." Journal of Applied Physiology 58, no. 5 (May 1, 1985): 1428–37. http://dx.doi.org/10.1152/jappl.1985.58.5.1428.
Повний текст джерелаАнотація:
Reports on measurement of muscle fiber conduction velocity in humans are scarce. Inferences on the behavior of conduction velocity have been drawn from the behavior of myoelectric spectral parameters. The present report contains information on conduction velocity and spectral parameters studied at various muscle contraction levels and during and after sustained contractions. The following results have been obtained from measurements on the tibialis anterior muscle. Conduction velocity demonstrated a positive correlation with limb circumference and with muscle force output. Thus we conclude that the diameters of the muscle fibers of high-threshold motor units are, on an average, larger than those of low-threshold motor units. The study of a sustained contraction and on the recovery after such a contraction revealed that conduction velocity consistently decreased during a strong contraction as did various myoelectric spectral parameters. However, the spectral parameters decreased approximately twice as much as did the conduction velocity, and we conclude that factors other than the conduction velocity along the muscle fibers affect the myoelectric signal during a high-level contraction. These other factors appertain to changes in the firing statistics of individual motor units as well as the correlation between the firings of different motor units.
33
Tulapurkara, E. G., and V. V. K. Bhalla. "Experimental Investigation of Morel’s Method for Wind Tunnel Contractions." Journal of Fluids Engineering 110, no. 1 (March 1, 1988): 45–47. http://dx.doi.org/10.1115/1.3243508.
Повний текст джерелаАнотація:
Based on a numerical study of the potential flow through contractions of chosen geometry, Morel (1975) has given a method to obtain the shape of contraction which gives small adverse pressure gradients and low nonuniformity in the velocity distribution at the exit. Two contractions with area ratios of 12 and 3.464 designed using this method are investigated experimentally. It is found that there is no separation of flow, the thickness of the boundary layer at the exit is small and the nonuniformity in velocity at the exit is smaller than the predicted value.
34
Vollestad, N. K., J. Wesche, and O. M. Sejersted. "Gradual increase in leg oxygen uptake during repeated submaximal contractions in humans." Journal of Applied Physiology 68, no. 3 (March 1, 1990): 1150–56. http://dx.doi.org/10.1152/jappl.1990.68.3.1150.
Повний текст джерелаАнотація:
We examine whether muscle oxygen consumption (VO2) increases gradually during repeated submaximal isometric contractions. Six subjects made two-legged isometric quadriceps contractions at 30% maximal voluntary contraction for 6 s with 4 s of rest between until exhaustion (58 +/- 8 min). Blood samples were taken from the femoral vein and artery, and blood velocity was recorded by ultrasound-Doppler technique in the femoral artery. Blood flow was calculated from velocity and artery diameter values. Leg VO2 increased sixfold within the 1st min of exercise. A further doubling of the VO2 was seen during the remainder of the exercise, reaching 307 +/- 22 ml/min at exhaustion. This latter increase was due to a 54% increase in blood flow and a 34% increase in oxygen extraction. After 20 min of recovery VO2 was still 75% higher than preexercise values. The results show a twofold increase in energy demand of the working muscle during repeated constant-force isometric contractions. The increased energy cost of contraction is probably localized at the cellular level, and it parallels fatigue determined as decreased force-generating capacity.
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Willems, Mark E. T., and William T. Stauber. "Effect of Contraction History on Torque Deficits by Stretches of Active Rat Skeletal Muscles." Canadian Journal of Applied Physiology 27, no. 4 (August 1, 2002): 323–35. http://dx.doi.org/10.1139/h02-018.
Повний текст джерелаАнотація:
Effects of contraction history on torque deficits by stretches of active skeletal muscles were examined. After three contractions using maximal and submaximal activation (80 and 20 Hz) at an ankle position of 40° (i.e., long muscle length) and with maximal activation at 120° (i.e., short muscle length), the isometric and stretch torques (15 stretches) of rat plantar flexor muscles (bout 1) were measured. Controls were unconditioned. Stretches (i.e., ankle rotation from 90° to 40°, velocity: 50°•s−1) were imposed on maximal isometric contractions at 90° (i.e. preloaded stretches). All groups performed a second bout following 2 hours of rest after bout 1. After maximal contractions at long muscle length, preload torque at 90° and stretch torque at 40° for stretch 1 of bout 1 were 25% and 18% lower than the other groups. However, for all groups, bout 1 ended and bout 2 began and ended with similar isometric and stretch torques. Stretches early in bout 2, with preloads similar to stretches in bout 1, had greater stretch torques resulting in larger torque deficits. Torque deficits, possibly caused by damage to muscle structures and excitation-contraction uncoupling, were not prevented by a history of isometric contractions. Different contraction histories can result in similar isometric torques but different stretch torques. Key words: injury, warm-up, isometric contractions, prevention, eccentric contractions
36
Seerden, T. C., W. J. E. P. Lammers, B. Y. De Winter, J. G. De Man, and P. A. Pelckmans. "Spatiotemporal electrical and motility mapping of distension-induced propagating oscillations in the murine small intestine." American Journal of Physiology-Gastrointestinal and Liver Physiology 289, no. 6 (December 2005): G1043—G1051. http://dx.doi.org/10.1152/ajpgi.00205.2005.
Повний текст джерелаАнотація:
Since the development of knockout animals, the mouse has become an important model to study gastrointestinal motility. However, little information is available on the electrical and contractile activities induced by distension in the murine small intestine. Spatiotemporal electrical mapping and mechanical recordings were made from isolated intestinal segments from different regions of the murine small intestine during distension. The electrical activity was recorded with 16 extracellular electrodes while motility was assessed simultaneously by tracking the border movements with a digital camera. Distension induced propagating oscillatory contractions in isolated intestinal segments. These propagating contractions were dictated by the underlying propagating slow wave with superimposed spikes. The frequencies, velocities, and direction of the propagating oscillations strongly correlated with the frequencies ( r = 0.86), velocities ( r = 0.84), and direction ( r = 1) of the electrical slow waves. Nω-nitro-l-arginine methyl ester decreased the maximal diameter of the segment and reduced the peak contraction amplitude of the propagating oscillatory contractions, whereas atropine and verapamil blocked the propagating oscillations. Tetrodotoxin had little effect on the maximal diameter and peak contraction amplitude. In conclusion, distension in the murine small intestine does not initiate peristaltic reflexes but induces a propagating oscillatory motor pattern that is determined by propagating slow waves with superimposed spikes. These spikes are cholinergic and calcium dependent.
37
Moreland, R. S., and S. Moreland. "Characterization of magnesium-induced contractions in detergent-skinned swine carotid media." American Journal of Physiology-Cell Physiology 260, no. 6 (June 1, 1991): C1224—C1232. http://dx.doi.org/10.1152/ajpcell.1991.260.6.c1224.
Повний текст джерелаАнотація:
Ca(2+)-calmodulin-dependent phosphorylation of the 20-kDa smooth muscle myosin light chain (MLC) results in high shortening velocities and rapid stress development. The stress maintained after a reduction in Ca2+ is associated with a decrease in MLC phosphorylation and velocity of shortening. This Ca(2+)-dependent stress without proportional MLC phosphorylation has been termed “latch” and has been postulated to reflect a population of dephosphorylated noncycling cross bridges or “latch bridges.” Mg2+ is necessary for contraction of smooth muscle, and in high concentrations, Mg2+ elicits contractions that are MLC phosphorylation independent. The purpose of this study was to test the hypothesis that high concentrations of Mg2+ directly induce latch-bridge formation. This was accomplished by comparing the characteristics of Mg(2+)-induced contractions of Triton X-100-skinned swine carotid media with the known characteristics of the Ca(2+)-dependent latch state. In the absence of Ca2+, free Mg2+ (3-20 mM) caused an increase in the velocity of shortening and a concentration-dependent increase in stress, with no detectable increase in MLC phosphorylation. Mg(2+)-induced contractions could be supported by CTP, which is a substrate for the actin-activated myosin adenosinetriphosphatase but not the MLC kinase. Stress development in response to Mg2+ was abolished at long tissue lengths, which also inhibit the expression of latch bridges. The calmodulin antagonist, trifluoperazine (TFP), inhibited the MLC phosphorylation-independent contractions elicited by Mg2+. TFP also inhibited the latch state. The results of this study support the existence of a regulatory system in vascular smooth muscle that is independent of the MLC phosphorylation system and can be directly activated by pharmacological levels of Mg2+.
38
Rosler, K., K. E. Conley, H. Howald, C. Gerber, and H. Hoppeler. "Specificity of leg power changes to velocities used in bicycle endurance training." Journal of Applied Physiology 61, no. 1 (July 1, 1986): 30–36. http://dx.doi.org/10.1152/jappl.1986.61.1.30.
Повний текст джерелаАнотація:
Increases in leg power production resulting from 8 wk of bicycle endurance training (30 min/day, 5 times/wk) were studied using an isokinetic dynamometer. In addition, biopsies of vastus lateralis were analyzed to characterize muscle ultrastructural changes. Performance increased on the dynamometer specifically near the estimated average knee angular velocity used during the bicycle training (200 degrees/s). Power measurements were made during the first 5 contractions (maximal power: Pmax) and last 5 contractions (final power: Pend) of 25 and 50 consecutive contractions (at 60 and 240 degrees/s, respectively). Pmax and Pend increased only at 240 degrees/s but not at 60 degrees/s. These increases in Pmax (86 W) and Pend (78 W) resulted primarily from longer torque maintenance but also from increased peak torque during each contraction and were close to the increase in mechanical power output maintained on the bicycle (Pb; 78 W) during the training sessions. The specificity of these changes to the angular velocities used in the bicycle training indicates a neural basis to these adaptations. We suggest that these neural adaptations, coupled with the observed enhancement of muscle mitochondrial and capillary density (+41 and +15%, respectively) underlie the increased ability to maintain power production on a bicycle after endurance training.
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Farina, Dario, Lars Arendt-Nielsen, and Thomas Graven-Nielsen. "Spike-triggered average torque and muscle fiber conduction velocity of low-threshold motor units following submaximal endurance contractions." Journal of Applied Physiology 98, no. 4 (April 2005): 1495–502. http://dx.doi.org/10.1152/japplphysiol.01127.2004.
Повний текст джерелаАнотація:
The motor unit twitch torque is modified by sustained contraction, but the association to changes in muscle fiber electrophysiological properties is not fully known. Thus twitch torque, muscle fiber conduction velocity, and action potential properties of single motor units were assessed in 11 subjects following an isometric submaximal contraction of the tibialis anterior muscle until endurance. The volunteers activated a target motor unit at the minimum discharge rate in eight 3-min-long contractions, three before and five after an isometric contraction at 40% of the maximal torque, sustained until endurance. Multichannel surface electromyogram signals and joint torque were averaged with the target motor unit potential as trigger. Discharge rate (mean ± SE, 6.6 ± 0.2 pulses/s) and interpulse interval variability (33.3 ± 7.0%) were not different in the eight contractions. Peak twitch torque and recruitment threshold increased significantly (93 ± 29 and 12 ± 5%, P < 0.05) in the contraction immediately after the endurance task with respect to the preendurance values (0.94 ± 0.26 mN·m and 3.7 ± 0.5% of the maximal torque), whereas time to peak of the twitch torque did not change (74.4 ± 10.1 ms). Muscle fiber conduction velocity decreased and action potential duration increased in the contraction after the endurance (6.3 ± 1.8 and 9.8 ± 1.8%, respectively, P < 0.05; preendurance values, 3.9 ± 0.2 m/s and 11.1 ± 0.8 ms), whereas the surface potential peak-to-peak amplitude did not change (27.1 ± 3.1 μV). There was no significant correlation between the relative changes in muscle fiber conduction velocity or surface potential duration and in peak twitch torque ( R2 = 0.04 and 0.10, respectively). In conclusion, modifications in peak twitch torque of low-threshold motor units with sustained contraction are mainly determined by mechanisms not related to changes in action potential shape and in its propagation velocity.
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Gilbert, R. J., and W. J. Dodds. "Effect of selective muscarinic antagonists on peristaltic contractions in opossum smooth muscle." American Journal of Physiology-Gastrointestinal and Liver Physiology 250, no. 1 (January 1, 1986): G50—G59. http://dx.doi.org/10.1152/ajpgi.1986.250.1.g50.
Повний текст джерелаАнотація:
In this study we examined the role of M1- and M2-muscarinic receptors in the mediation of circular smooth muscle esophageal contractions elicited by pharmacological cholinergic stimulation and during peristalsis in anesthetized opossums. Esophageal-body contractions were induced by bethanechol administration, whereas peristalsis was elicited by pharyngeal stroking or cervical vagal stimulation. Contractions were measured by a low-compliance manometric recording system. The incidence and amplitude of bethanechol-induced contractions were antagonized by 4-diphenylacetoxy-n-methylpiperidine (4-DAMP) and atropine but not pirenzepine. 4-DAMP and atropine caused an increased velocity, decreased amplitude, and preferential reduction of the incidence of primary peristaltic contractions in the proximal smooth muscle esophagus. During long-train vagal stimulation, intra-stimulus A-waves had a velocity similar to primary peristalsis, whereas poststimulus B-waves showed a velocity considerably faster than primary peristalsis. Short-train vagal stimulation produced a contraction sequence, termed an "S-wave," that had a velocity similar to that of the A-wave. At low doses 4-DAMP increased the velocity and decreased the amplitude of A-wave and S-wave contractions, and at high doses 4-DAMP abolished both the A-wave and S-wave contractions. B-wave contractions were minimally affected by 4-DAMP. Pirenzepine had no effect on contractions induced by swallows or vagal stimulation. We conclude that M2-muscarinic receptors mediate esophageal contractions in the circular smooth muscle during primary peristalsis and during A-waves and S-waves induced by vagal stimulation, and M1-receptors do not have any important role in the excitatory neural pathway to the esophagus.(ABSTRACT TRUNCATED AT 250 WORDS)
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Knaflitz, M., R. Merletti, and C. J. De Luca. "Inference of motor unit recruitment order in voluntary and electrically elicited contractions." Journal of Applied Physiology 68, no. 4 (April 1, 1990): 1657–67. http://dx.doi.org/10.1152/jappl.1990.68.4.1657.
Повний текст джерелаАнотація:
The relationship between surface myoelectric signal parameters and the level of voluntary or electrically elicited contractions was studied in 32 experiments on the tibialis anterior muscle of 22 healthy human subjects. Contractions were performed at 20 and 80% of the maximum voluntary contraction torque. Two levels of stimulation current were used, yielding, respectively, a maximum M wave and an M wave approximately 30% of the maximum. A four-bar electrode probe was used to detect single- and double-differential signals from which mean and median frequency of the power spectrum and average muscle fiber conduction velocity were estimated. Measurements obtained from voluntary contractions showed a positive correlation between contraction levels and both conduction velocity and spectral parameters. Conduction velocity increased by 21.2 +/- 10.9% when voluntary contraction level increased from 20 to 80% of the maximal value. Spectral parameters increased by similar amounts. Tetanic electrical stimulation was applied to a muscle motor point for 20 s via surface electrodes. Rectangular current pulses with 0.1-ms width and frequencies of 20, 25, 30, 35, and 40 Hz were used. Four types of behavior were observed with increasing stimulation level: 1) the two spectral parameters and conduction velocity both increased with stimulation in 15 experiments, 2) the two spectral parameters decreased and conduction velocity increased in 8 experiments, 3) the two spectral parameters and conduction velocity both decreased in 6 experiments, and 4) the two spectral parameters increased and conduction velocity decreased in 3 experiments. Conduction velocity increased with increasing stimulation current in 72% of the experiments, indicating a recruitment order similar to that of voluntary contractions, although it decreased in the other 28% of the cases, indicating a reverse order of recruitment. Contrary to what is observed in direct stimulation of nerves, motor units are not in general recruited in reverse order of size during electrical stimulation of a muscle motor point. This discrepancy may be the result of geometric factors or a lack of correlation between axonal branch diameter and the diameter of the parent motoneuron axon. Changes of conduction velocity and spectral parameters in opposite directions may be the result of the combined effect of the motor unit recruitment order and of the different tissue filtering function associated with the geometric location of the recruited motor units within the muscle.
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Oliveira, Anderson Souza, and Francesco Negro. "Neural control of matched motor units during muscle shortening and lengthening at increasing velocities." Journal of Applied Physiology 130, no. 6 (June 1, 2021): 1798–813. http://dx.doi.org/10.1152/japplphysiol.00043.2021.
Повний текст джерелаАнотація:
The control of movement velocity is accomplished by the modulation of the neural drive to muscle and its variation over time. In this study, we tracked motor units decomposed from HD-sEMG across shortening and lengthening contractions at increasing velocities in two submaximal load levels. We demonstrate that concentric and eccentric contractions of the tibialis anterior muscle at slow velocities are achieved by specific motor unit rate coding strategies rather than distinct recruitment schemes.
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Bullen, P. R., D. J. Cheeseman, and L. A. Hussain. "A study of Turbulent Flow in Pipe Contractions." Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering 210, no. 3 (October 1996): 171–80. http://dx.doi.org/10.1243/pime_proc_1996_210_312_02.
Повний текст джерелаАнотація:
An experimental study of turbulent water flow through abrupt contractions was carried out in order to determine the detailed flow field. Wall static pressure measurements enabled the calculation of pressure loss coefficients for a range of contraction area ratios from 0.13 to 0.67 over a Reynolds number range of 40000–200000. The effect of variations in contraction sharpness was also established. Measurements of mean velocities and turbulence intensities were made using a two-component laser Doppler anemometer for one area ratio of 0.332 to establish the detailed flow features.
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Kwantes, J., M. Rassmussen, J. A. Mercer, M. Hoffman, and L. R. Osternig. "FORCE VELOCITY RELATIONSHIP OF MUSCLE CONTRACTION AT SLOW ECCENTRIC ISOKINETIC VELOCITIES." Medicine & Science in Sports & Exercise 33, no. 5 (May 2001): S294. http://dx.doi.org/10.1097/00005768-200105001-01659.
Повний текст джерела
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Warren, Gordon L., John M. Fennessy, and Melinda L. Millard-Stafford. "Strength loss after eccentric contractions is unaffected by creatine supplementation." Journal of Applied Physiology 89, no. 2 (August 1, 2000): 557–62. http://dx.doi.org/10.1152/jappl.2000.89.2.557.
Повний текст джерелаАнотація:
This study's objective was to determine whether 14 days of dietary creatine supplementation preceding an injurious bout of eccentric contractions affect the in vivo strength loss of mouse anterior crural muscles. Three groups of nine mice each were fed a meal diet for 14 days, one group at each of three levels of creatine supplementation (i.e., 0, 0.5, and 1% creatine). Electrically stimulated concentric, isometric, and eccentric contraction torques produced about the ankle were measured both before and after a bout of 150 eccentric contractions. Tibialis anterior muscle creatine concentration was significantly increased by the supplementation, being 12% higher in the mice fed the 1% creatine diet compared with control mice. After the bout of eccentric contractions, the reductions in torque (i.e., 46–58%) were similar for the isometric contraction, all eccentric contractions, and the slow (i.e., ≤200o/s) concentric contractions; above 200o/s, the percent reduction in concentric torque increased progressively to 85–88% at 1,000–1,200o/s. However, there was no effect of creatine supplementation on the isometric torque loss or on the torque loss at any eccentric or concentric angular velocity ( P ≥ 0.62). In conclusion, a moderate increase in muscle creatine concentration induced by dietary supplementation in mice does not affect the strength loss after eccentric contractions.
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Murphy, Richard A., and Christopher M. Rembold. "The latch-bridge hypothesis of smooth muscle contraction." Canadian Journal of Physiology and Pharmacology 83, no. 10 (October 1, 2005): 857–64. http://dx.doi.org/10.1139/y05-090.
Повний текст джерелаАнотація:
In contrast to striated muscle, both normalized force and shortening velocities are regulated functions of cross-bridge phosphorylation in smooth muscle. Physiologically this is manifested as relatively fast rates of contraction associated with transiently high levels of cross-bridge phosphorylation. In sustained contractions, Ca2+, cross-bridge phosphorylation, and ATP consumption rates fall, a phenomenon termed "latch". This review focuses on the Hai and Murphy (1988a) model that predicted the highly non-linear dependence of force on phosphorylation and a directly proportional dependence of shortening velocity on phosphorylation. This model hypothesized that (i) cross-bridge phosphorylation was obligatory for cross-bridge attachment, but also that (ii) dephosphorylation of an attached cross-bridge reduced its detachment rate. The resulting variety of cross-bridge cycles as predicted by the model could explain the observed dependencies of force and velocity on cross-bridge phosphorylation. New evidence supports modifications for more general applicability. First, myosin light chain phosphatase activity is regulated. Activation of myosin phosphatase is best demonstrated with inhibitory regulatory mechanisms acting via nitric oxide. The second modification of the model incorporates cooperativity in cross-bridge attachment to predict improved data on the dependence of force on phosphorylation. The molecular basis for cooperativity is unknown, but may involve thin filament proteins absent in striated muscle.Key words: chemo-mechanical transduction, activation-contraction coupling, cross-bridge, myosin light chain kinase, myosin light chain phosphatase, phosphorylation, cooperativity.
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Sundberg, Christopher W., Andrew Kuplic, Hamidollah Hassanlouei, and Sandra K. Hunter. "Mechanisms for the age-related increase in fatigability of the knee extensors in old and very old adults." Journal of Applied Physiology 125, no. 1 (July 1, 2018): 146–58. http://dx.doi.org/10.1152/japplphysiol.01141.2017.
Повний текст джерелаАнотація:
The mechanisms for the age-related increase in fatigability during high-velocity contractions in old and very old adults (≥80 yr) are unresolved. Moreover, whether the increased fatigability with advancing age and the underlying mechanisms differ between men and women is not known. The purpose of this study was to quantify the fatigability of knee extensor muscles and identify the mechanisms of fatigue in 30 young (22.6 ± 0.4 yr; 15 men), 62 old (70.5 ± 0.7 yr; 33 men), and 12 very old (86.0 ± 1.3 yr; 6 men) men and women elicited by high-velocity concentric contractions. Participants performed 80 maximal velocity contractions (1 contraction per 3 s) with a load equivalent to 20% of the maximum voluntary isometric contraction. Voluntary activation and contractile properties were quantified before and immediately following exercise (<10 s) using transcranial magnetic stimulation and electrical stimulation. Absolute mechanical power output was 97 and 217% higher in the young compared with old and very old adults, respectively. Fatigability (reductions in power) progressively increased across age groups, with a power loss of 17% in young, 31% in old, and 44% in very old adults. There were no sex differences in fatigability among any of the age groups. The age-related increase in power loss was strongly associated with changes in the involuntary twitch amplitude ( r = 0.75, P < 0.001). These data suggest that the age-related increased power loss during high-velocity fatiguing exercise is unaffected by biological sex and determined primarily by mechanisms that disrupt excitation contraction coupling and/or cross-bridge function. NEW & NOTEWORTHY We show that aging of the neuromuscular system results in an increase in fatigability of the knee extensors during high-velocity exercise that is more pronounced in very old adults (≥80 yr) and occurs similarly in men and women. Importantly, the age-related increase in power loss was strongly associated with the changes in the electrically evoked contractile properties suggesting that the increased fatigability with aging is determined primarily by mechanisms within the muscle for both sexes.
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Zehr, E. Paul, and Digby G. Sale. "Ballistic Movement: Muscle Activation and Neuromuscular Adaptation." Canadian Journal of Applied Physiology 19, no. 4 (December 1, 1994): 363–78. http://dx.doi.org/10.1139/h94-030.
Повний текст джерелаАнотація:
Movements that are performed with maximal velocity and acceleration can be considered ballistic actions. Ballistic actions are characterized by high firing rates, brief contraction times, and high rates of force development. A characteristic triphasic agonist/antagonist/agonist electromyographic (EMG) burst pattern occurs during ballistic movement, wherein the amount and intensity of antagonist coactivation is variable. In conditions of low-grade tonic muscular activity, a premovement EMG depression (PMD; or silent period, PMS) can occur in agonist muscles prior to ballistic contraction. The agonist PMD period may serve to potentiate the force and velocity of the following contraction. A selective activation of fast twitch motor units may occur in ballistic contractions under certain movement conditions. Finally, high-velocity ballistic training induces specific neuromuscular adaptations that occur as a function of the underlying neurophysiological mechanisms that subserve ballistic movement. Key words: electromyography, motor control, training adaptation, motor unit
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Ameredes, B. T., W. F. Brechue, G. M. Andrew, and W. N. Stainsby. "Force-velocity shifts with repetitive isometric and isotonic contractions of canine gastrocnemius in situ." Journal of Applied Physiology 73, no. 5 (November 1, 1992): 2105–11. http://dx.doi.org/10.1152/jappl.1992.73.5.2105.
Повний текст джерелаАнотація:
The force-velocity (F-V) relationships of canine gastrocnemius-plantaris muscles at optimal muscle length in situ were studied before and after 10 min of repetitive isometric or isotonic tetanic contractions induced by electrical stimulation of the sciatic nerve (200-ms trains, 50 impulses/s, 1 contraction/s). F-V relationships and maximal velocity of shortening (Vmax) were determined by curve fitting with the Hill equation. Mean Vmax before fatigue was 3.8 +/- 0.2 (SE) average fiber lengths/s; mean maximal isometric tension (Po) was 508 +/- 15 g/g. With a significant decrease of force development during isometric contractions (-27 +/- 4%, P < 0.01, n = 5), Vmax was unchanged. However, with repetitive isotonic contractions at a low load (P/Po = 0.25, n = 5), a significant decrease in Vmax was observed (-21 +/- 2%, P < 0.01), whereas Po was unchanged. Isotonic contractions at an intermediate load (P/Po = 0.5, n = 4) resulted in significant decreases in both Vmax (-26 +/- 6%, P < 0.05) and Po (-12 +/- 2%, P < 0.01). These results show that repeated contractions of canine skeletal muscle produce specific changes in the F-V relationship that are dependent on the type of contractions being performed and indicate that decreases in other contractile properties, such as velocity development and shortening, can occur independently of changes in isometric tension.
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Merletti, R., M. Knaflitz, and C. J. De Luca. "Myoelectric manifestations of fatigue in voluntary and electrically elicited contractions." Journal of Applied Physiology 69, no. 5 (November 1, 1990): 1810–20. http://dx.doi.org/10.1152/jappl.1990.69.5.1810.
Повний текст джерелаАнотація:
The time course of muscle fiber conduction velocity and surface myoelectric signal spectral (mean and median frequency of the power spectrum) and amplitude (average rectified and root-mean-square value) parameters was studied in 20 experiments on the tibialis anterior muscle of 10 healthy human subjects during sustained isometric voluntary or electrically elicited contractions. Voluntary contractions at 20% maximal voluntary contraction (MVC) and at 80% MVC with duration of 20 s were performed at the beginning of each experiment. Tetanic electrical stimulation was then applied to the main muscle motor point for 20 s with surface electrodes at five stimulation frequencies (20, 25, 30, 35, and 40 Hz). All subjects showed myoelectric manifestations of muscle fatigue consisting of negative trends of spectral variables and conduction velocity and positive trends of amplitude variables. The main findings of this work are 1) myoelectric signal variables obtained from electrically elicited contractions show fluctuations smaller than those observed in voluntary contractions, 2) spectral variables are more sensitive to fatigue than conduction velocity and the average rectified value is more sensitive to fatigue than the root-mean-square value, 3) conduction velocity is not the only physiological factor affecting spectral variables, and 4) contractions elicited at supramaximal stimulation and frequencies greater than 30 Hz demonstrate myoelectric manifestations of muscle fatigue greater than those observed at 80% MVC sustained for the same time.