Littérature scientifique sur le sujet « Hamstring coactivation »

Context:Researchers have postulated that coactivation of the hamstrings during active knee extension assists the anterior cruciate ligament in maintaining knee joint stability by exerting an opposing force to anterior tibial translation.Objective:To compare the reciprocal coactivation of the hamstrings while performing low and high velocity isokinetic movements and two closed chain movements.Design:Within subject’s comparison of isokinetic and closed chain exercises.Setting:Biomechanics laboratory utilizing a Cybex norm isokinetic dynamometer and Biopac Data Collection system.Participants:12 healthy women.Main Outcome Measures:The root mean square of the Electromyogram (rmsEMG) was used as a measure of overall muscle activity.Results:The rmsEMG for hamstring coactivation during knee extension showed significant differences between the isokinetic movements and the closed chain exercises with greater coactivation when performing the isokinetic movements. In addition, greater activity was seen at the higher isokinetic velocity and during the one legged squat.Conclusions:These results suggest isokinetic movements, particularly at high speed, can more effectively increase the coactivation activity of the hamstrings when compared to two closed chain activities.

Background Closed kinetic chain exercises such as single-limb squats are preferred for knee rehabilitation. A complete understanding of the neuromuscular control of the knee during the single-limb squat is essential to increase the efficiency of rehabilitation programs. Hypothesis Performing a controlled single-limb squat with resistance to knee flexion and extension will increase the coactivation of the hamstring muscle group, thus reducing the quadriceps/hamstrings ratio. Study Design Descriptive laboratory study. Methods A total of 15 healthy human subjects (7 women, 8 men) performed controlled single-limb squats in a custom mechanical device that provided resistance to both flexion and extension. Subjects performed the task at 3 levels of resistance, set as a percentage of body weight. Surface electromyographic recordings from 7 muscles (gluteus medius, rectus femoris, vastus medialis oblique, vastus lateralis, biceps femoris, semitendinosus, and medial gastrocnemius) were collected during the task. Results Biceps femoris activity during knee flexion increased from approximately 12% maximum voluntary isometric contractions during low resistance (0% body weight) to approximately 27% maximum voluntary isometric contractions during high resistance (8% body weight). Although the quadriceps had greater activity than the hamstrings at all levels of resistance, the quadriceps/hamstrings ratio declined significantly with resistance (F2,27 = 29.05; P=. 012) from 3.0 at low resistance to 2.32 at the highest resistance. Conclusions Performing controlled resisted single-limb squats may help to simultaneously strengthen the quadriceps and facilitate coactivation of the hamstrings, thus reducing anterior tibial shear forces. The coactivation may also increase the dynamic control of the knee joint. Clinical Relevance The typical single-limb squat exercise performed in the clinic does not usually control for bidirectional resistance and knee joint excursion. As seen in this study, controlled single-limb squats at increased levels of resistance help to increase the coactivation of the hamstring muscles, which is essential to optimize neuromuscular control of the knee.

Twenty sedentary male university students were randomly assigned to an experimental or a control group. The experimental group trained the knee extensors of one leg by producing 30 isometric extension maximal voluntary contractions (MVC) per day, three times per week for 8 wk. After 8 wk of training, extensor MVC in the trained leg increased 32.8% (P less than 0.05), but there was no change in vastus lateralis maximal integrated electromyographic activity (IEMGmax). The most important finding was that the degree of hamstring coactivation during extension MVC decreased by approximately 20% (P less than 0.05) after the 1st wk of training. Less pronounced adaptations occurred in the untrained leg: extension MVC force increased 16.2% (P less than 0.05), hamstring coactivity decreased 13% (P less than 0.05) after 2 wk of training, and vastus lateralis IEMGmax was unchanged. The same measures in legs of the control group were not changed during the study. There were no changes in flexion MVC, biceps femoris IEMGmax, or the degree of quadriceps coactivity during flexion MVC in either leg of the control or experimental group. A reduction in hamstring coactivity in the trained and untrained legs indicates that these muscles provide less opposing force to the contracting quadriceps. We conclude that this small but significant decrease in hamstring coactivation that occurs during the early stages of training is a nonhypertrophic adaptation of the neuromuscular system in response to static resistance training of this type.

Context:Coactivation ratio of quadriceps to hamstring muscles (Q:H) and medial to lateral knee muscles (M:L) contributes to the dynamic stability of the knee joint during movement patterns recommended during rehabilitation and important for daily function.Objective:To compare the quadriceps-to-hamstring and medial-to-lateral knee muscles' coactivation ratios between men and women during the following closed kinetic chain exercises performed on a balance board: forward lunge, side lunge, single-leg stance, and single-leg squat.Design:Cross-sectional.Participants:20 healthy subjects (10 female and 10 male).Main Outcome Measures:Surface electromyography was used to measure the activation level of quadriceps (vastus lateralis and medialis) and hamstrings (biceps femoris and medial hamstrings) during forward- and side-lunge, single-leg-stance, and single-leg-squat exercises. Subjects were instructed during each exercise to move into the test position and to hold that position for 15 s. EMG was recorded during the 15-s isometric period where subjects tried to maintain a “set” position while the foot was on a balance board. Analysis of variance was used for statistical analysis.Results:There was a significant exercise-by-gender interaction for Q:H ratio (F3,48 = 6.63, P = .001), but the exercise-by-gender interaction for M:L ratio was not significant (F3,48 = 1.67, P = .18). Women showed larger Q:H ratio in side-lunge exercises than men (P = .002). Both genders showed larger M:L and lower Q:H ratio in a single-leg-stance exercise than in the other exercises.Conclusions:The results indicate that the forward- and side-lunge and single-leg-squat exercises should not be recommended as exercise where a balanced coactivation between quadriceps and hamstring muscles is warranted. Single-leg-stance exercise could be used when seeking an exercise where the ratio is balanced for both women and men.

Resistance training (RT) improves the skeletal muscle’s ability to generate maximal voluntary force and is accompanied by changes in the activation of the antagonist muscle which is not targeted primarily by RT. However, the nature and role of neural adaptation to RT in the antagonist muscle is paradoxical and not well understood. We compared moments, agonist muscle activation, antagonist activation, agonist-antagonist coactivation, and electromyographic (EMG) model-predicted moments generated by antagonist hamstring muscle coactivation during isokinetic knee extension in leg strength-trained (n = 10) and untrained (n = 11) healthy, younger adults. Trained vs. untrained adults were up to 58% stronger. During knee extension, hamstring activation was 1.6-fold greater in trained vs. untrained adults (p = 0.022). This hamstring activation produced 2.6-fold greater model-predicted antagonist moments during knee extension in the trained (42.7 ± 19.55 Nm) vs. untrained group (16.4 ± 12.18 Nm; p = 0.004), which counteracted (reduced) quadriceps knee extensor moments ~43 Nm (0.54 Nm·kg−1) and by ~16 Nm (0.25 Nm·kg−1) in trained vs. untrained. Antagonist hamstring coactivation correlated with decreases and increases, respectively, in quadriceps moments in trained and untrained. The EMG model-predicted antagonist moments revealed training history-dependent functional roles in knee extensor moment generation.

This study aimed to study the coactivation patterns of the hamstring and quadriceps muscle groups during submaximal strength exercises commonly used in injury prevention in soccer without the use of maximum voluntary isometric contraction testing. This was used to compare: (i) the inter-limb differences in muscle activation; (ii) the intra-muscular group activation pattern and (iii) the activation pattern during different phases of the exercise. Muscle activation was recorded by surface electromyography in 19 elite, male, youth soccer players. Participants performed the following: Bulgarian squat, lunge and squat. Electrical activity was recorded for the rectus femoris, vastus medialis, vastus lateralis, biceps femoris and semitendinosus. No significant inter-limb differences were found (F1, 13 = 619; p = 0.82; η2 = 0.045). Significant differences were found in the muscle activation between individual muscles within the quadriceps and hamstrings muscle group for each of the exercises: Bulgarian squat (F1,18 = 331: p < 0.001; η2 = 0.80), lunge (F4,72 = 114.5; p < 0.001; η2 = 0.86) and squat (F1,16 = 247.31; p < 0.001; η2 = 0.93). Differences were found between the different phases of each of the exercises (F2,26 = 52.27; p = 0.02; η2 = 0.80). The existence of an activation pattern of each of the muscles in the three proposed exercises could be used for muscle assessment and as a tool for reconditioning post-injury.

Abstract Background Dynamometry has been used extensively to measure knee extensor strength in individuals with cerebral palsy (CP). However, increased coactivation can lead to underestimation of knee extensor strength and, therefore, reduce validity of strength measurements. It is yet unknown to what extent coactivation occurs during dynamometry testing and whether coactivation is influenced by severity of CP, load levels, and muscle fatigue. Objectives The aims of this study were: (1) to investigate coactivation in adolescents with and without CP during dynamometer tests and (2) to assess the effect of Gross Motor Function Classification System (GMFCS) level, load level, and muscle fatigue on coactivation. Design A cross-sectional observational design was used. Method Sixteen adolescents with CP (GMFCS levels I and II: n=10/6; age range=13–19 years) and 15 adolescents without CP (n=15; age range=12–19 years) performed maximal isometric contractions (maximal voluntary torque [MVT]) and a series of submaximal dynamic contractions at low (±65% MVT), medium (±75% MVT), and high (±85% MVT) loads until fatigue. A coactivation index (CAI) was calculated for each contraction from surface electromyography recordings from the quadriceps and hamstring muscles. Results Adolescents with CP classified in GMFCS level II showed significantly higher CAI values than adolescents classified in GMFCS level I and those without CP during maximal and submaximal contractions. No differences were observed among load levels. During the series of fatiguing submaximal contractions, CAI remained constant in both the CP group and the group with typical development (TD), except for adolescents with TD at the low-load condition, which showed a significant decrease. Limitations Electromyography tracings were normalized to amplitudes during maximal isometric contractions, whereas previous studies suggested that these types of contractions could not be reliably determined in the CP population. Conclusion Coactivation was higher in adolescents with CP classified in GMFCS level II than in adolescents with TD and those with CP in GMFCS level I at different load levels. Within all groups, coactivation was independent of load level and fatigue. In individuals with CP, coactivation can lead to an underestimation of agonist muscle strength, which should be taken into account while interpreting the results of both maximal and submaximal dynamometer tests.

Background. Numerosi studi dimostrano che durante gli esercizi di estensione del ginocchio in catena cinetica aperta, la tensione sul legamento crociato anteriore (LCA) può essere ridotta considerevolmente con la co-contrazione intenzionale dei muscoli femorali, così come, attraverso l’utilizzo di una leg-extension che implementi o una compressione assiale esterna sul ginocchio, o una spostamento controllato del rullo mobile lungo la parte inferiore della gamba durante l’esercizio. Obiettivi. Gli obiettivi principali di questo studio sono: 1) determinare il possibile aumento della co-attivazione dei muscoli femorali causata dalla co-contrazione intenzionale durante esercizi di leg-extension in catena cinetica aperta, e valutare se una co-contrazione intenzionale dei muscoli femorali può ridurre completamente le forze di taglio tibiofemorali sul LCA durante l’esecuzione di questi esercizi; 2) costruire due prototipi di leg-extension: uno con compressione assiale sul ginocchio e uno con rullo mobile, e valutarne l’effettiva efficacia nel limitare la traslazione anteriore della tibia. Metodi. L’attività elettromiografica dei muscoli semitendinoso (ST), semimembranoso (SM), bicipite femorale (BF), e quadricipite femorale, e la cinematica del ginocchio sono state misurate su 20 soggetti (maschi adulti sani) durante esercizi isotonici di leg-extension con resistenza (R) compresa tra il 10% e l’80% del 1RM (Ripetizione Massimale). Gli stessi esercizi sono stati eseguiti sia sulla macchina standard, mentre i soggetti cercavano di aumentare la co-attivazione dei muscoli femorali attraverso una co-contrazione intenzionale, che sui due prototipi innovativi senza co-contrazione intenzionale. I dati ottenuti sono stati utilizzati come parametri di input di un modello per calcolare le forze tibiofemorali di taglio e di compressione negli esercizi di leg-extension per qualsiasi serie di co-attivazione dei diversi muscoli femorali. La tecnica radiografica è stata utilizzata per valutare se la compressione assiale esterna avesse la capacità di limitare la traslazione anteriore della tibia durante gli esercizi isometrici di leg-extension. Risultati. Con valori R≤40%1RM i livelli massimi di co-attivazione l ottenuti con co-contrazione intenzionale sono stati significativamente maggiori (P<0.001) di quelli l0 ottenuti senza co-contrazione intenzionale. Per ogni muscolo femorale, il livello massimo l è stato raggiunto con R=30%1RM, corrispondenti a 9.2, 10.5, e 24.5% della massima contrazione volontaria isometrica per i muscoli BF, ST, e SM, rispettivamente, dove il rapporto l/l0 ha raggiunto il massimo a R=20%1RM considerando approssimativamente 2, 3, e 4 volte, per BF, SM, e ST rispettivamente. I maggiori livelli l di co-attivazione intenzionale ottenuti per R≤30%1RM annullano completamente la forza tibiofemorale anteriore sviluppata dal quadricipite durante l’esercizio. La co-attivazione dei muscoli femorali non è stata significativamente influenzata né dal rullo mobile né dalla compressione assiale. La compressione assiale esterna sembra avere effetti limitati sulla traslazione anteriore della tibia solo ad alti livelli di compressione. Conclusioni. Negli esercizi di leg-extension con R≤40%1RM, la co-attivazione dei muscoli BF, SM, e ST può essere notevolmente aumentata (fino a 2, 3, e 4 volte, rispettivamente) dalla co-contrazione intenzionale dei muscoli femorali. I maggiori livelli di co-attivazione ottenuti con R≤30%1RM possono annullare completamente le forze tibiofemorali anteriori sviluppate dal quadricipite durante l’esecuzione dell’esercizio. Rilevanza Clinica. Questo studio suggerisce che gli esercizi di leg-extension con co-contrazione intenzionale dei muscoli femorali possono fornire un metodo sicuro per il rafforzamento del quadricipite nelle prime fasi dei programmi riabilitativi nei casi di infortunio o intervento chirurgico al LCA. Ulteriori studi, comprendenti trial clinici, sono tuttavia necessari al fine di supportare l’importanza terapeutica di questi esercizi nella pratica clinica.
Background. A number of research studies provide evidence that, during open kinetic-chain knee-extension exercises, the strain-force on the anterior cruciate ligament (ACL) can be considerably reduced by hamstring co-contraction, as well as by the use of leg extension equipment that implements either an external axial knee compression or a controlled radial displacement of the resistance pad along the lower-leg during the movement. Purpose. The purposes of this study were: 1) to determine the possible increase of hamstrings co-activation caused by a intentional co-contraction effort during open kinetic-chain leg-extension exercises, and to assess whether an intentional hamstring co-contraction can completely suppress the ACL-loading tibiofemoral (TF) shear force during these exercises; 2) to built two leg extension prototypes with knee axial compression and movable resistance pad, respectively, and to assess their effectiveness in limiting the anterior tibial translation. Methods. The electromyographic activity of semitendinosus (ST), semimembranosus (SM), biceps femoris (BF), and quadriceps femoris, and knee kinematics were measured in twenty healthy male subjects during isotonic leg-extension exercises with resistance (R) ranging from 10% to 80% 1RM (Repetition Maximum). The same exercises were also performed with the standard equipment while the subjects attempted to enhance hamstring co-activation through a intentional co-contraction effort, and with two innovative prototypes in absence of intentional co-contraction. The data served as input parameters for a model to calculate the shear and compressive TF forces in leg-extension exercises for any set of co-activation patterns of the different hamstring muscles. X-ray imaging was applied to assess whether the external axial compression has the potential to limit the anterior tibial translation during isometric leg-extension efforts. Results. For R≤40%1RM the peak co-activation levels l obtained with intentional co-contraction were significantly higher (P<0.001) than those l0 obtained without intentional co-contraction. For each hamstring muscle, the level l reached its maximum at R=30%1RM, corresponding to 9.2, 10.5, and 24.5% MVIC (maximum voluntary isometric contraction) for BF, ST, and SM, respectively, whereas the ratios l/l0 reached their maximum at R=20%1RM given approximately by 2, 3, and 4, for BF, SM, and ST respectively. The intentional enhanced co-activation levels l obtained for R≤30%1RM completely suppressed the anterior TF force developed by the quadriceps during the exercise. Hamstring co-activation was not significantly affected by movable pad and axial compression. The external axial compression seems to have an effect in limiting the anterior tibial translation only at high compression levels. Conclusions. In leg-extension exercises with resistances R≤40%1RM co-activation of the BF, SM, and ST can be significantly enhanced (up to 2, 3, and 4 times, respectively) by a intentional hamstring co-contraction effort. The enhanced co-activations levels obtained for R≤30%1RM can completely suppress the anterior TF force developed by the quadriceps during the exercise. Clinical Relevance. This study suggests that leg-extension exercise with intentional hamstring co-contraction may have the potential to be a safe and effective quadriceps strengthening intervention in the early stages of rehabilitation programs for ACL injury or reconstruction recovery. Further studies, including clinical trials, are needed to investigate the relevance of this therapeutic exercise in clinical practice.