The nervous system does not compensate for an acute change in the balance of passive force between synergist muscles

Electromechanical delay of the hamstrings following semitendinosus tendon autografts in return to competition athletes

PURPOSE

Knee flexor electromechanical delay (EMD) has been proposed as a contributing factor to non-contact anterior cruciate ligament (ACL) injury risk and the semitendinosus (ST) autograft technique has been shown to impair knee flexor torque at large angles of knee flexion. The purpose of this study was to analyse the effects of ACL reconstruction (ACLR) using the ST tendon autograft technique on knee flexor EMD across the knee flexion range of motion, in athletes who had returned to competition.

METHODS

Athletes with ACLR (n = 8 females, n = 3 males, 1.7 ± 0.5 years post-surgery) and non-injured control athletes (n = 6 females, n = 4 males) performed rapid maximal voluntary contractions of isometric knee flexion and extension at 30°, 50°, 70°, 90°,and 105° of knee flexion. Electrical activity of the ST, biceps femoris (BF), vastus lateralis, and vastus medialis was recorded using surface electromyography.

RESULTS

No change in EMD for the knee flexors or extensors was observed across joint angles. Greater EMD was found only for the BF in the ACLR limb of injured athletes compared to the contralateral limb (P < 0.05). In post-hoc analysis, evidence of ST tendon regrowth was noted for only 2/11 athletes.

CONCLUSION

While the EMD-joint angle relationship appeared to be unaffected by ST tendon harvest for ACLR, the absence of ST tendon regrowth should be considered. Despite return to competition, greater BF EMD was found, which may impair knee joint stabilization capacity by delaying the transfer time of muscle tension to the tibia after ST autograft.

Influence of low muscle activation levels on the ankle torque and muscle shear modulus during plantar flexor stretching

During stretching studies, surface electromyography (sEMG) is used to ensure the passive state of the muscle, for the characterization of passive muscle mechanical properties. Different thresholds (1%, 2% or 5% of maximal) are indifferently used to set "passive state". This study aimed to investigate the effects of a slight activity on the joint and muscle mechanical properties during stretching. The joint torque and muscle shear modulus of the triceps surae muscles were measured in fifteen healthy volunteers during ankle dorsiflexions: (i) in a "fully relaxed" state, (ii) during active conditions where participants were asked to produce an sEMG amplitude of 1%, 2% or 5% of their maximal sEMG amplitude of the triceps surae. The 1% condition was the only that did not result in significant differences in joint torque or shear modulus compared to the relaxed condition. In the 2% condition, increases in joint torque were found at 80% of the maximal angle in dorsiflexion, and in the shear modulus of gastrocnemius medialis and gastrocnemius lateralis at the maximal angle in dorsiflexion. During the 5% condition, joint torque and the shear modulus of gastrocnemius medialis were higher than during relaxed condition at angles larger than 40% of maximal angle in dorsiflexion. The results provide new insights on the thresholds that should be considered for the design of stretching studies. A threshold of 1% seems much more appropriate than a 2% or 5% threshold in healthy participants. Further studies are required to define similar thresholds for patients.

Coordination of hamstrings is individual specific and is related to motor performance

The torque-sharing strategies between synergistic muscles may have important functional consequences. This study involved two experiments. The first experiment ( n = 22) aimed 1) to determine the relationship between the distribution of activation and the distribution of torque-generating capacity among the heads of the hamstring, and 2) to describe individual torque-sharing strategies and to determine whether these strategies are similar between legs. The second experiment ( n = 35) aimed to determine whether the distribution of activation between the muscle heads affects endurance performance during a sustained submaximal knee flexion task. Surface electromyography (EMG) was recorded from biceps femoris (BF), semimembranosus (SM), and semitendinosus (ST) during submaximal isometric knee flexions. Torque-generating capacity was estimated by measuring muscle volume, fascicle length, pennation angle, and moment arm. The product of the normalized EMG amplitude and the torque-generating capacity was used as an index of muscle torque. The distributions of muscle activation and of torque-generating capacity were not correlated significantly (all P > 0.18). Thus, there was a torque imbalance between the muscle heads (ST torque > BF and SM torque; P < 0.001), the magnitude of which varied greatly between participants. A significant negative correlation was observed between the imbalance of activation across the hamstring muscles and the time to exhaustion ( P < 0.001); i.e., the larger the imbalance of activation across muscles, the lower the muscle endurance performance. Torque-sharing strategies between the heads of the hamstrings are individual specific and related to muscle endurance performance. Whether these individual strategies play a role in hamstring injury remains to be determined.

NEW & NOTEWORTHY The distribution of activation among the heads of the hamstring is not related to the distribution of torque-generating capacity. The torque-sharing strategies within hamstring muscles vary greatly between individuals but are similar between legs. Hamstring coordination affects endurance performance; i.e., the larger the imbalance of activation across the muscle heads, the lower the muscle endurance.

Neuromechanical coupling within the human triceps surae and its consequence on individual force sharing strategies

Little is known about the factors that influence the coordination of synergist muscles that act across the same joint, even during single-joint isometric tasks. The overall aim of this study was to determine the nature of the relationship between the distribution of activation and the distribution of force-generating capacity among the three heads of the triceps surae (soleus [SOL], gastrocnemius medialis [GM] and lateralis [GL]). Twenty volunteers performed isometric plantarflexions during which the activation of GM, GL and SOL was estimated using electromyography (EMG). Functional muscle physiological cross-sectional area (PCSA) was estimated using imaging techniques and was considered as an index of muscle-force generating capacity. The distribution of activation and PCSA among the three muscles varied greatly between participants. A significant positive correlation between the distribution of activation and the distribution of PCSA was observed when considering the two bi-articular muscles at intensities ≤50% of the maximal contraction (0.51&lt;r&lt;0.62). Specifically, the greater the PCSA of GM compared with GL, the stronger bias of activation to the GM. There was no significant correlation between monoarticular and biarticular muscles. A higher contribution of GM activation compared with GL activation was associated with lower triceps surae activation (−0.66 &lt;r&lt;−0.42) and metabolic cost (−0.74&lt;r&lt;−0.52) for intensities ≥30% of the maximal contraction. Considered together, an imbalance of force between the three heads was observed, the magnitude of which varied greatly between participants. The origin and consequences of these individual force-sharing strategies remain to be determined.