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<r<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 <r<−0.42) and metabolic cost (−0.74<r<−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.

In vivo assessment of muscle fascicle length by extended field-of-view ultrasonography

The present study examined the reliability and validity of in vivo vastus lateralis (VL) fascicle length ( Lf) assessment by extended field-of-view ultrasonography (EFOV US). Intraexperimenter and intersession reliability of EFOV US were tested. Further, Lf measured from EFOV US images were compared to Lf measured from static US images (6-cm FOV) where out-of-field fascicle portions were trigonometrically estimated (linear extrapolation). Finally, spatial accuracy of the EFOV technique was assessed by comparing Lf measured on swine VL by EFOV US to actual measurements from digital photographs. The difference between repeated VL Lf measurements by the same experimenter was 2.1 ± 1.7% with an intraclass correlation (ICC) of 0.99 [95% confidence interval (CI) = 0.95–1.00]. In terms of intersession reliability, no difference ( P = 0.48) was observed between Lf measured on two different occasions, with ICC = 0.95 (CI = 0.80–0.99). The average absolute difference between Lf measured by EFOV US and using linear extrapolation was 12.6 ± 8.1% [ICC = 0.76 (CI = −0.20–0.94)]; EFOV Lf was always longer than extrapolated Lf. The relative error of measurement between Lf measured by EFOV US and by dissective assessment (digital photographs) in isolated swine VL was 0.84% ± 2.6% with an ICC of 0.99 (CI = 0.94–1.00). These results show that EFOV US is a reliable and valid method for the measurement of long muscle fascicle in vivo. Thus EFOV US analysis was proven more accurate for the assessment of skeletal muscle fascicle length than conventional extrapolation methods.