This study aimed to identify characteristics of the triceps surae muscles and aponeuroses stiffness in vivo, during graded isometric submaximal plantarflexion efforts. A total of twelve healthy male subjects (age: 27 ± 4 years) participated and were required to stay at rest (0% MVC) and perform isometric submaximal plantar flexion contractions (20%, 40%, 60% of MVC) on a dynamometer. Young's modulus of triceps surae muscles and the adjoining aponeuroses between gastrocnemii and soleus at the proximal and distal sites were obtained in the longitudinal direction (along muscle's line of action) during at rest and submaximal plantar flexions. Additionally, Young's modulus of adjoining aponeuroses in the transverse direction at the distal sites was also calculated. Young's modulus of LG (lateral gastrocnemius), SOL-lat (lateral part of soleus) and LPS (superficial aponeurosis of LG) at the proximal site showed significant (p < 0.001) graded increase response to the submaximal contraction levels. Besides, in the lateral side, significant differences in the Young's modulus of aponeuroses were observed between longitudinal and transverse directions at rest and during contractions (p < 0.002). Changes of aponeuroses length were significantly correlated with Young's modulus changes of the proximal gastrocnemii muscle bellies (r = 0.43–0.45, p = 0.006–0.008) and superficial aponeuroses (r = 0.49–0.60, p < 0.002). The results further indicate that the triceps surae muscles and aponeuroses showed inhomogeneous and anisotropic mechanical properties during submaximal muscle contractions, and the stiffening effect of muscle belly possibly make influence on the mechanical properties of aponeuroses during muscle contractions, especially for the lateral gastrocnemius.
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