Shear Displacement Occurs in the Aponeuroses Between the Soleus and Gastrocnemius During Voluntary Contractions

Abstract

2368 The muscle-tendon complex of the human triceps surae is intricate in constitution and function with three separate muscle compartments merging via their aponeuroses into the Achilles tendon. The mechanical function of these connective tissue structures during muscular contraction is not well understood. PURPOSE: To evaluate ‘shear’ displacement between the medial gastrocnemius (MG) and soleus (SO) aponeuroses during plantarflexor contractions. METHODS: 8 male subjects (mean ± SEM; age; 30 ± 2 yrs, body mass; 77 ± 2 kg, height; 1.83 ± 0.02 m) performed isometric contractions with the plantarflexor muscles of one leg by grading the force from zero to maximum in 10 s. The contractions were performed in two positions: knee joint maximally extended and flexed. The joint moment was assessed with a strain gauge load cell, and corresponding displacement of the MG and SO aponeuroses was determined by ultrasonography. Shear displacement was quantified by subtracting displacement of SO from that of MG. Wilcoxon 2-tailed signed ranks tests were used for statistical analysis, P<0.05 was considered significant. Data are reported as means ± SEM. RESULTS: When aponeurosis displacement was analyzed to the greatest common force level between positions, displacement of MG exceeded that of SO in knee extension (9.0 ± 0.8 mm vs. 6.2 ± 0.6 mm), whereas SO displacement was greater than that of MG (9.6 ± 1.0 mm vs. 7.9 ± 1.2 mm) in knee flexion. The amount and ‘direction’ of aponeurosis shear differed significantly between the two positions throughout the force range. CONCLUSION: Shear displacement between the MG and SO aponeuroses was substantial, and its direction varied with knee joint position. This observation suggests that the free Achilles tendon may be subjected to intratendinous shear and stress gradients during human locomotion.