Influence of biceps brachii tendon mechanics on position-dependent elbow flexor force steadiness

Elbow flexor force steadiness (FS), measured as coefficient of variation (CV) of force, depends on forearm position and strength. Achilles tendon mechanical properties are associated with standing balance and plantar flexion FS; however, little is known about how tendon mechanics contribute to FS of an upper limb muscle such as the elbow flexors. The purpose of this study was to determine the influence of distal biceps brachii tendon mechanics on elbow flexor FS across supinated, neutral and pronated forearm positions. It was hypothesized that maximal voluntary contraction (MVC), stiffness, tendon force and stress would be higher in supinated and neutral, while strain would be higher in pronated, contributing to enhanced FS, as previously observed for supinated and neutral compared with the pronated position. Eleven males performed isometric elbow flexion tasks at low (5, 10% MVC) and high (25, 50, 75%) forces. Tendon elongation and cross-sectional area were recorded with ultrasound during contraction to quantify tendon mechanics among positions. MVC, FS, tendon force and stress were less in pronated (p<0.01). Tendon strain was greater in neutral compared to pronated at 25, 50 and 75% MVC, and compared to supinated at 75% MVC (p≤0.05). Tendon stiffness did not differ among positions (p>0.05). The associations and influence of MVC and tendon mechanics on CV of force were analyzed using Pearson’s correlations and forward multiple regressions, respectively, for low and high force levels. Associations of MVC (-0.61