Biceps Brachii Muscle Length Affects Force Steadiness with and Without Fatigue

Abstract

Purpose: The purpose of this study was to examine the influence of muscle length and fatigue on maximal force, submaximal force steadiness, and electromyographic (EMG) activity of the biceps brachii. Methods: Force and EMG responses were measured before and after a fatigue protocol consisting of maximal intermittent contractions of the elbow flexors until task failure (n = 20). The protocol was performed on two separate occasions in a randomized order. During one visit, the elbow joint was at 90° (EF90) and for the other, it was extended to 120° (EF120). Results: The results show a large effect size for greater force loss following fatigue at long muscle length (P = 0.067, ηp2 = 0.166). The fatigue-based decreases in force steadiness were not different between muscle lengths (P = 0.502, ηp2 = 0.024). Force steadiness was lower at long muscle length before and after fatigue (P < 0.01, d = 0.691). Following fatigue, muscle excitation decreased and increased during maximal and submaximal force tasks, respectively, yet there were no length-dependent EMG responses. Conclusions: The novel findings show fatigue at long muscle length likely affects force loss to a greater degree than fatigue-based decreases in force steadiness. These data show lower elbow flexion force steadiness when the biceps brachii is in a lengthened position.