Biceps Brachii Muscle Length Affects Force Steadiness with and Without Fatigue

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.

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).

The results show a large effect size for greater force loss following fatigue at long muscle length (P = 0.067, 𝜂_p² = 0.166). The fatigue-based decreases in force steadiness were not different between muscle lengths (P = 0.502, 𝜂_p² = 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 lengthdependent EMG responses.

The novel findings show fatigue at long muscle length likely affects force loss to a greater degree than fatiguebased decreases in force steadiness. These data show lower elbow flexion force steadiness when the biceps brachii is in a lengthened position.