Low‐Frequency Fatigue: Dependence on Contraction Mode, Movement Speed and Duty Cycle During Repetitive Tasks

To determine which movement variables most affect low‐frequency fatigue (LFF) during voluntary repetitive tasks, we compared LFF from pairs of exercise protocols in which one variable at a time was manipulated. Each exercise protocol included 4 hours of repeated wrist flexions against 25% maximum voluntary contraction. LFF was assessed from the ratio of forces from 20 Hz and 50 Hz stimulation. To test whether LFF depends on the mode of contraction, we compared LFF from protocols requiring either isotonic shortening of wrist flexors or isometric contraction. Repeated isotonic shortening contractions caused much more LFF than did repeated isometric contractions (28.7 ± 3.4% compared to 13.2 ± 3.2% below baseline, N = 12). To determine whether LFF depends on the movement speed, we compared protocol with fast and slow flexions (~0.5 s and ~1.0 s per flexion). Exercise with slow movements produced much more LFF than did the exercise with fast movements (47.6 ± 3.2% compared to 28.7 ± 3.4% below baseline, N = 12). To distinguish between movement speed and duty cycle, we used a third exercise protocol with slow movements, but rest periods comparable to the fast movement protocol. Fast and slow movements with the same amount of rest produced about equal degrees of fatigue (28.7 ± 3.4% and 34.8 ± 2.4% below baseline, N = 12). Significant LFF remained after two hours recovery from any of the exercise protocols. Most protocols also produced high‐frequency fatigue, but LFF was always a more sensitive measure, and persisted longer. Supported by NIOSH R01 OH07441