Neuromuscular fatigue reduces force responsiveness when controlling leg external forces

Agile movement requires precise control of ground reaction forces, yet the impact of neuromuscular fatigue on this control remains incompletely understood. We investigated how leg muscle fatigue affects the nervous system's ability to modulate vertical ground reaction forces, hypothesizing that fatigue would impair responsiveness, and accuracy. Eighteen healthy participants (11 W, 7 M; age: 26 ± 3.8 yr) performed force-matching tasks using a custom apparatus that constrained body movement while allowing isolated leg force production. Participants force matched visual step targets before and after fatigue, which we induced by sustained submaximal contractions. We then quantified responsiveness (rise time, bandwidth) and accuracy (overshoot, steady-state error, variability). Fatigue reduced MVC force by 26.1 ± 12.3% (from 2.95 ± 0.58 to 2.20 ± 0.47 body weights; p = 4.70e-6) and impaired responsiveness, with rise time increasing 23.3 ± 41.0% (from 303 ± 100 to 373 ± 94 ms; p = 6.00e-4) and bandwidth decreasing 25.2 ± 17.5% (from 1.33 ± 0.60 to 1.00 ± 0.25 Hz; p = 4.83e-2). Notably, accuracy remained largely unchanged, with only a transient decrease in overshoot from 4.30 ± 2.62% to 2.56 ± 1.19% (p = 0.047). These findings show neuromuscular fatigue selectively impairs the speed of leg force adjustments while preserving accuracy, possibly through compensatory strategies. This dissociation has implications for understanding motor control deficits and injury risk during fatigue.