The effect of muscle fatigue on neuromuscular activation and force production of the quadriceps muscle

Muscle performance declines with fatigue; EMG mean frequency (MNF) falls linearly with repetitions, whereas root mean square (RMS) shows inconsistent individual patterns. Prior knee-extension studies used high angular velocities (≈180°·s⁻¹), emphasizing explosive power rather than maximal strength. We evaluated strength-oriented fatigability at 90°·s⁻¹ and computed a Fatigue Index (FI) as the percent change between the first and last five repetitions for peak torque (PT), work per repetition (J/rep), MNF, and RMS. Thirty-one healthy adults (21 men, 10 women; 22–35 y) completed 30 concentric knee extensions while torque and surface EMG were recorded. Force-related FI showed large declines (PT −33.5%; J/rep −34.3%) and MNF decreased by 20.1% (all p<0.001). RMS was unchanged overall (FI −5.7%, p=0.88) but rose in women (FI −17.4%) versus men (−0.08%; p=0.015). Under strength-oriented loading, FI provides a concise within-muscle metric linking reductions in force with neuromuscular activation and reveals a sex-specific RMS response. We propose adding FI-based isokinetic testing to established anaerobic assessments (Wingate, vertical jump) to enrich athlete monitoring and rehabilitation profiling. Larger cohorts are required to confirm these findings and clarify mechanisms underlying the female-specific RMS increase.