Effect of fatigue on neuromuscular adaptations in endurance-trained and recreationally active males

Neuromuscular fatigue can be characterized by an exercise-induced reduction in force-generating capacity involving both neural and muscular mechanisms. Previous research has suggested that the functional organization of the neuromuscular system differs between endurance-trained individuals and sedentary or recreationally active individuals. This difference may lead to distinct neuromuscular responses to fatigue. The aim of this study was to compare neuromuscular fatigue responses between endurance-trained (ET) and recreationally active (RA) males during a sustained submaximal isometric knee extension contraction.

Eleven ET and 11 RA participants performed maximal voluntary isometric contractions (MVIC) of the knee extensors (KE), followed by a trapezoidal contraction (ascending to 60% MVIC) and an isometric fatiguing task at 30% MVIC sustained to task failure. An additional MVIC was completed immediately post-fatigue task. High-density surface electromyography (HDsEMG) was simultaneously recorded from the vastus lateralis, and HDsEMG root mean square (RMS), median frequency (MDF) and muscle fibre conduction velocity (MFCV) were estimated. The MFCV-torque relationship during the ascending phase of the trapezoidal contraction (up to 60% MVIC) was assessed using the mixed linear model.

Baseline MVIC of the KE did not differ between groups. The ET group showed a significantly lower rate of increase in MFCV (p < 0.001) during the ascending phase of the contraction and lower absolute MFCV at 60% MVIC (p < 0.001) compared to the RA group. During the sustained fatiguing task, both groups reached task failure at similar times with comparable MVIC reductions (∼25%). However, the RA group exhibited significant declines in both MDF (p < 0.001) and MFCV (p = 0.04), whereas these parameters remained unchanged in the ET group.

While ET and RA individuals exhibited similar levels of fatigue, the underlying neuromuscular mechanisms may differ. The ET group showed a lower rate of increase in MFCV with increasing voluntary force and unchanged MFCV and MDF during fatiguing contractions, whereas the RA group exhibited fatigue-induced decreases in both MFCV and MDF. These findings suggest that endurance training is associated with altered recruitment and/or muscle membrane properties, likely linked to differences in muscle fibre characteristics.