Abstract

Background. Neuromuscular fatigue can be characterized by an exercise-induced reduction in force generation capacity involving both neural and muscular mechanisms. Previous research has suggested that there is a difference in the functional organization of the neuromuscular system in endurance-trained and untrained individuals, which may lead to different neuromuscular responses to fatigue. The aim of this study was to access the neuromuscular fatigue in endurance-trained distance runners (DR) and untrained males (UM) during a sustained submaximal isometric knee extension contraction.
Methods. Eleven DR (20.1 ± 2.3 yr) and 11 UM participants (23 ± 1.7 yr) performed maximal voluntary isometric contractions (MVIC) of the knee extensors (KE) and one trapezoidal contraction to 60% MVIC followed by a submaximal isometric fatiguing contraction at 30% MVIC until task failure. An additional MVIC was then performed immediately after the fatiguing task. High-density surface electromyography (EMG) was simultaneously recorded from the vastus lateralis muscle. The EMG root mean square (RMS), median frequency (MDF) and the muscle fibre conduction velocity (MFCV) were estimated. The relationship between MFCV and voluntary force during the ascending phase of the trapezoidal contraction up to 60% MVIC was examined using the mixed linear model.
Results. At baseline (pre-fatigue), the maximal knee extension strength of the UM (368.8 ± 87.2 Nm) and DR (294.3 ± 92.1 Nm) groups was similar. During the ascending phase of the contraction, the MFCV increased linearly as a function of voluntary force in both groups. However, the DR group exhibited a less pronounced rate of increase (slope = 0.012 m·s -1 ·%MVIC; p < 0.001) compared to the UM group ( slope = 0.019 m·s -1 ·%MVIC; p < 0.001) groups. The DR group also showed a lower MFCV at the 60% MVIC (i.e., the plateau of the trapezoidal contraction) than the UM group (4.34 ± 0.23 m/s vs. 4.96 ± 0.32; p < 0.001) . During the fatiguing task, the MDF (p < 0.001) and MFCV (p = 0.04) decreased in the UM group, whereas no changes was observed in the DR group. Following the fatiguing task, both groups had similar time-to-failure (TTF; 106.6 ± 71.8 vs. 153.3 ± 128.6 s) and comparable reductions in post-fatigue MVIC (~25%). Furthermore, the UM group showed a significant decrease in post-fatigue MDF (p = 0.006).
Conclusion. The MFCV of the DR increased at a lower rate as a function of voluntary force, suggesting a possible decline in the recruitment strategies of high-threshold motor units and the alterations of sarcolemma excitability and fibre diameter, following long-term endurance training. Moreover, the fatiguing task induced comparable fatigue in both groups as evidenced by the similar TTFs and similar reductions in MVIC. However, the underlying neuromuscular fatigue mechanisms may differ, likely due to the distinct muscle fibre distributions in the two groups.