Vastus Lateralis Muscle Fiber Conduction Velocity During Heavy and Severe Cycling Exercise

: High-density surface electromyography has recently emerged as a tool to investigate neuromuscular function across different motor tasks. Muscle fiber conduction velocity, a biophysical marker estimated from high-density surface electromyography signals, quantifies the speed of action potential propagation along muscle fibers. Currently, little information is available on the responses of muscle fiber conduction velocity during dynamic exercise. We therefore explored how cycling within two distinct exercise-intensity domains (heavy and severe) elicited substantial changes in vastus lateralis muscle fiber conduction velocity. Furthermore, we compared muscle fiber conduction velocity with averaged rectified values obtained from the same high-density surface electromyography signals, and we tested differences in their respective intra-individual variability. In the severe-intensity domain, muscle fiber conduction velocity decreased from 4.44±0.22 to 4.03±0.22 m∙s-1, whereas in the heavy-intensity domain, it increased from 4.56±0.33 to 5.02±0.42 m∙s-1 (p<0.001). Although both muscle fiber conduction velocity and averaged rectified values changed significantly over time, indicating the myoelectric manifestation of fatigue in the severe domain, muscle fiber conduction velocity showed less variability within single time points. These findings suggest that muscle fiber conduction velocity is a more robust indicator of myoelectric fatigue than averaged rectified values, even in highly dynamic contractions like cycling.