Spectral dynamics of muscle fiber activation in response to exercise and acute fatigue

Skeletal muscles form an integrated multi-component system with complex dynamics of continuous myoelectrical activation of various muscle types with different functions and muscle fibers within muscles. To understand the multiscale dynamics of neuromuscular activity, we investigate spectral characteristics of different muscle types across timescales, and we track the evolution of the dynamics with progression of exercise and fatigue. We develop a protocol for young and elderly subjects of repeated squat exercise segments performed until exhaustion. We analyze differentiated spectral power responses over a range of frequency bands representing muscle fiber dynamics within leg and back muscle activation. We find that leg and back muscle are characterized by muscle-specific spectral profiles with differentiated frequency bands contribution and a muscle-specific evolution path in response to acute fatigue. Specifically, we observe that with increasing fatigue leg muscle spectral power increases for low, intermediate and high frequencies. In contrast, back muscle spectral power increases with fatigue only for low and intermediate frequencies below 50 Hz. Further, leg muscle shows higher spectral response to fatigue (35% and 40% for young and elderly subjects respectively; p < 0.05) compared to the back muscle (25% increase only for the young group; p = 0.03). The uncovered universality among subjects in the spectral profile, reveals a previously unrecognized multi-scale mechanism underlying differentiated response of distinct muscle types and muscle fibers to exercise-induced fatigue in young and elderly. Tracking changes in the spectral profiles could help quantify more precisely muscle adaptations after training interventions and may assist coaches with the selection of most appropriate and effective training programs.