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. 2020 Jul 16;129(3):419–441. doi: 10.1152/japplphysiol.00385.2020

Fig. 1.

Fig. 1.

Leg and back muscle myoelectrical activity during exercise at different time scales. Evolution of myoelectrical activity represented by electromyographical (EMG) amplitude profiles at large time scale of consecutive exercise segments (A) and short time scale of individual and consecutive squats (B), comparing the right (VL-R) and left leg vastus lateralis muscle (VL-L) with the right (ES-R) and left back erector spinae (ES-L), for a typical young subject. The exercise protocol is composed by 3 consecutive squat tests, each performed until exhaustion, separated by 10-min rest periods in the supine position; EMG data are recorded with 500-Hz sampling frequency. Leg and back muscles show different EMG amplitude profiles at both large (A) and short (B) time scales. A: within and across exercise segments, the leg EMG amplitude gradually increases, reflecting the effect of fatigue, in contrast to the back muscle, where the EMG amplitude does not change significantly. Note also the higher initial EMG amplitude at the beginning of exercises 2 and 3 compared with exercise 1 for the leg muscle, indicating residual fatigue, an effect that is not present for the back muscle. B: markedly different EMG amplitude profiles are observed for the leg and back muscles at short time scales of a few seconds, associated with individual squats, with bimodal profile for the leg muscle with 2 phases corresponding to the down (smaller amplitudes) and up (larger amplitude) squat movements. The observed differences between the leg and back muscles in the EMG amplitude profiles at both short and large time scales and the profile evolution with fatigue reflect different muscle fiber structure and role to squat movement. These empirical observations motivate our hypothesis that distinct muscles have specific spectral power profiles, with different roles of muscle fibers to the spectral power of high- and low-frequency components, and muscle-specific trajectory for the evolution of spectral profiles across short and long time scales in response to exercise-induced fatigue and aging.