Passive stretching effects on electromechanical delay and time course of recovery in human skeletal muscle: new insights from an electromyographic and mechanomyographic combined approach

Acute passive stretching has been shown to alter muscle-tendon unit (MTU) stiffness and to reduce peak tetanic force (pF). MTU mechanical properties and electro-mechanical delay (EMD) are closely related. Thus, EMD changes would be expected after stretching. The aim of the study was to assess the stretching-induced changes in both contractile and viscoelastic contributors to EMD. The time course of these changes will be also evaluated. Tetanic stimulations were delivered on the medial gastrocnemius muscle of 16 active males, before and after (every 15 min, for 2 h) passive stretching administration. During contractions, electromyographic (EMG), mechanomyographic (MMG) and force signals were recorded. The delays between EMG and force (Δt EMG-F, which corresponds to EMD), EMG and MMG (Δt EMG-MMG) and MMG and force (Δt MMG-F) signals were calculated, together with pF and EMG conduction velocity (CV). After stretching (i) pF decreased by 31% (P < 0.05) and remained depressed for the entire recovery period, while EMG CV did not change; (ii) Δt EMG-F, Δt EMG-MMG and Δt MMG-F increased significantly from 45.4 ± 3.0 ms, 2.2 ± 0.3 ms and 42.4 ± 3.1 ms to 52.7 ± 3.4 ms, 2.4 ± 0.3 ms and 50.3 ± 3.5 ms, respectively; (iii) Δt EMG-F and Δt MMG-F remained lengthened for the entire recovery period, while Δt EMG-MMG recovered to its pre-stretching condition within 15 min. These findings suggest that after stretching, the reduction in pF was accompanied by an elongation of the overall EMD. However, stretching had effects of short duration at the contractile level, but more persisting effects on MTU viscoelastic characteristics.