Fatigue effects on the electromechanical delay components during the relaxation phase after isometric contraction

AIM: By a combined electromyographic (EMG), mechanomyographic (MMG) and force analysis, the electromechanical delay during muscle relaxation (R-DelayTOT ) was partitioned into electrochemical and mechanical components. The study aimed to evaluate the effects of fatigue on R-DelayTOT components and to assess their inter-session and inter-day reliability. METHODS: During tetanic stimulations, EMG, MMG and force were recorded from the human gastrocnemius medialis muscle before and after fatigue. The latency between EMG and MMG ripple cessations (R-Δt EMG-MMGR , electrochemical R-DelayTOT component); between MMG ripple cessation and force decay onset (R-Δt MMGR -F, first R-DelayTOT mechanical component), and between force decay onset and maximum MMG negative peak (R-Δt F-MMGp-p , second R-DelayTOT mechanical component) were calculated. RESULTS: Before fatigue, R-Δt F-MMGp-p was the major contributor (61.9±1.7 ms, 75%) to R-DelayTOT (82.7±1.0 ms), while R-Δt EMG-MMGR and R-Δt MMGR -F accounted for 16% (13.3±1.2 ms) and 9% (7.5±1.0 ms), respectively. After fatigue, R-DelayTOT , R-Δt EMG-MMGR and R-Δt MMGR -F increased by 11%, 41% and 67%, respectively (P<0.05), whereas R-Δt F-MMGp-p didn't change. Consequently, the relative contribution of R-Δt EMG-MMGR , R-Δt MMGR -F and R-Δt F-MMGp-p , to R-DelayTOT changed to 20±2%, 12±1% and 68±2%, respectively. Measurement reliability was always from high to very high (0.705 to 0.959). CONCLUSION: Fatigue altered the processes between neuromuscular activation cessation and force decay onset, but not the second mechanical component (cross-bridges detachment rate and series elastic components release). This combined approach provided reliable measurement of the different R-DelayTOT components and it may represent a valid tool to get more insights on muscle electromechanical behaviour.