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

Aim: By a combined electromyographic (EMG), mechanomyographic (MMG) and force (F) analysis, the electromechanical delay during muscle relaxation (R-Delay_TOT) was partitioned into electrochemical and mechanical components. The study aimed to evaluate the effects of fatigue on R-Delay_TOT components and to assess their intersession and interday reliability Intraclass correlation coefficient (ICC). Methods: During tetanic stimulations, EMG, MMG and F were recorded from the human gastrocnemius medialis muscle before and after fatigue. The latency between EMG and MMG ripple cessations (R-Δt EMG-MMG_R, electrochemical R-Delay_TOT component); between MMG ripple cessation and F decay onset (R-Δt MMG_R-F, first R-Delay_TOT mechanical component); and between F decay onset and maximum MMG negative peak (R-Δt F-MMG_p-p, second R-Delay_TOT mechanical component) was calculated. Results: Before fatigue, R-Δt F-MMG_p-p was the major contributor (61.9 ± 1.7 ms, 75%) to R-Delay_TOT (82.7 ± 1.0 ms), while R-Δt EMG-MMG_R and R-Δt MMG_R-F accounted for 16% (13.3 ± 1.2 ms) and 9% (7.5 ± 1.0 ms) respectively. After fatigue, R-Delay_TOT, R-Δt EMG-MMG_R and R-Δt MMG_R-F increased by 11, 41 and 67%, respectively (P <0.05), whereas R-Δt F-MMG_p-p did not change. Consequently, the relative contribution of R-Δt EMG-MMG_R, R-Δt MMG_R-F and R-Δt F-MMG_p-p, to R-Delay_TOT changed to 20 ± 2, 12 ± 1 and 68 ± 2% respectively. Measurement reliability was always from high to very high (ICC 0.705-0.959). 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-Delay_TOT components and it may represent a valid tool to get more insights on muscle electromechanical behaviour.