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.