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

AimBy 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). MethodsDuring 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-t EMG-MMG(R), electrochemical R-Delay(TOT) component); between MMG ripple cessation and F decay onset (R-t MMG(R)-F, first R-Delay(TOT) mechanical component); and between F decay onset and maximum MMG negative peak (R-t F-MMG(p-p), second R-Delay(TOT) mechanical component) was calculated. ResultsBefore fatigue, R-t F-MMG(p-p) was the major contributor (61.91.7ms, 75%) to R-Delay(TOT) (82.7 +/- 1.0ms), while R-t EMG-MMG(R) and R-t MMG(R)-F accounted for 16% (13.3 +/- 1.2ms) and 9% (7.5 +/- 1.0ms) respectively. After fatigue, R-Delay(TOT), R-t EMG-MMG(R) and R-t MMG(R)-F increased by 11, 41 and 67%, respectively (P<0.05), whereas R-t F-MMG(p-p) did not change. Consequently, the relative contribution of R-t EMG-MMG(R), R-t MMG(R)-F and R-t 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). ConclusionFatigue 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.