Effects of fatigue on the electromechanical delay components in gastrocnemius medialis muscle

Under electrically evoked contractions, the time interval between the onset of the stimulation pulse (Stim) and the beginning of force (F) development can be partitioned (Delay(TOT)), by an electromyographic (EMG), mechanomyographic (MMG) and F combined approach, into three components each containing different parts of the electrochemical and mechanical processes underlying neuromuscular activation and contraction. The aim of the study was to evaluate inter- and intra-operator reliability of the measurements and to assess the effects of fatigue on the different Delay(TOT) components. Sixteen participants underwent two sets of tetanic stimulations of the gastrocnemius medialis muscle, with 10 min of rest in between. After a fatiguing protocol of 120 s, tetanic stimulations were replicated. The same protocol was repeated on a different day. Stim, EMG, MMG and F signals were recorded during contraction. Delay(TOT) and its three components (between Stim and EMG, Delta t Stim-EMG; between EMG and MMG, Delta t EMG-MMG and between MMG and F, Delta t MMG-F) were calculated. Before fatigue, Delay(TOT), Delta t Stim-EMG, Delta t EMG-MMG and Delta t MMG-F lasted 27.5 +/- A 0.9, 1.4 +/- A 0.1, 9.2 +/- A 0.5 and 16.8 +/- A 0.7 ms, respectively. Fatigue lengthened Delay(TOT), Delta t Stim-EMG, Delta t EMG-MMG and Delta t MMG-F by 18, 7, 16 and 22 %, respectively. Delta t Stim-EMG, Delta t EMG-MMG and Delta t MMG-F contributed to Delay(TOT) lengthening by 2, 27 and 71 %, respectively. Reliability was always from high to very high. The combined approach allowed a reliable calculation of the three contributors to Delay(TOT). The effects of fatigue on each Delay(TOT) component could be precisely assessed.