DelayTOT and its subcomponents: a new approach for the electromechanical delay partitioning before and after a fatiguing protocol during electrically-evoked contractions

Aim: DelayTOT is the time lag between the onset of the electrical stimulation (Stim) and the force (F) development. During this time frame the following several mechanisms take place to transduce the electrical pulse into a mechanical events: i) the propagation of the action potential from the site of excitation throughout the inner branches of the axons and from the pre- to the post-synaptic membrane of the motor endplate; ii) the propagation of the action potential along the sarcolemma, the dihydropyridine and ryanodine interaction and the excitation-contraction (E-C) coupling processes and iii) the tensioning of the muscle-tendon unit (MTU) with the force development. Several factors influence the neuromuscular processes involved in the muscular contraction among which temperature, diseases, joint angle, and fatigue. A combined electromyogram (EMG) and mechanomyogram (MMG) approach during electrically-evoked contractions allows to partition DelayTOT in three latencies, a synaptic, E-C coupling and mechanical component, and to evaluate each component in different conditions. The study aims to assess the DelayTOT and its subcomponents in fresh and fatigued muscles during stimulated contractions of the gastrocnemius medialis muscle. Method: Sixteen healthy students underwent two sessions (SA and SB) of three tetanic stimulations (50 Hz, 3s), with 5 min of rest in between. Moreover, after a fatiguing protocol (35 Hz, 120s), three tetanic stimulations were administered after 1, 2 and 7 minute, respectively. Stimulation current (Stim), surface electromyogram (EMG), mechanomyogram (MMG) and force signal (F) were acquired. The DelayTOT (the time lag between Stim and F development), the synaptic delay (∆t Stim-EMG, the time interval from Stim onset and the EMG artefact), the E-C coupling latency (∆t EMG-MMG, the time frame between the EMG artefact and the MMG onset) and the mechanical component (∆t MMG-F, the delay from the MMG onset and the F development) were calculated and expressed as mean ± standard error (SE). The statistical analysis were performed using one-way ANOVA for repeated measures and post-hoc multiple comparison. Results: DelayTOT, Δt Stim-EMG, Δt EMG-MMG and Δt MMG-F were 27.5±0.9 ms, 1.4±0.1 ms, 9.2±0.5 ms and 16.8±0.7 ms, respectively. Fatigue lengthened DelayTOT, Δt Stim-EMG, Δt EMG-MMG and Δt MMG-F by about 18%, 7%, 16% and 22%, respectively (P<0.05). Δt Stim-EMG, Δt EMG-MMG and Δt MMG-F contributed to DelayTOT increase by about 2%, 27% and 71%, respectively. Reliability analysis showed from high to very high values. Conclusion: The combined approach allowed a reliable calculation of synaptic, E-C coupling and mechanical contribution to DelayTOT. Moreover, the effects of fatigue on each DelayTOT component is differently distributed and could be precisely assessed.