Peripheral fatigue : partitioning the contributors during muscle contraction

Aim: Peripheral fatigue is a phenomenon involving mechanisms that are synaptic, electrochemical and mechanical in nature. An electromyographic (EMG), mechanomyographic (MMG) and force (F) combined approach may allow the evaluation of the changes in thesynaptic, electrochemical and mechanical components. Since prior investigations evaluated only the acute effects of a fatiguing protocol, the effects of fatigue on the different components during a fatiguing protocol were evaluated. Methods: EMG, MMG and F signals were detected in the medial gastrocnemius muscle in eighteen male participants (age: 25 ± 3 yrs; body mass: 77 ± 4 kg; stature: 1.81 ± 0.06 m; mean ± SD) during a fatiguing protocol (twelve continuous stimulations at 35 Hz, 9.5 s on/0.5 s off; total time: 120 s). Stimulation (Stim), EMG, MMG and F signals were recorded during contraction. The electromechanical delay during contraction (DelayTOT) and its three components (between Stim and EMG: Dt Stim-EMG, synaptic component; between EMG and MMG: Dt EMG-MMG, electrochemical component; and between MMG and F: Dt MMG-F, mechanical component) were calculated. Results: Before fatigue DelayTOT was 22.3 ± 0.9 ms. Dt Stim-EMG, Dt EMG-MMG and Dt MMG-F contribution was 6 %, 48 % and 46 %, respectively. During fatigue, DelayTOT and Dt MMG-F increased significantly in the first 40 s, then remaining stable, whereas Dt Stim-EMG and Dt EMG-MMG increased until the 10th and 30th s, respectively. At the end of the fatiguing protocol, F decreased by 44 %. DelayTOT was 31.2 ± 1.6 ms. Dt Stim-EMG, Dt EMG-MMG and Dt MMG-F significantly changed their contribution to 5 %, 43 % and 53 %, respectively. Conclusions: The EMG, MMG and F combined approach permitted to follow the changes in the synaptic, electrochemical and mechanical components during a fatiguing protocol.