Eccentric Contraction-Induced Muscle Fibre Adaptation

Abstract

Hard-strength training induces strength increasing and muscle damage, especially after eccentric contractions. Eccentric contractions also lead to muscle adaptation. Symptoms of damage after repeated bout of the same or similar eccentrically biased exercises are markedly reduced. The mechanism of this repeated bout effect is unknown. Since electromyographic (EMG) power spectra scale to lower frequencies, the adaptation is related to neural adaptation of the central nervous system (CNS) presuming activation of slow-non-fatigable motor units or synchronization of motor unit firing. However, the repeated bout effect is also observed under repeated stimulation, i.e. without participation of the CNS. The aim of this study was to compare the possible effects of changes in intracellular action potential shape and in synchronization of motor units firing on EMG power spectra. To estimate possible degree of the effects of central and peripheral changes, interferent EMG was simulated under different intracellular action potential shapes and different degrees of synchronization of motor unit firing. It was shown that the effect of changes in intracellular action potential shape and muscle fibre propagation velocity (i.e. peripheral factors) on spectral characteristics of EMG signals could be stronger than the effect of synchronization of firing of different motor units (i.e. central factors).

Keywords

• Repeated bout
• Adaptation
• EMG power spectrum
• Synchronization