On the nature of the electromyographic signals recorded during vibration exercise

Purpose : Surface electromyography (EMG) has been widely used to measure neuromuscular activity during vibration exercise (VE) to investigate the underlying mechanisms elicited by VE. However, the EMG spectrum recorded during VE shows sharp peaks at the vibration frequency whose interpretation remains controversial. Some authors considered those peaks as a result of motion artifacts, while others interpreted them as due to vibration-induced neuromuscular activity. The aim of the present study is to clarify the nature of those sharp peaks observed during VE. Methods : Three independent EMG measurements were performed during VE: in vitro (IVT), in vivo at rest ( IVV R ), and in vivo during voluntary contraction ( IVV C ). The amplitudes of the EMG vibration frequency components ( A VF ) were extracted for all measurements. The conduction velocity (CV) of the vibration frequency components and the full EMG spectrum were also estimated during voluntary contraction. Results : Our spectrum analysis revealed small A VF for IVT and IVV R , accounting for only 3.3 and 7.6 % of that obtained from IVV C . Moreover, the CV estimation indicated the EMG vibration components to propagate along the muscle fiber with CV ˜ 6.5 m/s, comparable to the CV estimated using the full EMG spectrum (5.7 m/s). Conclusion : We may therefore conclude that the sharp spectral peaks observed during VE are mainly due to vibration-induced muscle activity rather than motion artifacts.