EMG activity during whole body vibration: motion artifacts or stretch reflexes?

Abstract

The validity of electromyographic (EMG) data recorded during whole body vibration (WBV) is controversial. Some authors ascribed a major part of the EMG signal to vibration-induced motion artifacts while others have interpreted the EMG signals as muscular activity caused at least partly by stretch reflexes. The aim of this study was to explore the origin of the EMG signal during WBV using several independent approaches. In ten participants, the latencies and spectrograms of stretch reflex responses evoked by passive dorsiflexions in an ankle ergometer were compared to those of the EMG activity of four leg muscles during WBV. Pressure application to the muscles was used to selectively reduce the stretch reflex, thus permitting to distinguish stretch reflexes from other signals. To monitor motion artifacts, dummy electrodes were placed close to the normal electrodes. Strong evidence for stretch reflexes was found: the latencies of the stretch reflex responses evoked by dorsiflexions were almost identical to the supposed stretch reflex responses during vibration (differences of less than 1 ms). Pressure application significantly reduced the amplitude of both the supposed stretch reflexes during vibration (by 61 +/- 17%, p < 0.001) and the stretch reflexes in the ankle ergometer (by 56 +/- 13%, p < 0.01). The dummy electrodes showed almost no activity during WBV (7 +/- 4% of the corresponding muscle's iEMG signal). The frequency analyses revealed no evidence of motion artifacts. The present results support the hypothesis of WBV-induced stretch reflexes. Contribution of motion artifacts to the overall EMG activity seems to be insignificant.

On the nature of the electromyographic signals recorded during vibration exercise

“…They found an average EMG amplitude ratio of 7 % between the dummy electrodes and the contacted electrodes at the vibration frequency. In these studies the spectra and latencies of the EMG signals recorded during WBV matched those of the stretch reflex mechanically evoked by an ankle ergometer (Ritzmann et al 2010). Based on these findings, the same conclusion as in (Martin and Park 1997) was drawn.…”

“…Moreover, by use of the phase method, band-pass filtering to select the peaks corresponding to the vibration frequency is no longer necessary, therefore avoiding possible phase distortion introduced by filtering (Ritzmann et al 2010). However, the phase method presents some limitations for the assessment of conduction velocity in the presence of shape changes between EMG signals recorded in adjacent channels (Farina and Merletti 2004).…”

“…However, the electrodes placed on the patella may also register cross talk from surrounding muscles. In fact, a considerable amount of cross talk has been recorded from electrodes placed above osseous structures (Ritzmann et al 2010).…”

“…For instance, for a 40-N force modulation at 45 Hz we measured a displacement of 1 mm. Ritzmann et al (2010) was the first gathering evidence that the sharp EMG spectral peaks observed during WBV are due to stretch reflex. To this end, dummy electrodes were used and EMG signals recorded during WBV were compared with those evoked by an ankle ergometer.…”

“…The prominent peaks observed during voluntary contraction were therefore interpreted as TVR, representing the degree of motor unit synchronization. Some other studies compared the EMG signal recorded by contacted electrodes placed over the soleus muscle with those recorded by dummy electrodes placed on insulating tape fixed on top of the skin (Ritzmann et al 2010). They found an average EMG amplitude ratio of 7 % between the dummy electrodes and the contacted electrodes at the vibration frequency.…”