Increased voluntary drive is associated with changes in common oscillations from 13 to 60 Hz of interference but not rectified electromyography

During voluntary contractions, cortical drive to the motor neuron pool has been typically evaluated with coherent activation between signals recorded from the brain [electroencephalogram (EEG) or magnetoencephalogram (MEG)] and surface-recorded signals from contralateral muscles [electromyogram (EMG)]. MEG–EMG and EEG–EMG coherence has been used to demonstrate common oscillations between the motor cortex and muscle primarily at two frequency bands, known as the “beta” (15–30 HZ) and “Piper” (30–60 HZ; also known as the low-gamma band) bands.1 These bands are of interest because there is evidence that reflects the communication between higher centers (cortex) and the periphery (muscle)1,2 and because corticomuscular coherence in these bands differs between neurological patients and healthy adults.3–7 Recently, some studies have used coherent activation between EMG signals as an indicator of the oscillatory drive to the motor neuron pool from higher centers (EMG–EMG coherence).3,5,8–11 Interestingly, EMG–EMG coherence appears to provide similar results for EEG–EMG coherence,3,6,11 which means the two methods can be used interchangeably. Because the EMG–EMG coherence approach is easier to record, it will simplify the identification of the cortical drive to the motor neuron pool for movement-related research and clinical diagnosis.3 Rectification of surface EMG signals is a common processing step used prior to performing EEG–EMG coherence and EMG–EMG coherence.3,4,10–17 The experimental evidence for rectifying the EMG signal is limited to two studies in the literature.17,18 Specifically, Myers et al.,17 using simulations, provided evidence that rectification of the EMG signal can capture the mean discharge rate of motor units, whereas Yao et al.18 demonstrated that EEG–EMG coherence was similar when either interference EMG or rectified EMG signals were used. Recent evidence from our laboratory, however, questions the use of EMG rectification for EMG–EMG coherence.19 The findings of that study were based on manipulating EMG signals that were reconstructed from experimentally recorded EMG signals. Rectification of the EMG signal significantly impaired the accuracy of estimating the original input to the signal and the common oscillations between two EMG signals. In contrast, when the interference EMG signal was used, the known input to an EMG signal and the known common input between two EMG signals were accurately captured. Previous findings1,20 suggest that the strength of corticomuscular coherence in the beta band and Piper band changes differently with the level of voluntary force exerted. Specifically, the beta band decreases, and the Piper band increases with the level of voluntary force.1,20 Although these findings occurred with the use of rectified EMG, the changes at the muscle level should be similar with both interference and rectified EMG, because MEG–EMG coherence and EEG–EMG coherence are similar regardless of whether the EMG signal is rectified or not.18 The purpose of this experiment, therefore, was to compare the sensitivity of the interference and rectified EMG wavelet spectrum to detect changes in the strength of the beta and Piper bands when voluntary force increases from 15% to 50% MVC. Based on findings from our previous study,19 we hypothesized that the interference but not the rectified EMG would be sensitive to changes in the beta and Piper bands in muscle activity with an increased voluntary force level. A portion of the experimental recordings have been reported in a previous study.21