Abstract
Interference surface electromyogram (EMG) recorded from linear electrode arrays aligned to muscle fibres can be separated into propagating and non-propagating contributions. The first reflects the propagation of action potentials along muscle fibres. The non-propagating components are here shown to be related to the overall firing pattern of active motor units (MU). Indeed, in simulations, the power spectral density (PSD) of non-propagating components shows a low frequency peak corresponding to the mean firing rate, even when such a contribution is not visible in the PSD of the EMG (either monopolar or single differential configuration, either rectified or not). Moreover, it has a high correlation with the PSD of the cumulative firings of the MUs in the detection volume of the recording system. Applications to experimental data confirm that the low frequency peak is more evident for the non-propagating components than for the raw signals and is related to the MU firing frequency. Potential future applications are expected in the study of the MU control in different conditions (e.g., training, fatigue or pathology, inducing changes, or modulation of firing rate) and in the investigation of common synaptic inputs to motor neurons.
Highlights
Surface electromyogram (EMG) reflects the bioelectric command inducing muscle contraction, providing information both on the peripheral adaptations of motor units (MU) and on their control
Theoretical [8]–[10] and experimental [11], [12] works suggested that the low frequency portion is mainly affected by the firing statistics, whereas, at higher frequencies, the power spectral density (PSD) reflects the average shapes of MU action potentials (MUAPs)
LOW FREQUENCY PEAK FOR DIFFERENT FORCE LEVELS Preliminary tests are considered in Figure 1, showing the PSDs of simulated EMG at different force levels
Summary
Surface electromyogram (EMG) reflects the bioelectric command inducing muscle contraction, providing information both on the peripheral adaptations of motor units (MU) and on their control. The power spectral density (PSD) of surface EMG reflects many properties of the active MUs. Theoretical [8]–[10] and experimental [11], [12] works suggested that the low frequency portion (under about 40 Hz) is mainly affected by the firing statistics, whereas, at higher frequencies, the PSD reflects the average shapes of MU action potentials (MUAPs). In fatiguing contractions, a reduction of muscle fibre conduction velocity (CV) is associated to a proportional shrink of the portion of PSD reflecting MUAP shapes toward lower frequency. The low frequency peak is not directly affected by CV and was found to introduce problems in following
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