Fig 1.
Experimental setup for (A) estimation of surface MUAP waveforms and (B) recording surface EMG interference pattern at different force levels. (A) Experimental setup for estimation of motor unit action potential waveforms from surface electromyography (EMG) signals of four leg muscles: Soleus (SO), Medial Gastrocnemius (MG), Lateral Gastrocnemius (LG), and Tibialis Anterior (TA). Surface EMG signals were recorded, while the participant executed weak contractions. Intramuscular EMG signals were also recorded using two fine wire electrodes inserted in a region between a pair of surface electrodes. (B) Experimental setup for recording surface EMG interference pattern signals. The participant executed isometric plantar-flexion and dorsiflexion contractions at 5%, 10% and 20% of the maximum voluntary contraction force, while the right foot was strapped to a rigid pedal coupled to a strain gauge. Feedback of the force produced on the rigid pedal was displayed on a monitor placed in front of the participant. Surface EMG signals from MG, LG, and TA muscles were acquired using electrodes positioned according to the SENIAM convention, whereas myoelectric signals from SO muscle were acquired using the arrangement adopted in [32]. The most distal electrodes were connected to the “+” terminal of EMG machine, while the most proximal electrodes were connected to the “-” terminal.
Fig 2.
Estimation of surface motor unit action potentials waveforms from the decomposition of (A) surface and (B) intramuscular EMG signals. (A) Surface electromyography (EMG) signal recorded during minimal voluntary contraction. Trains of two surface motor unit action potentials (MUAP) standing out of the signal baseline are identified with rectangles. (B) Surface and intramuscular EMG signals simultaneously recorded during weak contractions. A single motor unit action potential train in the intramuscular EMG signal standing out of the signal baseline is identified, while the surface EMG signal is composed of considerable superpositions of surface MUAPs. (C) MUAP waveforms estimated from the surface EMG signals shown in (A) and (B). The surface MUAP waveforms 1 and 2 were estimated directly from the surface EMG signal using EMGLAB software [34]. The surface MUAP waveform 3 was estimated by averaging the surface EMG signals in time-windows centralized at the peak amplitudes of intramuscular MUAPs.
Fig 3.
Representative (A) surface MUAP waveforms and (B) EMG interference pattern signals of different leg muscles. (A) Spectrum median frequency (MDF), in Hz, and duration, in ms, of representative motor unit action potential (MUAP) waveforms estimated from surface electromyography (EMG) signals recorded from Soleus (SO), Medial Gastrocnemius (MG), Lateral Gastrocnemius (LG), and Tibialis Anterior (TA) muscles during weak voluntary contractions. The power spectral densities were normalized by the mean square value of the surface MUAP waveform amplitudes. (B) MDF, in Hz, estimated from surface EMG interference pattern signals obtained from SO, MG, LG, and TA muscles of a representative participant during isometric constant force plantarflexion and dorsiflexion contractions at 5%, 10%, and 20% of the maximum voluntary contraction (MVC). The power spectral densities were normalized by the mean square value of the surface EMG signal amplitudes.
Fig 4.
(A) duration and (B) spectrum median frequency of surface MUAP waveforms compared across leg muscles. (A) Box plots of durations, in ms, of the decomposed motor unit action potential (MUAP) waveforms obtained from Soleus (SO), Medial Gastrocnemius (MG), Lateral Gastrocnemius (LG), and Tibialis Anterior (TA) muscles. (B) Box plots of spectrum median frequencies (MDF), in Hz, of the decomposed surface MUAP waveforms obtained from SO, MG, LG, and TA muscles. The Kruskal-Wallis test was used to compared the MDF and durations across the four leg muscles (SO, MG, LG, and TA). The sample size for SO is nSO = 101, for MG is nMG = 75, for LG is nLG = 69, and for TA is nTA = 49. The horizontal bars indicate statistical differences across muscles. Legend: * p < .05, ** p < .01, and *** p < .001.
Fig 5.
Spectrum median frequency of surface EMG interference pattern signals compared across leg muscles.
Box plots of spectrum median frequencies (MDF), in Hz, estimated from the surface electromyography (EMG) interference pattern signals acquired from Soleus (SO), Medial Gastrocnemius (MG), Lateral Gastrocnemius (LG) and Tibialis Anterior (TA) muscles during isometric constant-force plantarflexion and dorsiflexion contractions at 5%, 10%, and 20% of maximum voluntary contraction force (MVC). The standard errors are indicated with vertical bars. The two-way ANOVA was used to compare the MDF across muscles (SO, MG, LG, and TA) and contraction intensities (5%, 10%, and 20% of MVC). The horizontal bars indicate statistical differences across muscles. The sample size for each muscle and contraction intensity is n = 20. Legend: * p < .05, ** p < .01, and *** p < .001.
Table 1.
Mean and standard deviation of the power spectrum median frequencies calculated from the first and second halves of surface EMG interference pattern signals.