Differences in EMG spike shape between individuals with and without non-specific arm pain

The purpose of this study was to determine whether spike shape analysis of surface electromyographic (SEMG) activity is a useful tool to study muscle disorders. This study investigated SEMG spike shape parameters at low levels of contraction and changes in SEMG spike shape across different levels of isometric wrist extension contractions in individuals with non-specific arm pain (NSAP), asymptomatic subjects deemed at-risk for repetitive strain injury, and asymptomatic control subjects. Twenty-two asymptomatic control subjects, 8 at-risk subjects, and 16 subjects with NSAP participated. Bipolar SEMG data were recorded from the ECRB muscle during isometric wrist extension contractions at 10, 20, 30, 40, 50, 60, and 70% of maximum voluntary contraction (MVC) force performed in a randomized order. Five criterion measures: mean spike amplitude (MSA), mean spike duration (MSD), mean spike slope (MSS), mean spike frequency (MSF), and mean number of peaks per spike (MNPPS) were computed from each SEMG signal. A one-way analysis of covariance (ANCOVA) of the spike shape parameters computed from the 10% MVC data, with group as a main effect and age as a covariate, revealed a significant group by age interaction for MSA, and significant group main effects for MSS and MNPPS, where the NSAP group had lower MSS and lower MNPPS than the control subjects. An ANCOVA including group as a main effect and contraction level and age as covariates revealed that all three groups showed predictable changes in the spike shape analysis criterion measures over increasing contraction force levels, where motor unit recruitment and rate coding appear to be the primary mechanisms for increasing force output of the muscle. Significant interactions between group and contraction level were observed for MSD, MSA, MSS, and MNPPS. The NSAP group presented with differences in how the spike shape measures change with increasing contraction level that may be indicative of myogenic changes, a result that is consistent with previous quantitative EMG findings. This work provides evidence that NSAP involves myogenic changes in the ECRB muscle and that spike shape analysis may be a valuable non-invasive tool in the evaluation of neuromuscular disorders.