Reproducibility of surface EMG during dynamic shoulder forward flexions: a study of clinically healthy subjects

There is a shortage of studies concerning the reproducibility of surface electromyography (EMG) during dynamic contractions. The movement per se introduces factors that could have the potential to affect the characteristics of the electromyogram. The aim of this study was to investigate the during-the-day reproducibility (using intra-class coefficient, ICC) of the peak torque (PT) and the EMG variables (without removing electrodes) of dynamic shoulder forward flexions. Eleven healthy women performed three sets of 10 dynamic maximum right-shoulder flexions at 1 h intervals using an isokinetic dynamometer. The PT of each flexion was determined. EMG signals were recorded from four muscles (trapezius, deltoid, infraspinatus and biceps brachii) using surface electrodes, and the mean frequency of the power spectrum (MNF [Hz]) and the signal amplitude (RMS [microV]), were computed. The ability to relax between maximum flexions was calculated as a ratio between the RMS during the passive extension phase and the RMS during the active flexion phase of each contraction cycle. This ratio is the signal amplitude ratio (SAR). The present study showed good reproducibility for PT, MNF and RMS, while the reproducibility of SAR was generally acceptable (fair) according to the criteria used.