Motor unit behavior of the lower trapezius and serratus anterior in individuals with scapular dyskinesis

Abstract Scapular dyskinesis results from weakness of the lower trapezius and serratus anterior, but no studies have investigated the motor unit (MU) behavior required for muscle exertion. This study aimed to classify scapular dyskinesis into raising (SDR) and lowering (SDL) phases and investigated the MU behaviors of the lower trapezius and serratus anterior. Fifty healthy young subjects underwent a scapular dyskinesis test and were divided into Normal, SDR, and SDL groups. The subjects performed submaximal voluntary contractions of the lower trapezius and serratus anterior, and surface electromyography signals were decomposed into the MU action potential amplitude (MUAP AMP ), mean firing rate (MFR), and recruitment threshold (RT). The average MUAP AMP and MFR and the slopes and y-intercepts of the linear MUAP AMP -RT and MFR-RT were compared. The MUAP AMP -RT slopes in the lower trapezius ( p = 0.049, r = 0.420) and serratus anterior were smaller ( p = 0.010, r = 0.490) and the MFR-RT y-intercept in the serratus anterior was lower ( p = 0.004, r = 0.540) in the SDR group than in the Normal group. Differences in other parameters between the SDL and Normal groups were not significant. Lower trapezius and serratus anterior MU behavioral changes may cause scapular dyskinesis in the raising phase.