Applicability of the coactivation method in assessing synergies of the scapular stabilizing muscles

Hypothesis: This study investigated the feasibility of quantifying the coactivation levels in the evaluation of synergisms of the scapular stabilizing muscles during the elevation and lowering of the arms. This method, which quantifies the overlapping area of the normalized electromyographic (EMG) activity, could be applicable in assessing such muscular synergies. Materials and methods: Both shoulders of 10 healthy volunteers were assessed. The coactivation of 3 synergic muscular groups (upper, middle and lower trapezius; upper trapezius and serratus anterior; middle trapezius and serratus anterior) and the isolated electromyographic activity of each muscle were analyzed. Results: Analysis of variance revealed that the coactivation values between the synergic groups were different from the isolated levels of each respective muscle for both movements (24.42 < F < 40.12; df=6; p < 0.001). The coactivation values of all groups during elevation were different from those during the lowering of the arms (13.31 < F < 959.92; df=1; p <= 0.002), with progressive increases in EMG activity during the elevation and decreases during the lowering of the arms. Conclusion: The significant differences in EMG activity between the isolated and the coactivation methods indicated that the coactivation method was adequate to assess the scapular muscular synergies. Level of evidence: Basic science study. (C) 2009 Journal of Shoulder and Elbow Surgery Board of Trustees.