This study aimed to clarify the difference in the onset of EMG activity between eight trunk muscles, including the anterior (QL-a) and posterior (QL-p) layers of the quadratus lumborum during rapid shoulder joint abduction. Thirteen healthy men participated in this study. Electromyography of the QL-a, QL-p, transversus abdominis (TrA), internal oblique (IO), external oblique (EO), rectus abdominis (RA), lumbar multifidus (LMF), lumbar erector spinae (LES) on non-movement side, and middle deltoid (MD) on the movement side were measured. Subjects who were standing in a relaxed position performed rapid shoulder abduction with the dominant hand after light stimulus with or without a 3 kg wrist weight. Two-way ANOVA (muscles × weight conditions) was used to compare the onset of trunk muscles relative to that of MD. There was a significant main effect of the muscles. The onset of the QL-a, QL-p, and TrA was significantly earlier than that of the IO, EO, LMF, and LES (P < 0.01). This result suggests that the activities of the QL-a, QL-p, and TrA have a crucial role in controlling the center of mass within the base of support and stabilizing the lumbar spine in the coronal plane during shoulder abduction.
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