Effect of Shoulder Position on Scapular Muscle Activity during Scapular Protraction

Purpose: This study was to evaluate several tasks performed at a high intensity in terms of their ability to elicit EMG activity in the serratus anterior by comparing the EMG activities of the serratus anterior, upper trapezius, and lower trapezius muscles during six tasks combined shoulder flexion with rotation. Methods: Fifteen healthy males were recruited to this study. Each subject was instructed to assume a sitting position without back support and asked to flex (90° or 120°) the right shoulder and protract the scapula in the sagittal plane with maximal external rotation; to assume a neutral position; or to internally rotate the glenohumeral joint. The EMG data were collected from the serratus anterior (SA), upper trapezius (UT), and lower trapezius (LT) muscles were normalized to maximum voluntary isometric contraction. The UT/LT and UT/SA muscle activity ratios in each task were assessed by calculating the surface EMG. Data were analyzed by two-way repeated-measures analysis of variance, with the level of significance set at p<0.05. Results: The results of this study, shoulder flexion with external rotation resulted in low upper trapezius/serratus anterior and upper trapezius/lower trapezius ratios and a relatively high level of serratus anterior activation. Conclusion: Shoulder flexion with external rotation used herein may be considered as important for clinical interventions aimed at selectively increasing SA strengthen and clinical selection of exercises for improving glenohumeral joint and scapulothoracic control.