Electromyographic Analysis of Shoulder Girdle Muscles during Common Internal Rotation Exercises

Authors:  Alizadehkhaiyat O, Hawkes DH, Kemp GJ, Frostick SP

High level throwing performance requires the development of effective muscle activation within shoulder girdle muscles, particularly during forceful internal rotation (IR) motions.  The purpose of this study was to investigate activation pattern of 16 shoulder girdle muscles/muscle sub-regions during three common shoulder IR exercises.  Electromyography (EMG) was used to record muscular activity in 30 healthy subjects from 16 shoulder girdle muscles/muscle sub-regions (surface electrode: anterior, middle and posterior deltoid, upper, middle and lower trapezius, serratus anterior, teres major, upper and lower latissimus dorsi, upper and lower pectoralis major; fine wire electrodes: supraspinatus, infraspinatus, subscapularis and rhomboid major) using a telemetric EMG system. Three IR exercises (standing IR at 0 degrees abduction and 90 degrees of abduction, and IR at Zero-Position [zero rotation of the humerus, arm elevated to 155 degrees]) were studied. EMG amplitudes were normalized to EMGmax (EMG at maximal IR force in a standard position) and compared using one-way repeated-measures analysis of variance (ANOVA).  The result demonstrated significant differences in muscles’ activation across IR exercises (p<0.05–p<0.001). Rotator cuff and deltoid muscles were highly activated during IR at 90° of Abduction. Latissimus dorsi exhibited markedly higher activation during IR at Zero-Position. While upper trapezius had the highest activation during IR at Zero-Position, middle and lower trapezius were activated at highest during IR at 90 degrees of abduction. The highest activation of serratus anterior and rhomboid major occurred in IR at Zero-Position and IR at 90 degrees of abduction, respectively.  Studied exercises have the potential to effectively activate glenohumeral and scapular muscles involved in throwing motions which helps to provide further evidence for developing rehabilitation, injury prevention, and training strategies.

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