EMG and strength correlates of selected shoulder muscles during rotations of the glenohumeral joint

Objective. To identify activation patterns of several muscles acting on the shoulder joint during isokinetic internal and external rotation. Design. Combined EMG and isokinetic strength analysis in healthy subjects, Background. EMG studies of the shoulder region revealed intricate muscular activation patterns during elevation of the arm but no parallel studies regarding pure rotations of the joint could be located, Methods. Fifteen (n = 30 shoulders) young, asymptomatic male subjects participated in the study. Strength production during isokinetic concentric and eccentric internal and external rotations at 60 and 180 degrees/s was correlated with the EMG activity of the rotator cuff, biceps, deltoid and pectoralis major. Analysis of the smoothed EMG related to the timing of onset of the signal and to the normalized activity at the angle of the peak moment. Determination of the association between the EMG and the moment was based on strength ratios. Results. Findings indicated that for both types of rotations, the rotator cuff and biceps were active 0.092 +/- 0.038-0.215 +/- 0.045 s prior to the initiation of the actual movement and 0.112-0.034 s prior to onset of deltoid and pectoralis major activity. These differences were significant in all of the eight conditions (P < 0.05). In terms of the strength ratios, strong association was found between electrical activity and moment production in the subscapularis and infraspinatus (r(2) = 0.95 and 0.72, respectively) at the low and high angular velocities. Conclusions. Prior to actual rotation of the shoulder joint, normal recruitment of the rotator cuff and biceps is characterized by a non-specific presetting phase which is mainly directed at enhancing the joint 'stiffness' and hence its stability. Once movement is in progress, the EMG patterns of these muscles become movement specific and are correlated with the resultant moment.