Evolution of muscular fatigue in periscapular and rotator cuff muscles during isokinetic shoulder rotations.

To this day, how shoulder muscles react to a strong fatigue stimulus during dynamic shoulder rotations remains unknown. The aims of this study were to assess the effect of repeated maximal internal-external isokinetic shoulder rotations on shoulder strength and muscle activity. Twenty-four individuals completed a 50-repetition fatiguing isokinetic protocol while electromyography was recorded on eleven muscles of the shoulder girdle. Time-frequency transformation and an ANOVA model using statistical parametric mapping methods were used to analyze shifts in instantaneous median frequency (MDF) between each 10-repetition Blocks. Peak torques decreased in both internal and external rotation (P < 0.01) by 24.8% on average which indicated the presence of fatigue. Significant decrease in MDF (P < 0.01) was observed for pectoralis, middle deltoid, upper, middle and lower trapezius, infraspinatus and subscapularis muscles. The observed fatigue to the periscapular and rotator cuff muscles suggests that shoulder stability could be compromised during repeated shoulder rotations, which could underlie the increased risk of scapular dyskinesis and shoulder impingement during fatiguing tasks. The present study provides a deeper understanding on the manifestations of fatigue within muscles of the shoulder girdle and the results could be applied toward improvements in athlete shoulder injury prevention and rehabilitation programs.