Upper body kinematic and muscular variability in response to targeted rotator cuff fatigue

The rotator cuff muscles are prominent stabilizers of the shoulder and are vulnerable to muscle fatigue. Rotator cuff fatigue may result in subacromial impingement (SAI) through the superior translation of the humeral head. Scapular changes have been reported inconsistently, but may prevent SAI. The purpose of this study was to quantify changes in scapular kinematics, as well as muscle activity during glenohumeral motions following targeted rotator cuff fatigue. Ten healthy men completed four planar glenohumeral motions (cross-flexion, frontal, scapular, and sagittal plane elevation) prior to and immediately following a rotator cuff fatigue protocol on two separate days. Scapular kinematics and muscle activity of thirteen muscles were recorded. Scapular protraction decreased significantly with fatigue during scapular plane elevation (p < 0.001; η_p² = 0.74). Although not significant, large effect sizes were found with decreased scapular protraction during elevation in the frontal (p = 0.012; η_p² = 0.52) and sagittal planes (p = 0.007; η_p² = 0.58), as well as decreased scapular medial rotation during cross-flexion (p = 0.008; η_p² = 0.56). Between-subject variability (standard deviations from 2.3° to 14.5°) and within-subject day-to-day differences (upwards of 10° deviation in the opposite direction) were high among all kinematic changes following fatigue. Considerable day-to-day differences in scapular stabilizer muscle activity in response to fatigue were present. Due to the degrees of freedom at the upper extremity, individuals can employ a variety of compensatory strategies to fatigue. The variable compensatory strategies across the scapular stabilizers resulted in individual-specific scapular kinematic changes that could act as either impingement-sparing or impingement-promoting. The high variance in day-to-day differences within-subjects indicates that kinematic and muscular responses to fatigue may be adaptive within individuals over time.