Comparison of scapular kinematics between elevation and lowering of the arm in the scapular plane

Objective. To compare scapular orientation during both the concentric (elevation) and eccentric (lowering) phases of scapular plane abduction in subjects with and without shoulder impingement. Design. Mixed model analysis of variance with one between-subjects factor (group) and within-subjects factors of phase, humeral angle, and trial. Background. Abnormal scapular kinematics have been identified in shoulder impingement patients during the concentric phase of arm elevation, and under static conditions. Because abnormal scapular motion is observed clinically during the eccentric phase of arm elevation, analysis of this phase of motion is warranted. Methods. Twenty-six symptomatic and 26 healthy subjects performed five repetitions of humeral scapular plane abduction. An electromagnetic tracking device described three-dimensional scapular kinematics during arm elevation and lowering. Angular values for scapular anterior/posterior tipping in the sagittal plane, upward/downward rotation in the scapular plane, and internal/external rotation in the transverse plane were calculated. Scapular orientation relative to the thorax at humeral angles of 40degrees, 60degrees, 80degrees 100degrees, and 120degrees was statistically tested for effects of phase and trial, or for interactions of phase with group or humeral angle. Results. Internal rotation was significantly increased in the eccentric phase for both groups at the 100degrees angle (P < 0.05) and for the symptomatic group only at the 120 degrees angle (P < 0.05). Scapular anterior tipping was significantly decreased during the eccentric phase in both groups at the 80degrees (P < 0.001), 100 degrees (P < 0.0001), and 120degrees (P < 0.0001) angles. Conclusions. Small but statistically significant differences in scapular tipping and internal rotation during the eccentric phase of arm elevation were identified at higher humeral angles in both subject groups, while no significant phase differences for scapular upward rotation or for scapular variables at lower humeral angles were found. Averaged across phases, the symptomatic group demonstrated significant reductions in upward rotation at lower humeral elevation angles, and significant increases in anterior tipping at higher elevation angles as compared to the healthy group.