Dynamic scapulohumeral rhythm: the effects of external resistance during elevation of the arm in the scapular plane.

The purpose of this study was to characterize the scapulohumeral rhythm in the normal shoulder during dynamic elevation of the arm in the scapular plane and to compare the scapulohumeral rhythm during three different states of loading: 1) arm completely unloaded and passively elevated, 2) light load consisting of active elevation against the weight of the limb, and 3) heavy loading against maximal resistance. Electromagnetic tracking was used to record three-dimensional scapular and humeral kinematics on 25 normal subjects. Cardan angles were calculated to assess the dynamic relationship of humeral elevation to scapular upward rotation. The results show that during dynamic humeral elevation the scapulohumeral rhythm changes depending on the phase of elevation and the external load on the arm. During passive range of motion, the scapulohumeral rhythm decreased from 7.9:1 to 2.1:1 as the arm was elevated. Light shoulder loads caused an increase of the scapulohumeral rhythm from 3.1:1 to 4.3:1 as the arm was elevated. Heavy shoulder loading resulted in an increasing scapulohumeral rhythm from 1.9:1 to 4.5:1 as the arm was elevated. The results suggest that the historical assumption of a simple linear 2:1 scapulohumeral rhythm ratio may be overly simplistic and may not accurately describe the scapulohumeral rhythm under varying dynamic conditions. Therapists need to understand the normal changing relationships of the scapulohumeral rhythm under different conditions for accurate interpretation of clinical observations.