The term “Scapulohumeral Rhythm” is a familiar term likely introduced to the clinician in the academic setting. Proper scapula motion and more specifically scapulohumeral rhythm is an essential “concept” in the appreciation of shoulder “function”. This concept describes the relationship of stability and motion between the scapula and the humerus during arm elevation.
Unloaded arm elevation has been traditionally described in three (3) phases:
The setting phase – With active humeral elevation up to 30 degrees in the coronal or scapula planes of the body (additional documentation states up to 60 degrees of sagittal plane flexion) the scapula seeks a position of stability.
30 – 90 Degrees of arm elevation – The scapula continues to rotate laterally at approximately 60 degrees of elevation stabilizing against the thoracic wall with little superior or inferior migration. The “classic” described ratio for this phase of arm elevation is 2:1 for humeral elevation to scapula upward rotation.
90 – 180 degrees of arm elevation – The scapula continues to rotate causing the center of rotation to shift toward the glenoid, resulting in medial and continued superior movement of the glenoid with lateral migration of the of the scapula inferior angle. Along with lateral rotation, the scapula orients the glenoid fossa anteriorly toward the sagittal plane. The scapula posteriorly tilts as the superior angle moves away for the thoracic wall with concomitant movement of the inferior angle toward the thoracic wall. The “classic” described ratio for this final phase of arm elevation is 1:1 for humeral elevation to scapula upward rotation.
Therefore the overall 180 degrees of unloaded arm elevation transpires due to 120 degrees of humeral elevation and 60 degrees of scapula motion. However, it is documented that an alteration in shoulder kinematics occurs during loaded arm elevation. McQuade and Smidt (1) have demonstrated that during the exercise performance of light loaded and heavy loaded arm elevation the scapulohumeral rhythm ratio changes from a 2:1 ratio to approximately 3:1 and 4.5:1 respectively. From my perspective this adjustment in the scapulohumeral rhythm ratio is reasonable as during loaded arm elevation an increased emphasis is likely placed upon the scapula in its role as the platform of stability for appropriate gleno‐humeral mobility to take place. The incorporation of an exercise load results in a change in the scapula’s relationship (a change in the proportion of upward rotation) with the humerous during arm elevation including the positioning of the acromion. This alteration in shoulder kinematics raises a concern for potential sub‐acromial impingement due the rotator cuff possibly not clearing the acromion. Therefore it is feasible for prudence to take place in regard to the patient’s program design. This includes the importance for incorporating peri‐scapula strengthening exercises as well as an appropriate volume (i.e. exercises and exercise repetitions) of work prescribed. This realization of critical thinking will assist in a positive progression of your patient’s rehabilitation as well as a successful outcome.
1. McQuade KJ, Smidt GL, “Dynamic scapulohumeral rhythm: the effects of external resistance during elevation of the arm in the scapula plane”. J Ortho Sports Phys Ther 27:125 – 133, 1998.