Roles of deltoid and rotator cuff muscles in shoulder elevation

OBJECTIVE: The objective of this study was to measure abduction moment arms of the supraspinatus, subscapularis, infraspinatus, and deltoid (anterior, middle, and posterior portions) muscles during humeral elevation in the scapular plane (abduction). DESIGN: Moment arms were measured by conducting an in vitro experiment. BACKGROUND: The moment arm of a muscle represents its mechanical advantage, which is an important determinant of muscle function. METHODS: Measurements were made on 10 fresh frozen cadaveric specimens. Tendon excursions were measured as the humerus was elevated in the plane of the scapula. The principle of virtual work was used to estimate the muscle moment arm of each muscle by computing the slope of the tendon excursion versus joint angle relationship. RESULTS: Moment arms were affected by joint angle in a non-linear fashion. The anterior deltoid, middle deltoid, subscapularis, and infraspinatus muscles had abduction moment arms throughout most of the range of motion studied. The posterior deltoid had an adduction moment arm. Internal and external humeral rotation affected the elevation moment arms of all six muscles. CONCLUSIONS: Abduction moment arm magnitudes of the muscles studies vary throughout the arc of elevation. This study was limited by considering broad muscles to have a single line of action. RELEVANCE: The positive elevation moment arms of the infraspinatus and subscapularis muscles indicate that they can elevate the arm in addition to acting as stabilizers. Thus this study suggests a biomechanical explanation for the clinical success of conservative treatment for rotator cuff tears.