CHANGES IN THE MOMENT ARMS OF THE ROTATOR CUFF AND DELTOID MUSCLES WITH ABDUCTION AND ROTATION

The behavior of the moment arms of the rotator cuff and deltoid muscle s was studied during simple and combined movements of abduction and ro tation about the glenohumeral joint. This was done by experimental mea surement of excursions of the muscles in an in vitro cadaver model and by use of a multiple-regression analysis to delineate the changes in the moment arms as a function of abduction and rotation. The results d emonstrated the potential of some rotator cuff muscles to contribute t o both abduction and rotation, the sensitivity of the abductor moment- arm lengths to internal and external rotation and of the rotator momen t-arm lengths to the degree of abduction, and the capacity of the abdu ctor moment-arm lengths of the deltoid to increase with increasing abd uction. Characterization of this behavior resulted in an increased und erstanding of the complex role of the rotator cuff and deltoid muscles about the glenohumeral joint and provided quantitative descriptions o f functional relationships. CLINICAL RELEVANCE: This study demonstrate s the capacity of the infraspinatus and subscapularis muscles to contr ibute not only to external and internal rotation, respectively, but al so to elevation of the arm in the plane of the scapula, a role for whi ch these muscles have been given little or no consideration. Furthermo re, it demonstrates that the contribution of the infraspinatus to abdu ction is enhanced with internal rotation while that of the subscapular is is enhanced with external rotation. Thus, dysfunction of the supras pinatus muscle need not preclude good elevation of the arm, and rehabi litation to reprogram and strengthen the remaining muscles becomes an important consideration.