Three-dimensional continuous muscle moment arm maps for the anatomical shoulder

A muscle's moment arm represents its mechanical advantage and indicates its role in joint actuation and rotational stability. The objective of this study was to use an ex-vivo simulator to map the moment arms of eight major shoulder muscles across a continuous range of motion. The three-dimensional moment arms for the deltoid (anterior, lateral, and posterior), subscapularis (inferior and superior), supraspinatus, infraspinatus, and teres minor were measured in eight specimens (57 ± 6 years) using the tendon excursion method. The anterior deltoid had a significantly larger elevation moment arm in anterior planes of elevation (p < 0.001) while the lateral deltoid had a significantly larger elevation moment arm in posterior planes (p < 0.001). The posterior deltoid was an antagonist to elevation with anterior arm orientations (p < 0.001). The supraspinatus had biphasic function; in anterior elevation planes it was a horizontal extensor and internal rotator but was a horizontal flexor and external rotator in posterior planes (p < 0.001). The infraspinatus and superior subscapularis were both arm elevators, but the infraspinatus was an external rotator and horizontal extensor while the superior subscapularis was an internal rotator and horizontal flexor. The inferior subscapularis was a horizontal flexor and internal rotator while the teres minor was an antagonist to elevation, horizontal extensor, and external rotator. Each muscle had a multifaceted function which changed significantly with arm orientation for all muscles except the inferior subscapularis. The muscle moment arms maps created in this study improve current understandings of the three-dimensional function of eight major muscles in the shoulder.