Abstract
The static implications of the fiber architecture, i. e., the orientation of a muscle's line of action, fiber bundle length and physiological cross-sectional area of the scapulohumeral muscles, were analyzed 3-dimensionally using five Japanese macaques. The results are as follows: 1. The teres major and deltoids, whose ∠ε (the angle formed between the humeral long axis Ax and CI, where C: geometrical center of humeral head; I: muscle insertion) is close to zero, work exclusively as a humeral circumducer irrespective of humeral orientational change. In contrast, in the remaining muscles whose ∠εis large, the actual humeral spatial orientation or ∠δ (the angle formed between Ax and Poic, the plane passing three points of O, I, and C; O; muscle origin) decides the action of a muscle. It was theoretically shown that at the humeral resting position the infraspinatus and subscapularis acts as a rotator, the supraspinatus as a circumducer, and the teres minor as an intermediator. 2. The muscle excursion range, and the certain limited rotatory range of OI about OC demarcate the potential maximum range of I (RmaxI) on Soc (the sphere centered at C with its axis passing O and C). Since Ax can rotate about CI, the potential maximum range of Ax (RHP) and the rotatory range of CI about Ax (RHR) are then geometrically given on Soc. Again, the potential maximum moment about C (MaxPot. |Nt|) is given as a function of I's site in RmaxI. In the muscles with a small ∠ε, RHP approximately overlaps RmaxI. Thus they are considered to have a higher maneuverability of Ax, with a value of MaxPot. |Nt| as an approximate function of the Ax' site. The knowledge obtained in the present study provides a theoretical basis for clearly defining how the shoulder musculoskeletal system of primates evolved through their arboreal lives.
