Morphological Change in the Biceps Brachii Muscles during Shoulder Rotation: A Cadaver Study

In the literature, the passive movement of the biceps brachii during shoulder motion is unclear. This study investigated the passive movement of the long head (LHBB) and short head (SHBB) of the biceps brachii during shoulder rotation. We included 18 shoulders from 15 cadavers. At three shoulder positions (internal rotation [IR]20°, 0°, external rotation [ER]20°), the three sides of the triangle consisting of the confluence of the SHBB and LHBB, coracoid processes, and lesser tubercles were measured. The confluence angle of the LHBB and SHBB (Angle A) was calculated using the cosine theorem. The mean values for Angle A and standardized three sides of the triangle were statistically compared among the three shoulder positions. Angle A values were 13.5° ± 7.6° at IR20°, 15.2° ± 5.8° at 0°, and 15.7° ± 4.3° at ER20° with no significant difference (p = 0.096). The lengths of all three sides and Angle A were the smallest at IR20° and highest at ER20°. All the three sides revealed significant differences among the three shoulder positions (p < 0.0001). These results imply that morphological changes of the biceps brachii occur during shoulder rotation. These morphological changes in biceps brachii may contribute to shoulder joint stabilization by adjusting the forces the act on the tendons of both heads.