To elucidate anatomic basis of susceptibility for contracture of the subscapularis muscle in Erb's palsy of the brachial plexus, we semiquantitatively studied the spinal nerve origins of the subscapular nerves innervating the subscapularis, with special reference to the contribution of C7 innervation to the subscapularis. Thirty-three sides of formalin-fixed upper extremities were dissected to obtain the intact brachial plexus. After immersed in 10% acetic acid for 2 weeks, the upper and lower subscapular nerves innervating the whole subscapularis, were dissected retrogradely to verify their spinal nerve origins. The cross-sectional area by C7 innervation and that by the upper trunk innervation was calculated respectively to obtain the constituent percentage of different components in the upper and lower subscapular nerves. In the upper subscapular nerve, fascicles of C7 accounted for 0% (interquartile range, 0–1.1%) of cross-sectional area and those of the upper trunk, 100% (98.9–100%). In the lower subscapular nerve, fascicles of C7 accounted for 40.5% (23.5–47.5%) and those of the upper trunk, 59.5% (52.5–76.5%). In total, 18.6% (13.3–27.3%) of fascicles in the subscapular nerves innervating the subscapularis originated from C7, while 81.4% (72.7–86.7%) of those came from the upper trunk. It is confirmed that innervation of the subscapularis originates from more spinal cord segments than that of infraspinatus and teres minor, and this may be the main reason for which in Erb's palsy, functional recovery of the subscapularis is often faster than that of lateral rotators of the shoulder, resulting in medial rotation contracture of the shoulder. Clin. Anat., 2013. © 2012 Wiley Periodicals, Inc.