Anatomy of the Lumbrical Muscles: Implications for Mechanical Advantage

Background: The lumbrical muscles comprise 4 intrinsic muscles of the hand and are involved in flexion of the metacarpophalangeal joint (MCPJ) and extension of the proximal interphalangeal and distal interphalangeal joints. The purpose of this study was to investigate the anatomical mechanics of the lumbrical muscles of the index, middle, ring, and small fingers. Methods: We evaluated 25 cadaver arms and measured the distance between the MCPJ and fingertip, the distance between the MCPJ and lumbrical muscle insertion, and the distance between the MCPJ and the most proximal lumbrical muscle origin. With these measurements we calculated the needed force, insertion ratio (length of the proximal, middle, and distal phalanx divided by the MCPJ to insertion distance), and lumbrical muscle length. Results: We found that the force was significantly different between all fingers, except for the comparison of the index and ring finger ( P = .34). In addition, we found that muscle length was significantly different between most the fingers, except for the comparison between the index and middle fingers ( P = .24), and index and ring fingers ( P = .20). There was no significant difference in insertion ratio. Conclusions: Our study suggests that the anatomical mechanics for the motor function of the lumbrical muscles are similar in all fingers. This could further imply that movements are equally precise in all fingers resulting in coordination with one another and, therefore, adequate hand function. Level of evidence: IV.