Three‐dimensional functional capacity of the intrinsic hand musculature in humans and chimpanzees

Abstract

Accurately interpreting the functional significance of variation in hand morphology has been central to discussions of the evolution of in hominoids, especially humans. Although there is an extensive literature on bony morphology, muscle size and strength, kinematics, and use of the hand, to date it has not been possible to visualize and quantify the mechanics of the of the hand in situ.This study compared the intrinsic hand muscles of Homo sapiens and Pan troglodytes in 3D. Each specimen was fixed, stained with Lugol’s iodine, and microCT scanned [<61 µm resolution]. Bones, muscles and the flexor retinaculum were segmented using Avizo. Avizo Xfiber was used to visualize and quantify fascicle orientation within muscles, which permits quantification of both whole muscle and within muscle fascicle orientations. Fascicle length, pennation angles and muscle volume were used to calculate PCSA and estimate force output of each muscle. Together, these data are used to compare functional capability of muscles individually and in aggregate.Data reveal differences in force‐generating capacity and force orientations of muscles, and with finer‐scale variation of within‐muscle fascicle orientations. The most significant differences were found in the adductor pollicis and opponens pollicis, which are larger and more transversely oriented in humans than in chimpanzees. Data also reveal differences in other thenar and hypothenar muscles, affecting the capacity to effect different grips.Our approach and results highlight the importance of evaluating of the musculoskeletal system of the hand in 3D which can then be used to compare to bone form across hominoids.