3D analysis of the forearm rotational efficiency variation in humans

Pronosupination is a component of the hominoid orthograde corporal plane that enables primates to execute efficient and sure locomotion in their habitat and is an essential movement for the development of manipulative capacities. We analyze human variability in the rotational efficiency of the pronator teres muscle by applying the biomechanical model created by Galtés et al. (Am J Phys Anthropol 2008; 135:293-300; Am J Phys Anthropol 2009a; 140:589-594) to skeletal remains of a human sample (N = 29) and three nonhuman hominoid specimens (chimpanzee, gorilla, and orangutan) by means of 3D technology. We aim to examine whether there is a distinctive human pattern of rotational efficiency and determine which structural features of the upper-limb bones have the greatest influence on the determination of rotational efficiency. Our results show that the human pattern differs from efficiencies observed in nonhuman hominoids, which may be interpreted in the light of morphofunctional adaptations. We identify medial epicondylar form as the key structure of the upper-limb bones for the determination of the rotational efficiency of the forearm. Results indicate that the more medially projected epicondyle of nonhuman hominoids relative to humans leads to higher values of maximum rotational efficiency. Moreover, the orientation of the medial epicondyle determines the pronounced differences in the position of the maximum efficiencies in the pronosupination range between humans and the studied nonhuman hominoids. Proximodistal orientation of the medial epicondyle is suggested to be a more appropriate feature for distinguishing between humans and nonhuman hominoids than anteroposterior orientation and, therefore, for inferring behavioral aspects from skeletal remains and fossils of primate upper-limb bones.