3D Analysis of the Forearm Rotational Efficiency Variation in Humans

Abstract

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 Galt es 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 ¼ 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.

Entheseal Changes and Functional Implications of the Humeral Medial Epicondyle

“…This form of locomotion explains the ape body plane that enabled the acquisition of bipedalism, and many aspects of forelimb anatomy can be explained as adaptations to vertical climbing (Fleagle et al, 1981). Moreover, the orientation of the medial epicondyle in nonhuman primates is related to other functional aspects (Fleagle, 1988;Lague & Jungers, 1996;Seiffert et al, 2000;Ciochon & Gunnell, 2004;MacPhee & Meldrum, 2006), and its form is suggested to be partially determined by the locomotor behaviour of each species (Ibáñez-Gimeno et al, 2012).…”

“…Only the best preserved extremity of each individual was included in the study, and the sample contained right and left specimens in similar proportions. As medial epicondylar orientation shows no differences between sexes (Ibáñez‐Gimeno et al , ), both male and female individuals were included in the sample. In total, 19 men (10 young and 9 mature adults) and 11 women (eight young and three mature adults) were analysed.…”

“…Point O′ is placed taking into account the distance (rotational radius of the pronosupination movement), calculated as a function of (radial curvature). As described by Ibáñez‐Gimeno et al (), φ is measured from the resulting projection of plane P1 on P3, the latter being a plane parallel to P1 that passes through the point of maximum curvature of the radius or point A (distal attachment site of pronator teres muscle). Here, we represent φ on plane P1 for better visualisation (Figure (C)).…”

“…Medial epicondyle orientation was assessed with angle '. This angle is a variable used in the biomechanical model created by Galtés et al (2008) to calculate forearm rotational efficiency (Galtés et al, 2009a;Ibáñez-Gimeno et al, 2012). When analysing forearm rotational efficiency in extension of the elbow (180 ), ' is a proxy for the anteroposterior orientation or retroflexion of the epicondyle.…”

“…The humeral medial epicondyle is a morphological feature of the primate elbow that has been widely used to reconstruct the locomotor habits of extant and fossil primate taxa (Fleagle & Simons, 1982;Lague & Jungers, 1996;Bacon, 2000;Ciochon & Gunnell, 2004;MacPhee & Meldrum, 2006). Our recent work showed that an increase in size of the medial epicondyle enhances forearm rotational efficiency (Ibáñez-Gimeno et al, 2012). The orientation of the medial epicondyle determines in which position of the pronosupination range the forearm has the maximum efficiency.…”