Despite considerable literature on the functional anatomy of the hominoid upper limb, there are no quantitative approaches relating to bone design and the resulting muscular-activity enhancement. The purpose of this study is to quantitatively analyze the relationship between the rotational efficiency of the pronator teres muscle and the design of the skeletal structures on which it acts. Using conventional scan images of a human forearm for three rotational positions, this study develops an original biomechanical model that defines rotational efficiency as a mathematical function expressing a geometrical relationship between the origin and insertion muscular sites. The results show that this parameter varies throughout the entire pronation range, being maximal when the forearm lies around its functional position. Moreover, the rotational-efficiency formula allows us to demonstrate, by several simulation conditions, that an improvement in pronation efficiency is derived from a large shaft radius curvature, a large humeral medial epicondyle, and a more proximal pronator teres radial attachment. The fact that forearm pronation efficiency can be inferred, even quantified, throughout the entire rotational range, by applying the biomechanical model developed here allows us to undertake anatomical approaches in the field of Evolutionary Anthropology, to interpret more precisely how skeletal design is related to upper-limb function in extant and fossil primate taxa. Am J Phys Anthropol, 2008. © 2007 Wiley-Liss, Inc.