Structural and molecular analysis of elbow flexor muscles in modern humans and common chimpanzees

Abstract

In the present study we calculated the muscle fascicle length (MFL) and the physiological cross-sectional area (PCSA) in the elbow flexor muscles of five Homo sapiens and five Pan troglodytes. We also assessed the expression of the myosin heavy chain (MyHC) isoforms with real-time quantitative polymerase chain reaction in the same muscles and, finally, we analyzed the muscle insertion sites in the proximal epiphyses of the ulna and the radius with 3D geometric morphometrics. Our main objective is to identify quantitative differences in the elbow flexor muscles of Homo sapiens and Pan troglodytes in terms of muscle architecture, expression of MyHC isoforms, and bone insertion sites and to explore the association of these differences with the types of locomotion developed by these two species of hominoid primates. We observed that the elbow flexor muscles had a greater PCSA and a higher expression of the fast MyHC-II isoforms in Pan troglodytes, while they had a greater MFL and a higher expression of the slow MyHC-I isoform in Homo sapiens. The insertion site of the brachialis was larger in Pan troglodytes, while that of the biceps brachii was larger in Homo sapiens. Our findings may be related to the greater capacity of force generation in the elbow flexor muscles of Pan troglodytes, as an adaptation to vertical climbing, and to the greater contraction speed and resistance to fatigue of the muscles in Homo sapiens, as an adaptation to the manipulative function of the upper extremities.