Anatomical and molecular analyses of the deltoid muscle in chimpanzees (Pan troglodytes) and modern humans (Homo sapiens): Similarities and differences due to the uses of the upper extremity

In the deltoid muscles of Pan troglodytes and Homo sapiens, we have analyzed the muscle architecture and the expression of the myosin heavy chain (MHC) isoforms. Our aim was to identify differences between the two species that could be related to their different uses of the upper limb. The deltoid muscle of six adult Pan troglodytes and six adult Homo sapiens were dissected. The muscle fascicle length (MFL) and the physiological cross‐sectional area (PCSA) of each muscle were calculated in absolute and normalized values. The expression pattern of the MHC‐I, MHC‐IIa and MHC‐IIx isoforms was analyzed in the same muscles by real‐time polymerase chain reaction. Only the acromial deltoid (AD) presented significant architectural differences between the two species, with higher MFL values in humans and higher PCSA values in chimpanzees. No significant differences in the expression pattern of the MHC isoforms were identified. The higher PCSA values in the AD of Pan troglodytes indicate a greater capacity of force generation in chimpanzees than in humans, which may be related to a greater use of the upper limb in locomotion, specifically in arboreal locomotion like vertical climbing. The functional differences between chimpanzees and humans in the deltoid muscle are more related to muscle architecture than to a differential expression of MHC isoforms. Chimpanzees use the upper limb in different forms of terrestrial and arboreal locomotion, while humans use it preferentially in manipulative tasks. These functional differences translate into changes in the architecture of the deltoid muscle, mainly affecting its acromial portion. However, the different uses of the upper limb that exist between these two species of hominoid primates do not translate into changes in the expression patterns of MHC isoforms in the deltoid muscle.