Structural and molecular study of the supraspinatus muscle of modern humans (Homo sapiens ) and common chimpanzees (Pan troglodytes )

Objectives To analyze the muscle architecture and the expression pattern of the myosin heavy chain (MyHC) isoforms in the supraspinatus of Pan troglodytes and Homo sapiens in order to identify differences related to their different types of locomotion. Materials and methods We have analyzed nine supraspinatus muscles of Pan troglodytes and ten of Homo sapiens. For each sample, we have recorded the muscle fascicle length (MFL), the pennation angle, and the physiological cross?sectional area (PCSA). In the same samples, by real?time quantitative polymerase chain reaction, we have assessed the percentages of expression of the MyHC?I, MyHC?IIa, and MyHC?IIx isoforms. Results The mean MFL of the supraspinatus was longer (p?=?0.001) and the PCSA was lower (p?<?0.001) in Homo sapiens than in Pan troglodytes. Although the percentage of expression of MyHC?IIa was lower in Homo sapiens than in Pan troglodytes (p?=?0.035), the combination of MyHC?IIa and MyHC?IIx was expressed at a similar percentage in the two species. Discussion The longer MFL in the human supraspinatus is associated with a faster contractile velocity, which reflects the primary function of the upper limbs in Homo sapiens—the precise manipulation of objects—an adaptation to bipedal locomotion. In contrast, the larger PCSA in Pan troglodytes is related to the important role of the supraspinatus in stabilizing the glenohumeral joint during the support phase of knuckle?walking. These functional differences of the supraspinatus in the two species are not reflected in differences in the expression of the MyHC isoforms