The single-fiber and MHC datasets are then used to parameterize chimpanzee and human muscle models. We compare our chimpanzee data to similar data from humans and a wide range of other terrestrial mammals. In this study, we present direct measurements of single-fiber contractile properties and MHC isoform distributions of chimpanzee skeletal muscle to test these hypotheses. Yet, if one or more of these hypotheses are correct, it would indicate a significant (and previously unappreciated) evolutionary shift in the force and/or power-producing capabilities of skeletal muscle in either Pan or Homo since these two lineages diverged about 7–8 million years ago (Mya) ( 13). However, to date there have been no direct measurements of these parameters in the skeletal muscle of chimpanzees. Hypotheses for the muscular basis of the chimpanzee–human performance differential have included higher isometric force-producing capabilities ( 6 – 8, 11), faster maximum shortening velocities ( 7, 11), and/or a different distribution of myosin heavy chain (MHC) isoforms than human skeletal muscle ( 10, 11). A critical review of experiments (i.e., pulling and jumping tasks) carried out between 19 suggests that chimpanzee mass-specific muscular performance consistently exceeds that of humans, with a differential of about 1.5 times, on average ( SI Appendix, SI Discussion). This has led to the now long-standing proposal that chimpanzees are “super strong” compared with humans. Since at least the 1920s, both anecdotal reports and more controlled experiments have indicated that the strength of a chimpanzee can exceed that of a human ( 6 – 12). Subsequent evolution in brain size ( 3) and cognition as well as advancements in tools and other material culture ( 4, 5) have reduced our strict dependence on muscular strength for survival and fitness. Whereas chimpanzees are proficient tree climbers and arborealists ( 1), our hominin ancestors gave up a reliance on the forest canopy after the emergence of the genus Homo ( 2). Modern humans-with some exceptions-are often characterized as a weak and unathletic species compared with our closest living relatives, the chimpanzees. We propose that the hominin lineage experienced a decline in maximum dynamic force and power output during the past 7–8 million years in response to selection for repetitive, low-cost contractile behavior. Thus, the superior mass-specific muscular performance of chimpanzees does not stem from differences in isometric force-generating capabilities or maximum shortening velocities-as has long been suggested-but rather is due in part to differences in MHC isoform content and fiber length. Computer simulations of species-specific whole-muscle models indicate that maximum dynamic force and power output is 1.35 times higher in a chimpanzee muscle than a human muscle of similar size. Unlike humans, chimpanzee muscle is composed of ∼67% fast-twitch fibers (MHC IIa+IId). Here, we show that chimpanzee muscle is similar to human muscle in its single-fiber contractile properties, but exhibits a much higher fraction of MHC II isoforms. Hypotheses for the muscular basis of this performance differential have included greater isometric force-generating capabilities, faster maximum shortening velocities, and/or a difference in myosin heavy chain (MHC) isoform content in chimpanzee relative to human skeletal muscle. A mix of anecdotal and more controlled studies provides some support for this view however, a critical review of available data suggests that chimpanzee mass-specific muscular performance is a more modest 1.5 times greater than humans on average. Since at least the 1920s, it has been reported that common chimpanzees ( Pan troglodytes) differ from humans in being capable of exceptional feats of “super strength,” both in the wild and in captive environments.
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