![]() ( C) The main effect of MHC isoform content on single-fiber V o n = 22 error bars, SD P value is the result of an ANOVA F (2,19) = 97.16. Paired comparisons indicate that the MHC I ( n = 31), IIa ( n = 15), and IId ( n = 9) P o samples all differ significantly from each other ( P < 0.05, Tukey’s honest significant difference tests). ( B) The main effect of MHC isoform content on single-fiber P o n = 55 error bars, SD P value is the result of an ANOVA F (2,52) = 21.20. Insets show a chimpanzee single muscle fiber as well as the identification of the fiber MHC isoform content using gel electrophoresis after P o and V o measurements. ( A) Chimpanzee single fibers were sampled from m. Given these cellular-level results, humans and chimpanzees can be expected to exhibit commonalities in the molecular properties that affect single-fiber performance, such as actin–myosin kinetics.įig. Indeed, the P o and V o of chimpanzee muscle are not significantly different from humans ( P > 0.05, one sample t test) and are generally consistent with expectations based on body size scaling ( Fig. These results, which are taken to be representative of limb and trunk skeletal muscle in general, indicate that chimpanzee muscle is similar to humans and other terrestrial mammals in its single-fiber contractile properties ( Fig. 1 and SI Appendix, SI Methods and Tables S1 and S2). The maximum isometric force ( P o) of chimpanzee skeletal muscle ranged from 96 kN⋅m −2 to 150 kN⋅m −2, and the maximum shortening velocity ( V o) ranged from 0.64 to 4.96 L⋅s −1, depending on MHC type ( Fig. ![]() ![]() Data were collected at 15 ☌ from fibers containing pure MHC I, IIa, and IId isoforms. Using an isolated muscle fiber preparation, we directly measured the maximum isometric force and maximum shortening velocity of the skeletal muscle of the common chimpanzee ( Pan troglodytes). 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). 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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