Though they might not seem like much, chimps are stronger than humans — pound-for-pound of course. Actually, until not too long ago, chimps were thought of as ‘superstrong’, up to five times stronger than a human by some estimates. Recent work suggests there are only modest differences with humans in terms of sheer strength but our closest relatives still score better than humans on several measures. A new study headed by Matthew O’Neill, an anatomy and evolution researcher at the University of Arizona College of Medicine in Phoenix, reveals why — chimps have the most fast-twitch fibers.
Humans — the weakest of all great apes
The team performed biopsies on thigh and calf muscles collected from three anesthetized chimps housed at the State University of New York at Stony Brook. The samples were painstakingly separated into individual fibers then stimulated so the force they generate could be measured. This initial reading showed that the muscle output was about the same with that of humans, at an individual muscle fiber basis.
Another analysis was performed on muscle tissue harvested from the pelvic and hind limb muscles of three chimpanzee cadavers. Using a technique called gel electrophoresis, the team broke down the muscles into individuals muscle fibers and, again, compared the results with human muscle fiber data. They found was there was nothing special about chimp muscle. “Chimpanzee muscle is really no different than human muscle in terms of the force that individual fibers exert,” says O’Neill.
The analysis of the muscle fibers from the chimp cadavers, however, offered some clues as to why chimps are stronger than humans. Two-thirds of their muscle consists of fast-twitch fibers, whereas more than half of human fibers are slow-twitch. These are the two types of muscle fiber and fast-twitch fibers (myosin heavy chain II) contract very fast and generate more force in quick bursts than slow-twitch fibers. The downside is these fibers fatigue more quickly than MHC I. Chimps also seem to have longer fibers on average, which also enhances their strength.
“Our work is the first detailed study of the biology and mechanics of chimpanzee muscle tissue,” O’Neill told Gizmodo. “Our results show that the main difference between chimpanzee and human muscle is in fiber distribution, with chimpanzees having a much higher fraction of fast fibers than humans, on average,” adding that “all of our measurements of chimpanzee muscle are new.”
Later, data was run through a computer program that simulated the virtual muscles of human and chimps based on fiber composition. The model revealed chimp muscle is about 1.35 times more powerful than the human variety, as reported in the journal Proceedings of the National Academy of Sciences. But because humans are much heavier than a chimp, it’s safe to say that in absolute terms a typical human is more powerful than a typical chimp. There goes this myth, too.
” 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,” the authors wrote.
O’Neill says it’s likely other apes have similar muscle strength to chimpanzees. Humans seem to be the odd ones. Indeed, when the team compared the muscle fiber in various mammals like mice, cats, dogs, horses or macaques, they found that only two animals had more slow-twitch fibers: the lethargic slow loris and humans.
Fast-twitch fibers give mammals a competitive advantage when it comes to doing high-intensity tasks like climbing a tree or lifting a heavy bolder. We’re not feeble, though. Because we humans have more slow-twitch fibers, we’re better suited for endurance tasks like distance running, and during mankind’s great exodus out of Africa this certainly helped. Not to mention that human hunters could simply tire their pay around while hunting. Another benefit of slow-twitch fibers is they consume less metabolic energy, freeing energy that enabled humans’ bigger brains. Without this feature, you could say humans might have never come to conquer the world.
“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,” the authors concluded.
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