Analysis of the previous experimental studies showed that chimps, on average, outperform humans by a factor of approximately 1. The computer models, which combined the experimental data with the simulations, showed that the maximum force and power output of chimp muscles is 1. Chimpanzee muscle is composed of approximately 67 percent fast-twitch fibers, compared to about 40 percent in humans.
If you go back and look at all the data in those earlier studies, in many cases humans pull a similar amount of mass or jump with similar power as chimpanzees in absolute terms.
But humans also tend to be bigger than chimpanzees in these studies, so we account for this difference by dividing force or power by body mass. It also turns out that the secret to their superior strength isn't stronger muscle fibres, as suspected, but a specific ratio of different kinds of muscle tissues, providing insight into how our respective bodies evolved.
Since the s, the notion of chimpanzees possessing phenomenal strength has been a feature of pop-culture and a common subject for study in biology. A number of studies across the decades have suggested that pound-for-pound, chimpanzees could be as much as 3 to nearly 5 times stronger than a strapping human, or as little as 2. Even recent conservative calculations place their relative strength at around double our own, but most of the studies have been based on a chimp trying to move a heavy mass, and then having a human do the same thing.
This makes it hard to separate the effects of muscle arrangement, the contributions of tendons, or the effect of overall body shape from the actual strength of the muscles. Fast-twitch fibres are more powerful , but use more energy and become fatigued faster. This adds to the evidence that walking is considerably more energy-costly for chimps than for people. The results fit neatly with the idea that early humans evolved to walk or run long distances.
It seems that we sacrificed some strength for greater endurance. An earlier study found that our jaw muscles are particularly weak , which may have helped our brains grow larger. But it may have been fuelled by a study that claimed one chimp could pull nine times its own body weight. The only animal the researchers could find which mirrored the slow-twitch fibre pattern seen in humans was the slow loris - a sluggish nocturnal primate indigenous to southern Asia.
To the scientists, this was something that probably evolved in the lineage leading to humans after its divergence from the ancestral line leading to chimpanzees. The shorter muscle fibres and greater percentage of slow-twitch fibres in humans may have enhanced our endurance capabilities. These changes may coincide with evolutionary shifts in human locomotion, as human ancestors became better at upright walking and were required to travel longer distances.
But if this is true, it remains unclear why the dominance of slow-twitch fibres extends to the upper body as well as the lower. It may be that the distribution of different muscle types across the body is dialled up or down by the regulation of different genes. But Dr O'Neill says that only more research can answer that question.
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