Image copyright AFP Image caption Chimps do appear to be stronger than us, but not by as much as we often think

The greater strength of chimpanzees, relative to humans, may have been explained by American scientists.

A study suggests the difference is mostly due to a higher proportion in chimps of a muscle fibre type involved in powerful, rapid movements.

The findings do not support previous work suggesting mechanical aspects of chimp muscles are responsible.

But the difference in chimp-human muscle performance is more modest than sometimes depicted in popular culture.

In the 1920s, anecdotal evidence along with investigations by the biologist John Bauman, helped feed a perception that chimps were between four and eight times stronger than an adult human.

But subsequent studies failed to replicate these figures, as later researchers found that chimps did not greatly outperform adult males when given physical tasks.

Writing in PNAS journal, Dr Matthew C O'Neill, from the University of Arizona College of Medicine-Phoenix, and colleagues reviewed the literature on chimp muscle performance and found that, on average, they are 1.5 times more powerful than humans in pulling and jumping tasks.

Puny human?

"My sense of it was there had not been a critical review of all the experiments that had been done up until our study," Dr O'Neill told BBC News.

"It was something we needed to do before getting into the substance of our paper."

According to ideas put forward in previous work, the difference might be accounted for if chimpanzee muscles were able to generate more force per area, or, alternatively, if chimp muscle was able to shorten faster than human muscle - helping increase its power output.

Dr O'Neill and his colleagues set out to test these ideas and others, by directly measuring the properties of muscle fibres taken from chimps that had been frozen after death.

"We really wanted to get a handle on the basic properties of chimpanzee skeletal muscle - and find out whether they were different from those of human muscle," he explained.

Along with cardiac muscle and smooth muscle, skeletal muscle is one of the three main muscle types, and is mostly found attached to bones via the bundles of collagen known as tendons.

"What we found was that there was really no difference in the fundamental contractile properties of chimp muscle fibres and human muscle fibres, for any of the individual fibres," Dr O'Neill said.

However, they did find key differences in the length of the fibres - chimp muscle fibres tend to be longer than corresponding ones in humans - and in the distribution of different muscle fibre types.

Fast vs slow

Chimps possess about twice the amount of "fast-twitch" muscle fibre. This type of fibre contracts quickly and is useful for rapid movements such as sprinting. But fast-twitch fibres have a downside: they quickly tire.

By contrast, corresponding human muscles are dominated by "slow-twitch" muscle fibres, which contract more slowly, but keep going for longer. They are useful for activities that require endurance.

Computer simulations suggest that these differences increase the maximum dynamic force and power-producing capacity of chimp skeletal muscle by a factor of 1.35 compared with a human muscle of a similar size. The 1.35 figure corresponds well with the 1.5 times figure reached by reviewing the scientific literature.

The team members suggest this may reflect chimps' greater reliance on tree climbing and suspension for their survival.

In fact, the dominance of fast-twitch fibres appears to be a default setting in mammals, from mice to horses. 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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