Why are chimpanzees so much stronger than us? Despite what historical accounts of their strength would have you believe, they aren’t actually that much more powerful than humans. But the small difference there is, which researchers have now measured, is related to how humans evolved and what makes our species special.

Scientists have analyzed the muscle makeup and strength of chimps and compared that to humans, finding that chimp muscles are about 35 percent stronger than ours, pound for pound. That has something to do with the types of muscle fibers. A study in the journal Proceedings of the National Academy of Sciences explains chimpanzees have more of the muscle fibers known as fast-twitch while humans have more slow-twitch fibers.

The former are quick and powerful but run out of gas quickly while the latter are built for endurance. One way to think about them is to compare sprinting and long-distance running: Sprinters go faster for shorter periods of time while distance runners move more slowly but can last longer.

Read: Can Wild Chimps Live as Long as Humans?

Chimp muscle fibers are also longer, but there are otherwise no real differences in muscle makeup between the species on a cellular level — it’s how everything comes together that counts.

Researcher Brian Umberger, an expert in musculoskeletal biomechanics from the University of Massachusetts Amherst, said to analyze the muscle fibers from the two species, the group hooked them up to a device that measures how much force they can generate. The results showed on a muscular level, chimps were 1.35 times stronger than humans. It’s a value that’s hard to put into real-world terms such as by describing a specific show of strength, like what size weights a chimpanzee could lift at the gym.

“It’s important for chimps because they live primarily in an arboreal environment, in the trees,” he told International Business Times. Their lifestyle requires them “to rapidly climb up trees, swing around in the branches.” In that respect, they benefit from their strength and from the fast-twitch fibers that give it to them.

That’s where the researchers’ analysis starts to touch human evolution.

“Our ancestors long ago gave up that reliance on existing in the trees,” Umberger said.

Throughout their history, human ancestors had to be able to cover long distances, finding food along the way, both as those prehistoric people expanded their range slowly on a more regional level and as they migrated more widely around the world. Such activities were better performed by muscles with endurance that “operate at a lower cost,” Umberger said, and now more than two-thirds of human muscle fibers are slow-twitch.

In this way, humans are the outlier. The researchers also reviewed scientific information about the muscle makeup of other animals, including other primates, and found that although there was some variation, the chimpanzees fit in with that group — other mammals predominantly have the fast-twitch muscle fibers. It suggests humans are the ones who during their evolution moved away from the muscle traits that were present in the last common ancestor we shared with chimpanzees and other modern primates. It didn’t happen overnight. Chimps and humans diverged several million years ago.

“Humans are rather unusual,” Umberger said.

Superchimp

The idea that chimpanzees are amazingly strong may have originated in the 1920s, the study noted, and persisted despite research showing only slight advantages over humans. There are historical anecdotes describing great power, and at least as early as the Roaring ‘20s, scientists have been testing it out.

Umberger describes these old experiments as typically involving a chimp and a human taking turns at pulling on a rope or a lever attached to some gauge to see who exerted more force. Often they concluded chimps were absurdly strong.

Still, they are indeed stronger than the people poking at them. This new research shows the muscles themselves are 35 percent stronger, and when you take the entire body as a whole, the chimp strength jumps up to 50 percent more powerful than humans.

Read: Chimps Have Their Own Cultural Traditions Too

That latter number is based on the team’s review of scientific literature and describes how the chimps’ skeletal system combines with the muscles — the bone joints give the muscles leverage and thus increase their power beyond what they could achieve on their own. It's another area where the apes have an advantage over humans although that advantage is still nowhere near the enormous gap that has been portrayed historically.

“Contrary to some long-standing hypotheses, evolution has not altered the basic force, velocity or power-producing capabilities of skeletal muscle cells to induce the marked differences between chimpanzees and humans in walking, running, climbing and throwing capabilities,” UMass Amherst said. “Instead, natural selection appears to have altered more global characteristics of muscle tissue, such as muscle fiber type distributions and muscle fiber lengths.”

Natural selection determining the course of human muscle evolution was exactly the point the researchers were trying to get at.

Although it is “certainly is a nice idea” to dispel a myth about an animal with whom we share the overwhelming majority of our DNA, the reason the team took on this project, to begin with, was to open a window into the human past and figure out how the species became what it is.

“Humans are really the unusual one here,” Umberger said. “It adds a really important piece to that story where there was a lot of speculation but little data.”