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Weakness: it's what makes us human

Chimpanzees are about 1.35 times stronger than humans, according to a new study. But despite tales of superpower-like strength from chimps, this difference might be because humans are comparatively weak, not because our cousins are preternaturally strong.

Why it matters: The researchers think humans might be weaker than chimps because, as our ancestors became bipeds, our muscles were selected for chasing prey for long distances rather than quick feats of strength.

The authors found that human muscles are structurally different from chimpanzee muscles — and the muscles of most other primates. The chimpanzees studied had balanced amounts of slow-twitch and fast-twitch muscle fiber, which was consistent with past studies of primates. Humans, in contrast, contain have much more slow-twitch muscle, which is associated with endurance running, not strength.

How they did it: A review of studies comparing human and chimpanzee muscle strength found that on average, the studies showed chimps were only slightly stronger than humans. Researchers then studied muscle fibers in the lab to measure how much chimpanzee muscles could shorten and how much force they could exert, but found chimps and humans were similarly strong by this method. But when they looked at the length of the muscle fibers and the fibers they were composed of, chimps were better adapted for quick strength than humans were.

Appropriately, the slow loris is the only other animal studied so far with as much slow-twitch muscle fiber as humans.