If true, it's probably because humans have mostly slow twitch muscle fibers (e.g., great for endurance and manual dexterity) while great apes are mostly fast twitch muscle fibers (good for raw strength). That's why a chimpanzee, who is much smaller than a person, can perform feats pf strength that would embarrass most strong-man competitors. OTOH, humans evolved to run; the kind of long-distance running that some people do for fun would kill other primates.