Essentially. IIRC there are a couple of negative feedback loops, one local and one involving the brain, and they usually limit you to about a 1/3 duty cycle fo

Essentially. IIRC there are a couple of negative feedback loops, one local and one involving the brain, and they usually limit you to about a 1/3 duty cycle for each muscle fiber. There are certainly ways that ratio can be increased, just yelling can increase the maximum force you can consciously exert by about 15%. I have no idea what the maximum is, but I'd imagine that going much further than what you're normally limited to drastically increases your odds of tearing something during exertion. The reason chimpanzees can exert more force is anchoring, their muscles are anchored to the bone further form the joint so they have better mechanical advantage. A human has a much wider range of movement, though, and I believe we better maintain force under movement. So which a chimp might be able to lift themselves more easily, a human ends up being able to throw a rock or spear much further. EDIT: Also, ekianjo is right that lifting cars is an exaggeration.