alright, so i'm going to nerd it up here a bit and try to answer this underlying question: why are chimps stronger than humans?
the primary reason for a chimp's relative upper body strength is in its skeletal anatomy. as an example, the chimp's elbow protrudes out quite a bit further than that of a human's. this allows the muscular connections of the chimps arms to connect further away from the elbow joint;s center of rotation, which allows the chimp to generate more torque than the typical human.
this might not make sense at first, but once you realize that nearly all human (and other primate) motions result by articulating joints about their centers of rotations (knee, hip, elbow, shoulder) then you realize that the forces we produce are really twisting forces...in other words, torques. since torque is defined as a force times a distance, it's easy to understand that applying a muscular force further away from a joint's center of rotation will result in greater torque. it's like the difference between using a long-handled crowbar and a short-handled crowbar to pry something. while you might be pushing against both with the same force, the long-handled crowbar will produce more torque and will be better at prying. this is precisely how the chimp produces more torque despite the fact that its muscles are the same size as ours.
