That's what I was thinking I'd heard too, that there were points of physics involved, in addition to connective tissue and such. I wonder what competitive advantage came from us becoming weaker. Perhaps the fine movements of the hands meant a need for sacrificing strength. Who knows.
The chimpanzee distal humerus(elbow) contrasts with the human. The human lacks the robust lateral supracondylar ridge, a high and robust lateral epicondyle, and the steep, sharp, lateral margin of the olecranon fossa . The chimpanzee forearm is relatively long in comparison to humans . Chimp radius and ulna are more curved than in humans and the chimpanzee distal radius has a radiocarpal joint surface that diverges medially. The major differences between human and chimpanzee limbs are contrasts in relative proportion. Chimpanzees have large powerful arms, slightly longer than their very short legs. Human arms are about 70% as long as human legs.
Scroll down approx 2/3 of length of page to 'Appendix B'.
Chimps can lock their elbow joint; humans can't.