How are arms different than legs?

Question

Ok, this is a bit of a tangent question, but it came up yesterday and I didn't know the answer: How are arms and legs defined physiologically? For example, we say humans have two arms and two legs, while cats have four legs, and some monkeys (appear) to have four arms (although I guess they could be legs too). It's really unclear to me how we make these distinctions. So: **What is the physiological definition of both "arm" and "leg"? How are they different?**

Answer 1

If you were to look at just humans, there are great differences between the limbs of the lower body (legs) compared to the upper body (arms). Being bipedal we have greater amount of muscle and movement based physiological adaptations of the lower body, compared to the upper body (less muscle, but greater development for refined movements). Size of phalanges and tarsals differs, mobility of digits differs, weight bearing and adaptive posture differs.