The philosophy behind it is also beautiful -- for the parts of the body to mimic weapons of war:
The hands become the sword and dagger; the shins and forearms were hardened in training to act as armor against blows, and the elbow to fell opponents like a heavy mace or hammer; the legs and knees became the axe and staff. The body operated as one unit.
Source: https://en.wikipedia.org/wiki/Muay_Thai