When I asked a well-known actor this same question he told me that the "leg" referred to was the rope which opens the curtains and that to "break a leg" was to have so many curtain calls that the rope broke. It is the accepted way of wishing an actor "good luck"
The wings or curtains on the sides of the stage were once refered to as legs.
so to break a leg or rather to break out of the legs
was a saying of good luck to make an impression