sentences:  One theory for this is the fact that each of the two heads are innervated by different branches of the sciatic nerve.In states of fatigue or when the muscle is not fully warmed up, uncoordinated firing of the nerves may cause the muscle to contract inappropriately during movement, leading to injury.Biceps femoris tendon avulsion may also be associated with an avulsion fracture which occurs when a piece of the bone is pulled away with the tendon, during forceful contraction.