Adult Arm Brachial Plexus British Medical Research Council muscle strength scale Female Humans Male Muscle Strength Muscle, Skeletal Nerve Transfer Nerve grafts Nerve transfer Oberlin technique Recovery of Function Retrospective Studies Roots Ulnar Nerve Upper brachial plexus palsy adult article biceps brachii muscle brachial plexus injury clinical article elbow flexion female follow up human male muscle function muscle graft