I am unsure of the actual mechanism of injury, but my CP was injured doing a running BHS about 6 months ago. I thought the arm was broken as she was hysterically crying and refused to move it, but we went to the ER and the x-ray was clean. She had some swelling and a slight bruise. She was able to move it that night, and was "normal" within 48 hours. Working theory is that it was probably a slight dislocation that she popped back in when she drew her arm up to her body immediately after she injured it. The only reason we think this is because of a) the clean x-ray, obviously b)the severe amount of pain she was in immediately following the injury and c)how quickly she recovered. She most likely would have taken a while longer to recover if it was a true sprain.