Thirty cases of humeral shaft fracture sustained during arm wrestling were analyzed. Twenty eight were men and two were women. Eighteen were fractures of the right arm and 12 were of the left. The patients ranged in age from 15 to 60 years old, averaging 30 years. Although there was no prominent tendency for the injuries to occur at a certain phase of the match, they happened in all cases while full force was being exerted in an effort to win or to change the tide of the contest. All cases were spiral fractures, of which 23% presented with a medial butterfly fragment, and also 23% of which had radial nerve palsy. We infer that, through a shift in body weight and subsequent counterattack from the opponent, the shoulder internal rotator muscles suddenly changed from their maximum concentric contraction to eccentric contraction, resulting in development of an intense rotational force leading to the subsequent fracture. Therefore, this type of fracture may occur in anyone of any age engaging in this type of sport.