Humans often use the fist as a weapon when fighting. Previous work has shown that forming a buttressed fist decreases the risk of injury that may occur when striking an opponent. Sexual dimorphism can evolve when enhancement of specific musculoskeletal traits improve the effectiveness of using a weapon. We tested the hypothesis that selection on male fighting performance in humans is associated with sexual dimorphism in the musculoskeletal system that powers using the fist as a weapon. We measured arm cranking power output in males and females using an inertial load ergometer. We used forward arm cranking as a proxy for the power production component of striking with a fist and backward arm cranking as an unselected control. Our results show pronounced male-biased sexual dimorphism in power production for protracting the arm to propel the fist forward. Sexual dimorphism was greater in forward arm cranking than backward arm cranking, supporting the hypothesis that humans exhibit male-biased sexual dimorphism in the components of the musculoskeletal system that provide the power for throwing a punch. We also measured overhead pulling force in order to test an alternative hypothesis that sexual dimorphism in the upper body of humans is a result of selection on male overhead throwing ability. We found weaker support for this hypothesis, with less pronounced sexual dimorphism in overhead arm pulling force. These results provide further support for the hypothesis that sexual selection on male aggressive performance has played a role in the evolution of the human musculoskeletal system and the evolution of sexual dimorphism in hominins.