Fracture of the first rib usually results from high-impact, direct trauma. Stress fractures are less common and are associated with minimal morbidity. The case of a patient with a stress fracture resulting from the use of an exercise machine is reported. Previous reports have attributed stress and fatigue fractures of the first rib to the forces exerted by the scalene muscles. A new pathophysiologic mechanism involving the serratus anterior muscle is introduced and is supported by T2 relaxation times from magnetic resonance imaging. Stress and fatigue fractures of the first rib have minimal complications. An aggressive diagnostic evaluation of first rib fractures occurring by this mechanism is not warranted.