While first rib fractures typically occur from direct, traumatic injuries and have high rates of serious complication, less commonly, non-traumatic first rib fractures are encountered in young athletes. This paper presents the cases of a teenage basketball player with a first rib fracture after stretching, and that of a teenage cheerleader with a first rib fracture from straining to lift another cheerleader. Both patients became asymptomatic following conservative treatment and returned to sports. These cases were then evaluated next to the existing literature. The mechanism for these non-traumatic fractures is thought to be due to sudden contractions of opposing muscle forces on developing bone. Because of the biomechanics of the first rib's opposing muscle attachments, first rib fractures should be considered in patients who report pain deep to the medial clavicle, even without a history of direct, severe trauma.