One of the most common low back injuries in adolescent baseball players is a pars stress fracture, also known as spondylolysis. A pars fracture is a stress injury to a small bridge of bone in the back of the spine. The important thing to understand is that these injuries rarely happen from one pitch. They’re stress injuries from thousands of high-velocity throws that create repetitive loading on the lumbar spine. Normally, bone adapts and repairs itself. But when stress accumulates faster than recovery can occur, a stress reaction can develop. If loading continues, that stress reaction can progress to a fracture. This is one reason pars fractures are so common in adolescent athletes. They’re often growing, throwing more, playing year-round, and recovering less. The research also suggests that hip mobility may play an important role. Several studies have found that athletes with pars fractures often demonstrate reduced hip internal rotation compared to healthy athletes. Why does ...