In recent years, organized distance races have reported increases in children and young adult participants. In addition, female high school athletes generally experience higher rates of injury, including those due to overuse and specialization. These early injuries can lead to a higher likelihood of future injuries, growth plate disruption, and psychological outcomes like burnout. However, there is no general consensus among coaches, physicians, or athletic bodies about safe cumulative running loads at younger ages.

The purpose of this study was to investigate how kinematics, kinetics, and muscle activations changed with fatigue in a group of young female distance runners. Motion and ground reaction force data was collected before and after a 5-kilometer run at the subjects’ personal best pace for eleven healthy girls aged 8-17. The resultant data was processed and characteristics such as joint angles, moments, and powers, ground reaction forces, and muscle forces were compared for pre and post run, as well as for the younger runners compared to the older runners. Ankle joint mechanics were most significantly altered by fatigue, and knee kinetic changes were most dependent on the runners’ age. In addition, knee flexor forces increased and extensor forces decreased with fatigue, while changes to muscle forces around the hip and ankle were more dependent on the age of the runner. These results suggest that performance and injury avoidance in these young runners can be aided by strength programs including the involved muscles to avoid imbalances.