Background: The effects of training on the biomechanical risk factors of ACL injuries have been investigated, but no research has studied the effects of detraining on the knee biomechanics during sport-related movement.

Hypothesis: After one-month detraining, subjects would demonstrate greater biomechanical risk factors associated with ACL injuries.

Study Design: Longitudinal pre-post intervention study.

Methods: Twelve NCAA Division I female volleyball players participated in two stop-jump tests before and after a one-month season interval. Knee kinematics, kinetics, and electromyographic activity were assessed during the tasks.

Results: Subjects demonstrated significantly lower jump height, smaller initial knee flexion, smaller maximum knee flexion angle, and smaller pre-landing biceps femoris EMG at the time of post-detraining compared with pre-detraining.

Conclusion and Clinical Relevance: The decreased biceps femoris muscle activation could be the cause of the decreased initial knee flexion angle which consequently resulted in a decreased maximum knee flexion angle. The decreased knee flexion angle indicates a possible increased strain on the ACL and thus an increased risk for ACL injury. Proper neuromuscular training programs should be implemented for highly trained volleyball players after detraining for preventing ACL injuries.