Background Previous studies have suggested that increased valgus knee motion and decreased hamstring/quadriceps (H/Q) ratios can be risk factors of anterior cruciate ligament (ACL) injury.

Objective The objective of this study was to evaluate the effects of an ACL injury prevention program on these risk factors.

Design Randomised controlled trial.

Setting The evaluation was performed in a rehabilitation laboratory.

Participants 23 healthy females with no history of leg injuries participated in this study. They were randomly allocated to the intervention group (13 subjects) and the control group (10 subjects).

Intervention An ACL injury prevention program modified from previous studies was used with the intervention group for 6 weeks.

Main outcome measurements All subjects performed a drop vertical jump from a 30 cm high box at pre and post training to assess knee alignment and the knee separation distances normalised by the hip separation distances (K/H ratio). Measurements were obtained from video data and these were used as parameters. Surface electromyography (EMG) data were analyzed to assess muscle activation of the vastus medialis, vastus lateralis, semitendinosus, and biceps femoris for 50 ms immediately after initial ground contact. Isokinetic peak torque of knee extension and flexion at 60°/s were measured to calculate H/Q ratios.

Results The K/H ratio and the H/Q ratio significantly increased in the intervention groups (K/H ratio: 44.7% to 56.2%; H/Q ratio: 47.8% to 51.2%; p<0.05). However, there was no significant difference in EMG activity. No parameters showed significant changes in the control group.

Conclusion This study suggests that the ACL injury prevention program is useful to decrease risk factors such as valgus knee motion and H/Q ratio.