Effect of verbal instructions on muscle activity and risk of injury to the anterior cruciate ligament during landing.

BACKGROUND Minimising the likelihood of injury to the anterior cruciate ligament (ACL) during abrupt deceleration requires proper synchrony of the quadriceps and hamstring muscles. However, it is not known whether simple verbal instructions can alter landing muscle activity to protect the knee. OBJECTIVE To assess the efficacy of verbal instructions to alter landing muscle activity. METHODS Twenty four athletes landed abruptly in single limb stance. Sagittal plane motion was recorded with an optoelectric device, and ground reaction force and surface electromyographic data were recorded for the rectus femoris, vastus lateralis, biceps femoris, and semimembranosus muscles. Subjects performed 10 landings per condition: normal landing (N); repeat normal landing (R); landing after instruction to increase knee flexion (K); and landing after instruction to recruit hamstring muscles earlier (M). Muscle bursts immediately before landing were analysed relative to initial foot-ground contact (IC). RESULTS The K condition resulted in significantly (p</=0.05) greater knee flexion at IC compared with the other conditions. The M condition did not result in earlier hamstring muscle activity, but instead caused significantly (p</=0.05) earlier rectus femoris onset relative to IC, with a similar trend for the vastus lateralis. As these muscles are ACL antagonists, earlier onset times would be detrimental to the ACL. CONCLUSIONS Subjects successfully increased knee flexion during landing following the K condition instruction. However, further research is warranted to establish the efficacy of more extensive lower limb muscle retraining programmes to ensure landings that decrease susceptibility to ACL injury.