Can lower limb muscles be retrained to decrease the risk of ACL injury?

Rupture of the anterior cruciate ligament (ACL) of the knee joint frequently occurs in sports that require the athlete to land or change direction suddenly. The structural integrity of this ligament during such maneuvers is heavily dependent upon coordinated quadriceps and hamstring muscle activation. Therefore, strategies used to recruit these muscles have a pivotal role in equipping the knee to withstand the high forces experienced during dynamic landing to reduce the risk of ACL injury. Despite extensive research pertaining to the ACL, no research has examined whether athletes can change the way they recruit their lower limb muscles to better protect the knee during dynamic landing. In Study 1, 24 skilled netball players performed 10 trials of a dynamic landing task, for each of four conditions: (i) normal landing (N); (ii) repeat normal landing (R); (iii) landing following an instruction to bend their knees more (K); and (iv) landing following an instruction to turn their hamstring muscles on earlier and more before foot-ground contact (M). During each trial the sagittal plane motion, ground reaction forces, and muscle activity for rectus femoris (RF), vastus lateralis (VL), semimembranosus (SM), and biceps femoris (BF) of the landing limb were recorded. Results showed that simply asking players to bend their knees more on landing (K condition) was somewhat effective in increasing knee flexion. However, players were unable to selectively recruit their hamstring muscles upon request during this abrupt landing task. It was concluded that more extensive muscle training was required to alter the subjects hamstring-quadriceps muscle recruitment patterns during landing. Therefore, the purpose of Study 2 was to assess the efficacy of an electromyographic biofeedback retraining program to alter hamstring muscle recruitment during landing. For Study 2, 28 netball players performed the same landing action as described in Study 1 for 10 trials before (PRE) and after (POST) a 6-week interval in which the 14 experimental athletes completed hamstring biofeedback training for three 30-minute sessions per week, while the control subjects maintained their normal playing routines. Results confirmed that the biofeedback training was ineffective in training the subjects to significantly alter the timing of their hamstring muscles. Further a priori analysis of the data using single-subject design (Study 3) revealed that, although individuals displayed a variety of changes in muscle recruitment patterns that were masked when only examining the experimental group mean data, the biofeedback program was still not successful in achieving earlier hamstring muscle recruitment. In conclusion, it is still unknown how to best train individuals to equip them to perform dynamic landings without sustaining ACL ruptures. Further research is required to elucidate benefits of verbal instructions in landing training, to understand whether electromyographic biofeedback training can be used to train muscle recruitment patterns during dynamic landings, and to ascertain whether combining group mean data and single-subject analysis gives a more complete picture of how a group of individuals responds to a training intervention. With further research attention, it is anticipated that the risk of ACL rupture may be reduced