PURPOSE: This study evaluated the effectiveness of plyometric compared to resistance training on the magnitude of hamstring and quadriceps activation ratios during movements that are similar to those that cause anterior cruciate ligament (ACL) injuries. METHODS: Subjects were randomly assigned to a plyometric training (P), resistance training (RT), or a non-training control (C) group. Subjects in the P and RT groups participated in a 6 week training program. Plyometric program was periodized from 100 foot contacts to 60, and intensity was increased according to research quantifying plyometric intensity. Resistance program was periodized with sets of 12, 9, and 6 repetitions with intensity ranging from 74-85% of subjects estimated 1 RM. Resistance training exercises included deadlifts and variations therin, Russian curls, and back squats. Pre- and post-testing sessions consisted of 2 repetitions each of maximum voluntary isometric contraction's (MVIC) for the hamstrings and quadriceps, 2 repetitions each of the drop jump and sprint and cut at a 45 degree angle (cut), a maximum countermovement jump test, and a 6 RM back squat test. Electromyography (EMG) was used to quantify rectus femoris (RF), vastus lateralis (VL), vastus medialis (VM), lateral hamstring (LH) and medial hamstrings (MH) activation prior to and after foot contact for the drop jump and cut, and normalized to each subject's hamstring and quadriceps MVIC. Hamstrings to quadriceps activation ratios (H:Q) were calculated from the collective average of the hamstring muscles divided by the collective average of the quadriceps muscles EMG activity. Difference scores were calculated as the change in H:Q from pre- to post-testing sessions. A one-way ANOVA was used to compare H:Q difference scores between the P, RT, and C groups. Significant main effects were further analyzed using Bonferroni adjusted pair-wise comparisons to assess the differences between groups. RESULTS: RESULTS of the ANOVA revealed significant changes in H:Q for drop jump post-contact (p = 0.009), cut pre-contact (p = 0.001), and cut post-contact (p = 0.010). Post-hoc analysis are presented in table 1 and revealed that the RT group displayed greater increases in H:Q from pre- to post-training, compared to P and C groups for all of the aforementioned variables. No significant differences in H:Q were found between the P and C groups. Conclusion: This study reveals that 6 weeks of resistance training had a significant effect on H:Q prior to and after movements that are thought to cause ACL injury, whereas plyometric training did not improve H:Q. The increase in H:Q following the resistance training may have a positive effect on limiting anterior tibial translation and dynamic stabilization of the knee, thus decreasing the risk for ACL injury. Practical Application: Resistance training programs should include hamstring based exercises in order to improve H:Q activation ratios and potentially reduce the risk for ACL injury.
(C) 2011 National Strength and Conditioning Association