Improving the Q:H Strength Ratio in Women using Plyometric Exercises

Kavin Tsang and Angela DiPasquale, J Strength Cond Res. 2011 Oct; 25(10): 2740-5.

Since the initiation of Title IX in 1972 female participation in sports has increased. Over time the increased level of participation and level of competition has contributed to a larger incidence of injury among women, particularly injury to the anterior cruciate ligament (ACL). Several factors contributing to the increased rate of injury have been evaluated. A primary area of focus has been strength imbalances of the quadriceps and hamstring muscles measured as Q:H strength ratio. Previous literature has found that females demonstrate a lower Q:H strength ratio compared to males, and a lower Q:H strength ratio in the ACL injured leg compared to the uninjured leg. The purpose of this study was to evaluate the Q:H strength ratio in female subjects after 6 weeks of plyometric training compared to age matched healthy control subjects. Twenty-five female subjects participated in this study (11 plyometric, 14 control) consisting of isokinetic testing at baseline, 2, 4 and 6 weeks during the training program for knee flexion and extension at 60 and 120°/sec. The plyometric subjects participated in training 3 days a week for 6 weeks, involving 7 of 13 possible lower extremity plyometric exercises; control subjects did not. The training program was adapted from a previously established program by Hewett et al, consisting of exercises such as tuck jumps, scissor jumps, squat jumps and 180 jumps. The training sessions were led by the primary investigator who explained and demonstrated each exercise. Focus on proper technique, and correct take-off and landing mechanics was stressed. The results demonstrated a significant difference over time in the plyometric training group for hamstring strength at 120°/sec (Avg Power, PeakTorque). There was no significant different between the plyometric group and the control group at any time during the training sessions for quadriceps strength measures at 120°/sec. No significance was seen for either quadriceps or hamstring strength at 60°/sec in all participants. These findings demonstrate that over time, this plyometric training program increased hamstring strength, while maintaining quadriceps strength. This trend in strength allows for an improved Q:H strength ratio.

Muscle imbalance is thought to be a risk factor for ACL injury, particularly in females. Improving the Q:H strength ratio has the potential to decrease the risk for ACL injury. These strength improvements were only seen during isokinetic testing at higher speeds (120°/sec). This suggests that eccentric plyometric training improved hamstring strength for more rapid, dynamic tasks only. Emphasis of this training program was placed on proper mechanics and technique instead of intensity of exercise such as increasing the jump distances and height which might have allowed for a change in strength when tested at 60°/sec. I am not surprised by the results of this study. Over 10 years ago Hewett et al found that plyometric training influenced landing kinetics in female athletes. The next question is, “Does plyometric training decrease the risk for ACL injury?” A review of the literature by Grindstaff et al determined that 89 people needed to complete preventative plyometric training to prevent 1 ACL injury within a single sports season. Yes, plyometric training improves Q:H strength ratios, but does that decrease the risk of ACL injuries? Should all young female athletes participate in prophylactic plyometric training?

Written by: Kathleen White
Reviewed by: Stephen Thomas