A recent study: Anterior cruciate ligament laxity and strength of quadriceps, hamstrings, and hip abductors in young pre-pubescent female soccer players over time: a three-year prospective longitudinal pilot study.
…whoo, long title… as I was saying, a recent study was published in Orthopedic Physical Therapy Practice that clearly demonstrates the timeframe and magnitude to which the strength balance of young females’ bodies begins to become unbalanced.
Here’s the abstract:
Purpose: This was a longitudinal study to determine the effects of maturation on anterior cruciate ligament (ACL) laxity and muscle strength in pre-pubescent female soccer players. Methods: ACL laxity and quadriceps, hamstrings, and abductors strength were measured annually from 2006 through 2008 in 22 pre-pubescent female soccer players, ages 7-12yrs. Results: ACL laxity increased 2.2 mm (p < 0.0002) in 2007 and 1.7 mm (p < 0.005) in 2008. Quadriceps strength increased 1.9 kg (p < 0.01) in 2007 and 2.1 kg (p < 0.009) in 2008. No significant change was noted in the hamstrings. Abductor strength decreased 3.0 kg (p < 0.0001) in 2007 and 2.3 kg (p < 0.0001) in 2008. Quadriceps to hamstring (Q/H) ratio decreased 0.4 kg (p < 0.02) in 2008. Conclusion: ACL laxity increased with age in pre-pubescent girls. The high Q/H ratio, and decreased abductor muscle strength, indicates an increased risk of ACL injury. Significant changes at age 11.5 occur both in ACL laxity and muscle strength, just one year prior to average age of menses. Girls may be approaching puberty with preexisting muscle weakness and imbalance that may expose them to ACL injury.
The critical pieces to pull from the abstract refer to the combined effects of a high strength ratio between the quadriceps and hamstrings (ideally, you want them to be well balanced and fairly even), the decreasing strength of the abductors (they keep the knee from "caving"), and ever increasing strength of the quadriceps.
It’s worth noting that this study was conducted on girls who are athletes, female soccer players to be exact. So, the increase in ACL laxity was not due to inactivity.
I think it is fantastic that the exact age – 11.5 years – has been pinpointed as the most significant time when this shift towards imbalance is occurring.
What should you do? Well, if you have a daughter, I’d suggest getting her started in a program that has a strong (and highly successful) ACL tear prevention protocol. Training to prevent ACL tears is serious business and, in the long run, it will cost a lot less to PREVENT a tear that to surgically repair and rehab a tear.