1) Girls typically grow faster than boys (measured in height) at the same age
2) Girls will have less muscle mass = power/strength at the same age
3) Jumping and landing
A) These particular athletes are being videotaped during the test and it appears these kids will land from a jump with the knees extended (straight) more often than flexed (knee bent). When this occurs the ACL is at more risk. Now, girls are more prone to this because usually they lack the muscle mass (Quads/Hams/Gluts) to slow/control the landing. When they lack control, then increase risk for ACL problems. So, the lesson here is this:
1) Land with hips and knees flexed to allow for stability with muscle control
2) Build dynamic muscle strength of Quads/Hams/Gluts/hip flexors etc
3) Proprioceptive training – agility/coordination etc

4) Anatomy – simply put, the girls have a larger "Q" angle at the knee (when you look at young girls, they will appear more knock-knee than boys) and it continues to increase with age until around 16-18 when they are fully grown. This knee angle allows for more ACL instability.

Shane Lawler D.P.T.
Clinic Director Heritage Physical Therapy
1050 Old Des Peres Rd., St. Louis, MO 63136 314-821-0200