It’s clear that women athletes have a much greater risk of ACL injury than do men. However, in young athletes, before reaching puberty, the risks are similar. This has led researchers to speculate that puberty may be associated with changes in boys and girls that raise the risk for one group but not the other. A new study from the Cincinnati Children’s Hospital looked at knee movements in boys and girls as they grew through the pubertal stage. As girls progress through puberty, they adopt a landing strategy that may place them a risk of ACL injury. Boys tend to adopt a different strategy. This difference could explain why teenage girls are at greater risk for ACL injury. It also suggests a key period during growth for targeted ACL injury prevention training.
The researchers examined landing mechanics in two groups of boys and girls, based on their pubertal status – pubertal and post-pubertal. The focus here was pubertal age, not necessarily calendar age. In both groups, two sets of measurements were made approximately one year apart. The measurements focused on knee movements during a depth jump. Subjects jumped off of a 31cm (12 inch) box and immediately performed a maximal vertical jump.
During the landing and take-off phase of the jump, the degree of knee flexion and knee abduction was measured. The terms knee valgus and abduction mean roughly the same thing. Valgus is a position, often referred to as knock-kneed. Abduction, in the other hand is the actual movement with the knee moving from outward to inward or from normal to knock-kneed (click here for a figure of knee valgus). Essentially, the researchers wanted to know which athletes landed in a knock-kneed or not.
The ages of the pubertal girls was 12-13 years and the post-pubertal girls 14-15 years. The boys ranged from 13-14 and 15-16. Over the course of the study, the growth of both boys and girls was roughly the same.
At the pubertal ages, there were no differences in knee mechanics between boys and girls. However, as the girls grew, they tended to land with more knee abduction. After ground contact, the knee tended to move inward towards a more valgus or knock-kneed position. On the other hand, as the boys developed, they tended to land with less knee abduction. By the time the athletes reached post-pubertal age, there was a clear difference in the degree of knee abduction between the boys and girls. The amount of knee movement during the depth jump translated into greater valgus stress placed on the knee in the pubertal girls.
The results show that during the pubertal period or rapid growth, girls develop a landing / jumping strategy that increases knee abduction and valgus stress. From a clinical perspective, increased valgus stress places more stress on the ACL. It is also associated with greater risk of ACL injury. That is, landing, stopping or cutting with the knee “collapsing” into a knock-kneed position increases stress on the ACL and places it at risk for a sprain, tear or rupture.
Why does this occur? The authors point out that the explanation is more complicated than simply growth. They suggest that changes in neuromuscular control may underlie landing mechanics. Males tend to undergo a “neuromuscular spurt” during rapid growth. This period is associated with increased strength and power - changes translate greater sprint and power performances in post-pubescent males compared to females. It is also thought that this neuromuscular spurt may provide greater neuromuscular control of the knee and lower ACL injury risk. Some think that this neuromuscular spurt may also increase strength of the muscle stat stabilize the knee and those that prevent valgus collapse during landing, stopping and cutting.
This concept raises the idea that there may be a period of rapid growth that should be targeted for neuromuscular training. Researchers at the Cincinnati Children’s Hospital have long advocated neuromuscular training programs for female athletes as a means of lowering ACL injury risk. They have published several studies showing that theses programs are very effective. In this paper, they emphasize the need for neuromuscular training early within pubertal development (as young a 12 years of age). Developing muscular strength, core strength, flexibility and agility at an early age can provide protection for the ACL. Also, training and instruction on how to properly land, stop, turn and cut may help girls avoid valgus collapse and ACL stress during activity.
The Science of Soccer Online has also reviewed the effectiveness of these intervention programs. A few of these articles are listed below.
ACL Injury Prevention – The Evidence Mounts
Cutting Edge Research: A Modified Version of the FIFA 11 Reduces Injury Rate In Young Women.
Cutting-Edge Research: ACL Injury Prevention - Neuromuscular Training
Perhaps implementing one of these programs early on in a female’s athletic career can lower the risk and incidence of ACL. By developing greater strength and neuromuscular control at a young age and teaching young players proper technique, the number of ACL injuries suffered by teenage players may be reduced.
Ford KR, Shapiro R, Myer GD, van den Bogert AJ, Hewett TE (2010) Longitudinal sex differences during landing in knee abduction in young athletes. Medicine and Science in Sports and Exercise, Epublished ahead of print - doi: 10.1177/03635465103674.
Note: This is not the fist paper to suggest that differences in knee mechanics between boys and girls begin to develop ding puberty. For example, see: Yu B, McClure SB, Onate JA, Guskiewicz KM, Kirkendall DT, Garrett WE (2005) Age and gender effects on lower extremity kinematics of youth soccer players in a stop-jump task. American Journal of Sports Medicine, 33:1356-1364. It is, however, one of the first longitudinal studies of young athletes.