Adolescent and adult female athletes have a 4-8 times greater incidence of serious non-contact knee ligament injury than male athletes participating in the same sport.

There has been a great deal of speculation and research to try and find out why this is so, with researchers describing in adolescent and adult populations gender differences in neuromuscular indices, such as muscle strength, running, cutting sidestepping, and landing characteristics that are believed to play some role in the gender difference.

For some time our own research team at the Cincinnati Sportsmedicine and Research and Education Foundation (Cincinnati, Ohio) have been systematically exploring the subject, and in this study we decided to go further back than most other researchers and study the period from pre-adolescence (from age 9) to the age of 17. We specifically decided to explore whether or not, in this age frame, there was any gender difference in lower limb strength - particularly of the quadriceps and hamstrings muscle groups - or in the alignment of the limb on landing from a jump or a hop.

Ours was the largest study of its kind. 1140 (916 female and 224 male) healthy young athletes were recruited, aged between 9 and 17 years - all participating in a variety of organized sport. None had had previous knee injury of any significance. All had participated in organized sport for a mean of 4 years before the date of testing.

We tested their quadriceps and hamstrings strength using an 'isokinetic' machine (1030 subjects), videoed their lower limb alignment on a drop-jump (536 subjects) and also compared their limb symmetry in single-leg hop tests (324 subjects).

With regard to strength testing, what we were particularly interested in were the:

  • normalized peak torque of quadriceps and hamstrings - that is the maximum strength of each of these muscle groups, if a correction were made to account for their weight (in other words, we calculated what the maximal strength would be if the children had all been the same weight).
  • hamstring: quadriceps ratio - that is the strength of the muscle group which bends (flexes) the knee compared to the strength of the muscle group which straightens (extends) the knee (in other words, in other words whether one muscle group is too strong compared to the other).

The first major finding from this study was that girls reach their maximum hamstring strength by age 11 - a new detection in the medical literature. This is particularly concerning in light of the fact that hamstrings play a major role in protecting the anterior cruciate ligament from injury. Boys reach their maximum hamstring strength by age 14 and, as expected, had significantly greater strength than age-matched girls. For quadriceps strength, girls reached maximum values by age 13 and boys by age 14 and again, boys had much greater strength in this muscle group than age-matched girls.

Actually, we found no statistically significant difference in the hamstrings:quadriceps ratio between genders at any age. Both the boys and the girls showed a decrease in the ratio from 9-17 but we did not find the dramatic difference in genders that other researchers have found in this ratio - which was most likely due to the tremendous number of athletes we tested among the age groups and the fact that most of the athletes were involved in sports year-round.

The second important finding from our study was that there was no difference in lower limb alignment on landing from a drop jump - a high percentage of both males and females in all age categories landed in an overall knock-kneed (valgus) lower extremity alignment. As well, both boys and girls had poor lower limb symmetry during single leg hop tests. While other investigators have speculated that an improper valgus landing position may predispose females to ACL injury, this finding reveals that other factors are more likely to be responsible for the gender disparity in knee ligament injury rates.