Associations between physical characteristics and landing biomechanics in adolescent females

Females tear their anterior cruciate ligament (ACL) at a rate of four to six times that of similarly trained males. This greater risk of ACL injury in females begins to emerge around age 12, then peaks and remains elevated from age 15 into adulthood. While the specific underlying factors that lead to this increased risk in females are yet unknown, this is the time that sex differences in physical characteristics and landing biomechanics begin to emerge. During adolescence, females develop higher risk landing strategies that are thought to place them at greater risk for ACL injury. As females mature, they perform landing maneuvers with greater knee valgus, a more extended knee, and increased reliance on the knee extensor muscles to dissipate landing forces. While these biomechanical patterns are associated with greater strain on the ACL, it is unclear what causes females to develop these higher risk knee biomechanics. The trend towards higher risk landing strategies in adolescent females occurs during a time of steady growth, when strength, body composition, and fitness levels are disparately changing in females and males. Maturing females develop greater fat mass which is associated with a plateau in relative strength to body weight and decreased cardiovascular fitness. Therefore, the purpose of this study was to examine the extent that physical measures of strength, body composition and fitness affect knee joint biomechanics during a landing task in adolescent females. Physical characteristics were assessed using a battery of field based assessments for cardiorespiratory fitness, musculoskeletal fitness (strength), and body composition that has been demonstrated to be reliable and valid in children and adolescents. Because both physical measures (e.g. fitness, strength), and knee biomechanics may be affected by fatigue (when knee injury is more likely to occur), the relationship between physical characteristics and knee biomechanics was examined both before and after an exercise challenge. Fifty adolescent females between the ages of 11 and 15 were used for analyses (Age: 12.7±1.4yrs, Tanner stage: 3.4±0.8, Height: 160.7±7.8 cm, Mass: 52.3±10.2 kg). The primary findings were that a greater Tanner stage of maturation was related to less predicted initial knee valgus angle following exercise (R2=0.082, p=0.04), while greater functional lower extremity strength was related to greater predicted peak internal tibial rotation angle both before (β=0.18, p=0.01) and after exercise (β=0.17, p=0.03). There were no associations between physical characteristics and relative energy absorption at the knee. Furthermore, exercise had little to no effect on these associations. These results indicate that the measures of maturation and strength are related to landing biomechanics both before and after exercise. Thus, it appears that landing mechanics that have been shown to change during adolescence cannot solely be attributed to potential changes in strength, body composition and fitness based on the field tests used. However, each of these physical characteristic warrants further inclusion in future studies investigating changing landing biomechanics in populations of adolescent females that participate in athletics. Though only functional strength was statistically related to at-risk landing biomechanics in this representative population of adolescent females, the strength of the relationships with other variables suggests that with more subjects, particularly in Tanner stages 1 and 5, additional relationships may emerge.