Longitudinal Effects of Maturation on Lower Extremity Joint Stiffness in Adolescent Athletes

Background: Yearly changes in active joint stiffness may help explain when neuromuscular sex differences emerge in adolescent athletes that may relate to increased anterior cruciate ligament injury risk in females. Hypothesis: Pubertal males would demonstrate increases in knee stiffness while pubertal females would not. Second, postpubertal female athletes would have significantly lower knee joint stiffness than postpubertal male athletes. Study Design: Cohort Study; Level of Evidence 2 and Cross-Sectional Study; Level of Evidence 3. Methods: Two hundred sixty-five females and 50 males participated in 2 testing sessions approximately 1 year apart. The subjects were classified as either pubertal (n = 182, age 12.4 +/- 0.9 years) or postpubertal (n = 133, age 14.5 +/- 1.4 years) based on the modified Pubertal Maturational Observational Scale at each visit. Active joint stiffness of the ankle, knee, and hip was estimated during a drop vertical jump. Stiffness was calculated as the slope of the moment-angle curve from a least squares linear regression during the stance phase. Results: All athletes showed increased active knee stiffness during the span of a year (P<0.05). However, this increase was not different when stiffness was normalized to body mass. Only males demonstrated greater magnitudes of ankle and hip active stiffness (P<.05). Peak ankle and hip moments, but not knee moments, in postpubertal males were significantly greater than postpubertal females (P<.05). Females had a higher knee to hip moment ratio than males (P<.05). Conclusion: Both males and females showed increased active knee stiffness during the span of a year; males demonstrated increased ankle and hip active stiffness as well. Differences in hip joint posture at initial contact (greater flexion in males) and external hip flexion moment (greater flexion magnitude in males) may indicate that males use a different hip recruitment strategy during drop vertical jumps than females.