Is Knee Separation During a Drop Jump Associated With Lower Extremity Injury in Adolescent Female Soccer Players?

Am J Sports Med. 2016 Feb;44(2):318-23. doi: 10.1177/0363546515613076. Epub 2015 Nov 30.


Background: Knee injuries are common in older adolescent and adult female soccer players, and abnormal valgus knee appearance characterized by low normalized knee separation (NKS) is a proposed injury risk factor. What constitutes normal NKS in younger adolescents and whether low NKS is an injury risk factor are unknown.

Purpose: To determine the normal range of NKS using a drop-jump test in female perimenarchal youth soccer players and whether low NKS contributes to lower extremity injuries or knee injuries.

Study design: Cohort study; Level of evidence, 2.

Methods: From 2008 to 2012, a total of 351 female elite youth soccer players (age range, 11-14 years) were followed for 1 season, with complete follow-up on 92.3% of players. Baseline drop-jump testing was performed preseason. Lower extremity injuries during the season were identified using a validated, Internet-based injury surveillance system with weekly email reporting. Normalized knee separation at prelanding, landing, and takeoff was categorized 2 ways: as ≤10th percentile (most extreme valgus appearance) compared with >10th percentile and as a continuous measure of 1 SD. Poisson regression modeling with adjustment for clustering by team estimated the relative risk (RR) and 95% confidence interval (CI) of the association between the NKS and the risk of lower extremity and knee injury, stratified by menarche.

Results: Of the study participants, 134 players experienced 173 lower extremity injuries, with 43 (24.9%) knee injuries. For postmenarchal players (n = 210), those with NKS ≤10th percentile were at 92% increased risk of lower extremity injury (RR, 1.92; 95% CI, 1.17-3.15) and a 3.62-fold increased risk of knee injury (RR, 3.62; 95% CI, 1.18-11.09) compared with NKS >10th percentile at prelanding and landing, respectively. Among postmenarchal players, there was an 80% increased risk of knee injury (RR, 1.80; 95% CI, 1.01-3.23) with a decrease of 1 SD in landing NKS and a 66% increased risk of knee injury (RR, 1.66; 95% CI, 1.04-2.64) with a decrease of 1 SD in takeoff NKS. Among premenarchal players (n = 141), there was no statistically significant association between the NKS at prelanding, landing, and takeoff and the risk of lower extremity or knee injury.

Conclusion: Low NKS was associated with increased risk of lower extremity and knee injury only among postmenarchal players.

Keywords: female; knee injury; knee separation distance; lower extremity injury; pediatric; soccer.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Adolescent
  • Athletic Injuries* / epidemiology
  • District of Columbia / epidemiology
  • Female
  • Follow-Up Studies
  • Humans
  • Knee Injuries / epidemiology
  • Knee Injuries / etiology*
  • Knee Injuries / prevention & control
  • Knee Joint / pathology*
  • Lower Extremity / injuries*
  • Lower Extremity / pathology
  • Prospective Studies
  • Random Allocation
  • Risk Factors
  • Soccer / injuries*