title: Is Knee Separation During a Drop Jump Associated With Lower Extremity Injury in Adolescent Female Soccer Players?
abstract: 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.
  
  
  OBJECTIVE
  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.
  
  
  METHODS
  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.
  
  
  CONCLUSIONS
  Low NKS was associated with increased risk of lower extremity and knee injury only among postmenarchal players.
authors.affiliation: Department of Family Medicine and Orthopedics and Sports Medicine, School of Medicine, University of Washington, Seattle, Washington, USA jokane@uw.edu.
authors.forename: John W
authors.initials: JW
authors.lastname: O'Kane
authors.affiliation: Department of Orthopedics and Sports Medicine, School of Medicine, University of Washington, Seattle, Washington, USA.
authors.forename: Allan
authors.initials: A
authors.lastname: Tencer
authors.affiliation: The Mountain-Whisper-Light Statistics, Seattle, Washington, USA.
authors.forename: Moni
authors.initials: M
authors.lastname: Neradilek
authors.affiliation: The Mountain-Whisper-Light Statistics, Seattle, Washington, USA.
authors.forename: Nayak
authors.initials: N
authors.lastname: Polissar
authors.affiliation: Department of Orthopedics and Sports Medicine, School of Medicine, University of Washington, Seattle, Washington, USA.
authors.forename: Lori
authors.initials: L
authors.lastname: Sabado
authors.affiliation: Department of Epidemiology, School of Public Health, University of Washington, Seattle, Washington, USA Harborview Injury Prevention and Research Center, Seattle, Washington, USA.
authors.forename: Melissa A
authors.initials: MA
authors.lastname: Schiff
mesh_terms: D000293:Adolescent; D001265:Athletic Injuries / Q000453:epidemiology; D004219:District of Columbia / Q000453:epidemiology; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D007718:Knee Injuries / Q000453:epidemiology / Q000209:etiology* / Q000517:prevention & control; D007719:Knee Joint / Q000473:pathology*; D035002:Lower Extremity / Q000293:injuries* / Q000473:pathology; D011446:Prospective Studies; D011897:Random Allocation; D012307:Risk Factors; D012916:Soccer / Q000293:injuries*
keywords: female; knee injury; knee separation distance; lower extremity injury; pediatric; soccer