title: Effects of Sex and Fatigue on Biomechanical Measures During the Drop-Jump Task in Children.
abstract: BACKGROUND
  Female athletes have a higher rate of anterior cruciate ligament (ACL) injury than males from adolescence and into maturity, which is suggested to result from sex-specific changes in dynamic movement patterns with maturation. Few studies have studied movement strategies and response to fatigue in children.
  
  
  OBJECTIVE
  To evaluate the effect of fatigue on biomechanical variables associated with increased risk for ACL injury during a drop-jump (DJ) performance in children.
  
  
  METHODS
  Controlled laboratory study.
  
  
  METHODS
  A total of 116 children (mean age, 10.4 years) were recruited from local sports clubs and performed 5 repetitions of a DJ task before and after a fatigue protocol. Kinematic and kinetic data from initial contact (IC) to the first peak vertical ground reaction force (vGRF) were analyzed for both limbs, including limb and fatigue as within-subject factors for analyses between boys and girls. Pearson correlation coefficients were calculated to identify associations between variables of interest.
  
  
  RESULTS
  Girls demonstrated greater peak vGRF values than boys (by 8.1%;P< .05), there were greater peak vGRF values for the right limb than the left (by 6.2%;P< .001), and fatigue led to slightly greater values (P< .05). Although weak, the correlation between peak vGRF values and knee flexion excursion was stronger for girls (r= -0.20) than boys (r= -0.08) (P< .006). Fatigue resulted in greater knee flexion angles at IC and less excursion during landing, more so for girls (by 6.1° vs 1.4°; interaction,P< .001), although the knee flexion moment was generally lowered by fatigue (P< .001). Limb asymmetry in knee flexion moments was more pronounced for boys than for girls (interaction,P< .05), contrary to that seen in frontal plane knee moments, where asymmetry was much greater in girls than boys (interaction,P< .001).
  
  
  CONCLUSIONS
  Even as young athletes, girls and boys seem to adopt dissimilar movement strategies and are differently affected by fatigue.
  
  
  CONCLUSIONS
  Injury prevention programs should be considered at an earlier age in an effort to lower the risk of ACL injury in athletes.
authors.affiliation: Department of Physical Therapy, School of Health Sciences, University of Iceland, Reykjavik, Iceland.; Research Centre of Movement Science, University of Iceland, Reykjavik, Iceland.
authors.forename: Kristín
authors.initials: K
authors.lastname: Briem
authors.affiliation: Department of Physical Therapy, School of Health Sciences, University of Iceland, Reykjavik, Iceland.
authors.forename: Kolbrún Vala
authors.initials: KV
authors.lastname: Jónsdóttir
authors.affiliation: Department of Physical Therapy, School of Health Sciences, University of Iceland, Reykjavik, Iceland.; Research Centre of Movement Science, University of Iceland, Reykjavik, Iceland.
authors.forename: Árni
authors.initials: Á
authors.lastname: Árnason
authors.affiliation: Department of Physical Therapy, School of Health Sciences, University of Iceland, Reykjavik, Iceland.; Research Centre of Movement Science, University of Iceland, Reykjavik, Iceland.
authors.forename: Þórarinn
authors.initials: Þ
authors.lastname: Sveinsson
keywords: ACL; biomechanics; injury prevention; knee; motion analysis; pediatric sports medicine