title: Contribution of knee flexor and extensor strength on sex-specific energy absorption and torsional joint stiffness during drop jumping.
abstract: BACKGROUND
  Lower extremity injury often occurs during abrupt deceleration when attempting to change the body's direction. Although sex-specific biomechanics have been implicated in the greater risk of acute knee injury in women than in men, it is unknown if sex differences in thigh strength affect sex-specific energy absorption and torsional joint stiffness patterns.
  
  
  OBJECTIVE
  To determine sex differences in energy absorption patterns and joint stiffnesses of the lower extremity during a drop jump and to determine if these sex differences were predicted by knee extensor and flexor strength.
  
  
  METHODS
  Cross-sectional study.
  
  
  METHODS
  Laboratory environment.
  
  
  METHODS
  Recreationally active, college-aged students (41 women: age  =  22.1 ± 2.9 years, height  =  1.63 ± 0.07 m, mass  =  59.3 ± 8.0 kg; 40 men: age  =  22.4 ± 2.8 years, height  =  1.77 ± 0.1 m, mass  =  80.9 ± 14.1 kg).
  
  
  METHODS
  Participants performed knee flexor and extensor maximal voluntary isometric contractions followed by double-leg drop-jump landings.
  
  
  METHODS
  Lower extremity joint energetics (J × N(-1) × m(-1)) and torsional joint stiffnesses (Nm × N(-1) × m(-1) × degrees(-1)) were calculated for the hip, knee, and ankle during the initial landing phase. Body weight was measured in newtons and height was measured in meters. Sex comparisons were made and sex-specific regressions determined if thigh muscle strength (Nm/kg) predicted sagittal-plane landing energetics and stiffnesses.
  
  
  RESULTS
  Women absorbed 69% more knee energy and had 36% less hip torsional stiffness than men. In women, greater knee extensor strength predicted greater knee energy absorption (R(2)  =  0.11, P  =  .04), and greater knee flexor strength predicted greater hip torsional stiffness (R(2)  =  0.12, P  =  .03).
  
  
  CONCLUSIONS
  Sex-specific biomechanics during the deceleration phase of a drop jump revealed that women used a strategy to attempt to decrease system stiffness. Additionally, only female strength values were predictive of landing energetics and stiffnesses. These findings collectively demonstrated that the task may have been more difficult for women, resulting in a different movement strategy among those with different levels of thigh strength to safely complete the task. Future researchers should look at other predictive factors of observed sex differences.
authors.affiliation: Department of Kinesiology, University of North Carolina at Greensboro, NC 27402, USA. rjschmit@uncg.edu
authors.forename: Randy J
authors.initials: RJ
authors.lastname: Schmitz
authors.forename: Sandra J
authors.initials: SJ
authors.lastname: Shultz
mesh_terms: D000704:Analysis of Variance; D001696:Biomechanical Phenomena; D003430:Cross-Sectional Studies; D003654:Deceleration; D004735:Energy Transfer; D005260:Female; D006621:Hip Joint; D006801:Humans; D007592:Joint Diseases / Q000209:etiology* / Q000503:physiopathology; D007718:Knee Injuries / Q000209:etiology* / Q000503:physiopathology; D007719:Knee Joint; D008297:Male; D053580:Muscle Strength / Q000502:physiology*; D012307:Risk Factors; D012737:Sex Factors; D013334:Students; D054159:Torsion, Mechanical; D055815:Young Adult