title: Anterior knee stiffness changes in laxity "responders" versus "nonresponders" across the menstrual cycle.
abstract: BACKGROUND
  Although changes in anterior knee laxity (AKL) across the menstrual cycle have been reported, the effects of cyclic knee laxity changes on the underlying characteristics of the load-displacement (stiffness) curve generated during anterior loading of the tibia relative to the femur are relatively unknown.
  
  
  OBJECTIVE
  To describe the anterior load-displacement curve during anterior loading of the tibia relative to the femur using incremental stiffnesses and to compare underlying stiffness measures between days of the cycle when AKL is at its minimum and maximum.
  
  
  METHODS
  Descriptive laboratory study.
  
  
  METHODS
  University laboratory.
  
  
  METHODS
  Fifty-seven recreationally active women.
  
  
  METHODS
  Anterior knee laxity and 6 incremental stiffness measures (N/mm) were obtained with an instrumented knee arthrometer on days 1-6 of menses and days 0-8 postovulation during 2 consecutive menstrual cycles. Participants were then classified in tertiles based on the maximum change (difference between maximum and minimum) in AKL, and incremental stiffness was compared on days of minimum versus maximum laxity between the lowest (<1.24 mm cyclic laxity change = laxity "nonresponders" [n = 19]) and highest (>1.75 mm cyclic laxity change = laxity "responders" [n = 19]) tertiles.
  
  
  RESULTS
  All participants displayed decreasing stiffness initially (0-20 N > 20-40 N and 40-60 N), followed by incrementally increasing stiffness (40-60 N < 60-80 N < 80-100 N < 100-130 N) (P ≤ .05). Responders demonstrated decreased stiffness between the days of minimum and maximum AKL at the 10-130-N increment versus the 0-20-N and 20-40-N increments (P ≤ .05); nonresponders had no change in stiffness.
  
  
  CONCLUSIONS
  Participants who experienced larger magnitudes of cyclic changes in AKL also experienced decreases in terminal (100-130 N) stiffness during anterior knee joint loading. Decreases in incremental stiffness at higher anterior directed loads may adversely affect passive restraint systems, resulting in altered arthrokinematics during functional activity.
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: D000328:Adult; D016118:Anterior Cruciate Ligament / Q000503:physiopathology*; D001696:Biomechanical Phenomena; D005260:Female; D006801:Humans; D007593:Joint Instability / Q000503:physiopathology*; D007719:Knee Joint / Q000503:physiopathology*; D008597:Menstrual Cycle / Q000502:physiology*; D015203:Reproducibility of Results