title: Vertical stiffness is not related to anterior cruciate ligament elongation in professional rugby union players.
abstract: BACKGROUND
  Novel research surrounding anterior cruciate ligament (ACL) injury is necessary because ACL injury rates have remained unchanged for several decades. An area of ACL risk mitigation which has not been well researched relates to vertical stiffness. The relationship between increased vertical stiffness and increased ground reaction force suggests that vertical stiffness may be related to ACL injury risk. However, given that increased dynamic knee joint stability has been shown to be associated with vertical stiffness, it is possible that modification of vertical stiffness could help to protect against injury. We aimed to determine whether vertical stiffness is related to measures known to load, or which represent loading of, the ACL.
  
  
  METHODS
  This was a cross-sectional observational study of 11 professional Australian rugby players. Knee kinematics and ACL elongation were measured from a 4-dimensional model of a hopping task which simulated the change of direction manoeuvre typically observed when non-contact ACL injury occurs. The model was generated from a CT scan of the participant's knee registered frame by frame to fluoroscopy images of the hopping task. Vertical stiffness was calculated from force plate data.
  
  
  RESULTS
  There was no association found between vertical stiffness and anterior tibial translation (ATT) or ACL elongation (r=-0.05; p=0.89, and r=-0.07; p=0.83, respectively). ATT was related to ACL elongation (r=0.93; p=0.0001).
  
  
  CONCLUSIONS
  Vertical stiffness was not associated with ACL loading in this cohort of elite rugby players but a novel method for measuring ACL elongation in vivo was found to have good construct validity.
authors.affiliation: Trauma and Orthopaedic Research Unit, Canberra Hospital, Woden, Australian Capital Territory, Australia; Medical School, Australian National University, Canberra, Australian Capital Territory, Australia.
authors.forename: Benjamin G
authors.initials: BG
authors.lastname: Serpell
authors.affiliation: Trauma and Orthopaedic Research Unit, Canberra Hospital, Woden, Australian Capital Territory, Australia; University of Canberra Health Research Institute, Bruce, Australian Capital Territory, Australia.
authors.forename: Jennie M
authors.initials: JM
authors.lastname: Scarvell
authors.affiliation: Trauma and Orthopaedic Research Unit, Canberra Hospital, Woden, Australian Capital Territory, Australia; School of Information Technology and Electrical Engineering, University of New South Wales, Canberra, Australian Capital Territory, Australia.
authors.forename: Mark R
authors.initials: MR
authors.lastname: Pickering
authors.affiliation: Research Institute for Sport and Exercise, University of Canberra , Bruce, Australian Capital Territory , Australia.
authors.forename: Nick B
authors.initials: NB
authors.lastname: Ball
authors.affiliation: Trauma and Orthopaedic Research Unit, Canberra Hospital, Woden, Australian Capital Territory, Australia; Medical School, Australian National University, Canberra, Australian Capital Territory, Australia; University of Canberra Health Research Institute, Bruce, Australian Capital Territory, Australia.
authors.forename: Diana
authors.initials: D
authors.lastname: Perriman
authors.affiliation: Trauma and Orthopaedic Research Unit , Canberra Hospital , Woden, Australian Capital Territory , Australia.
authors.forename: John
authors.initials: J
authors.lastname: Warmenhoven
authors.affiliation: Trauma and Orthopaedic Research Unit, Canberra Hospital, Woden, Australian Capital Territory, Australia; Medical School, Australian National University, Canberra, Australian Capital Territory, Australia.
authors.forename: Paul N
authors.initials: PN
authors.lastname: Smith
keywords: ACL; Injury; Rugby