title: Unstable Surface Improves Quadriceps:Hamstring Co-contraction for Anterior Cruciate Ligament Injury Prevention Strategies.
abstract: BACKGROUND
  Increasing quadriceps:hamstring muscular co-contraction at the knee may reduce the risk of anterior cruciate ligament (ACL) injury. The purpose of this investigation was to examine muscle activation in the quadriceps and hamstrings and peak kinematics of the knee, hip, and trunk when performing a single-leg drop (SLD) on to a Bosu ball (unstable surface) compared with on to the floor (stable surface).
  
  
  OBJECTIVE
  (1) The SLD on an unstable surface would lower the quadriceps to hamstrings electromyographic (EMG) activation ratio (Q:H EMG activation ratio) compared with being performed on the floor. (2) Lower Q:H EMG activation ratio would be caused by a relative increase in hamstring activation, with no significant change in quadriceps activation.
  
  
  METHODS
  Controlled laboratory study.
  
  
  METHODS
  Thirty-nine Division I National Collegiate Athletic Association (NCAA) female athletes performed 3 SLDs per leg onto a Bosu ball and onto the floor. Muscle activity of the vastus lateralis and lateral hamstrings were used to estimate peak quadriceps and hamstring activation, along with the Q:H EMG activation ratio. Kinematic measures at the knee, hip, and trunk were also estimated. Differences between landings were assessed using a 2-level analysis of variance (limb and surface).
  
  
  RESULTS
  The maximum Q:H EMG activation ratio was significantly reduced when athletes performed an SLD onto the Bosu ball (20%, P < 0.001) compared with the floor. Peak hamstring activity was higher when athletes landed on a Bosu ball (18% higher, P = 0.029) compared with when they landed on the floor.
  
  
  CONCLUSIONS
  Compared with landing on the floor (a stable surface), landing on a Bosu ball (unstable surface) changed the athlete's co-contraction at the knee and increased hamstring activity. However, landing on a Bosu ball also decreased the athlete's knee flexion, which was an undesired effect.
  
  
  CONCLUSIONS
  These findings highlight the potential utility of unstable surfaces as a training tool to reduce the risk of ACL injury in female athletes.
authors.affiliation: Human Performance Lab, Sports Medicine Center, Department of Orthopaedic Surgery, Stanford University, Palo Alto, California ; Division of Sports Medicine, Department of Orthopaedic Surgery, Stanford University, Palo Alto, California.
authors.forename: Rebecca
authors.initials: R
authors.lastname: Shultz
authors.affiliation: Human Performance Lab, Sports Medicine Center, Department of Orthopaedic Surgery, Stanford University, Palo Alto, California ; Department of BioEngineering, Stanford University, Palo Alto, California.
authors.forename: Amy
authors.initials: A
authors.lastname: Silder
authors.affiliation: Human Performance Lab, Sports Medicine Center, Department of Orthopaedic Surgery, Stanford University, Palo Alto, California.
authors.forename: Maria
authors.initials: M
authors.lastname: Malone
authors.affiliation: School of Medicine, University of California San Francisco, San Francisco, California.
authors.forename: Hillary Jane
authors.initials: HJ
authors.lastname: Braun
authors.affiliation: Human Performance Lab, Sports Medicine Center, Department of Orthopaedic Surgery, Stanford University, Palo Alto, California ; Division of Sports Medicine, Department of Orthopaedic Surgery, Stanford University, Palo Alto, California.
authors.forename: Jason Logan
authors.initials: JL
authors.lastname: Dragoo
keywords: anterior cruciate ligament (ACL); intervention; perturbation; prevention; sports injuries