title: Musculotendon translational stiffness and muscle activity are modified by shear forces.
abstract: BACKGROUND
  In this study we investigate whether the nervous system is responsive to anterior-posterior shearing forces applied to the knee. Specifically, we examine whether the nervous system will increase musculotendon stiffness along the anterior-posterior degree-of-freedom via relative changes in muscle activation.
  
  
  METHODS
  While seated on a Biodex, 12 male participants performed knee extensor moments against a cuff at three different distances (265mm, 310mm, and 355mm) from the knee joint. This was done for each combination of knee angle (20°, 45°, 70°), moment (25%, 75% of maximum), and angular velocity (0°/s, 10°/s). By altering cuff position, the magnitude of the shear force could be changed while holding moment constant. We then calculated the 6-degree-of-freedom, musculotendon stiffness matrix of the knee and interpreted the eigenvalues and eigenvectors of this matrix using stiffness ellipses.
  
  
  RESULTS
  Anterior-Posterior knee stiffness significantly increased with an increase in shear force at 20° and 45° of knee flexion. This coincided with significant increases in semitendinosus and vastus medialis activity.
  
  
  CONCLUSIONS
  We found that the nervous system can significantly increase stiffness along a translational degree-of-freedom in response to translational loading, and did so by activating muscles geometrically oriented to provide stiffness along the anterior-posterior degree-of-freedom.
authors.affiliation: Department of Kinesiology, McMaster University, Hamilton, ON, L8S 2K1, Canada. Electronic address: cashabackjga@gmail.com.
authors.forename: Joshua G A
authors.initials: JG
authors.lastname: Cashaback
authors.affiliation: Department of Kinesiology, McMaster University, Hamilton, ON, L8S 2K1, Canada.
authors.forename: Kayla
authors.initials: K
authors.lastname: Fewster
authors.affiliation: Department of Kinesiology, McMaster University, Hamilton, ON, L8S 2K1, Canada.
authors.forename: Jim R
authors.initials: JR
authors.lastname: Potvin
authors.affiliation: School of Rehabilitation Science, McMaster University, Hamilton, ON, L8S 2K1, Canada.
authors.forename: Michael
authors.initials: M
authors.lastname: Pierrynowski
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena / Q000502:physiology; D004576:Electromyography / Q000379:methods; D017276:Friction / Q000502:physiology*; D006801:Humans; D007719:Knee Joint / Q000294:innervation / Q000502:physiology*; D007866:Leg / Q000502:physiology; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology*; D009424:Nervous System Physiological Phenomena / Q000502:physiology; D013710:Tendons / Q000502:physiology*
keywords: Impedance; Knee; Musculotendon; Stability; Stiffness