title: Muscle force-stiffness characteristics influence joint stability: a spine example.
abstract: BACKGROUND
  The muscle force-stiffness relationship has often been modeled as linear, while in situ muscle research has clearly demonstrated non-linearity. Estimation of rotational joint stability relies on both a muscle's instantaneous pre-perturbation force and stiffness. Under conditions of static equilibrium, a muscle's stiffness will function in a stabilizing manner, while its force can function in either a stabilizing or destabilizing manner depending on the muscle's orientation about the joint.
  
  
  METHODS
  A single muscle (rectus abdominis) was modeled and its individual direct stabilizing potential about the L4-L5 spine joint was analyzed. Three force-stiffness relationships were examined: (1) linear; (2) non-linear with moderate stiffness magnitudes; (3) non-linear with higher stiffness magnitudes.
  
  
  RESULTS
  With a linear force-stiffness relationship, stability increased proportional to muscle force; with a non-linear relationship, stability peaked and subsequently decreased at submaximal muscle forces. When considering the lower, as opposed to the higher non-linear stiffness magnitudes, the stabilizing potential of the muscle peaked at a lower muscle force level and actually became negative (destabilizing) at a critical stiffness magnitude.
  
  
  CONCLUSIONS
  It was concluded that a non-linear muscle force-stiffness relationship greatly alters the individual stabilizing potential of the muscle throughout its progression of force development. A muscle's stabilizing contribution may actually peak at and subsequently decrease above a critical submaximal force level. Incorporating this knowledge into stability models may assist in recognizing unstable events that lead to injury at higher levels of muscle activation.
authors.affiliation: Department of Kinesiology, University of Waterloo, Waterloo, Ont., Canada N2L 3G1.
authors.forename: Stephen H M
authors.initials: SH
authors.lastname: Brown
authors.forename: Stuart M
authors.initials: SM
authors.lastname: McGill
mesh_terms: D001696:Biomechanical Phenomena; D006801:Humans; D008159:Lumbar Vertebrae; D008433:Mathematics; D018482:Muscle, Skeletal / Q000502:physiology*; D012026:Reflex, Stretch / Q000502:physiology; D012399:Rotation; D013131:Spine / Q000502:physiology*