title: Lumbar spine mechanical response to axial compression load in vivo.
abstract: METHODS
  Lumbar spine kinematic response to a 1.0 body weight compressive load was measured in vivo by comparison of relaxed and loaded magnetic resonance image sets in the sagittal plane.
  
  
  OBJECTIVE
  To identify and measure acute response mechanisms of the lumbar spine during compression loading.
  
  
  BACKGROUND
  The isolated ligamentous spine buckles under small loads (88 N); yet, the spine supports >10 times that load in daily activities. Mechanical function of the lumbar spine in vivo is not well understood, and only a few studies examined the spine during in vivo loading.
  
  
  METHODS
  Magnetic resonance imaging scans of subjects were taken while subjects were relaxed and while supporting a 1.0 body weight compressive load. Vertebral bodies and disc perimeters were digitized, and relative centroid positions were measured and compared between conditions. Lumbar rotation, bending, compression, and disc translation were determined. Two parameter ensembles were analyzed to describe mechanisms of "spine shrinkage" (decrease of projected spine length) and lumbosacral response.
  
  
  RESULTS
  All subjects underwent spine shrinkage (-3.9 +/- 1.2 mm) dominated by cumulative bending, except in three subjects where the rotation component dominated. Levels L2-L4 extended, while L5 flexed, and dL2 through dL4 translated anterior, while dL5 translated posterior. Significant segmental deformations were as follows: L3 extension (-3.3 +/- 3.1 degrees ), dL5 disc translation (-1.4 +/- 1.4 mm), and posterior sacral rotation (3.2 +/- 4.7 degrees ).
  
  
  CONCLUSIONS
  Spine shrinkage occurred mainly from spine bending and rotation, with only small contribution from spine compression (shortening along the spine curvature). Response pattern groupings indicated at least two unique subgroups, but the cause remains to be determined.
authors.affiliation: Department of Health, Leisure, and Exercise Science, Appalachian State University, Boone, North Carolina 28608-2071, USA. wislederd@appstate.edu
authors.forename: D
authors.initials: D
authors.lastname: Wisleder
authors.forename: M B
authors.initials: MB
authors.lastname: Smith
authors.forename: T J
authors.initials: TJ
authors.lastname: Mosher
authors.forename: V
authors.initials: V
authors.lastname: Zatsiorsky
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena; D019245:Compressive Strength / Q000502:physiology*; D006801:Humans; D007091:Image Processing, Computer-Assisted; D007403:Intervertebral Disc / Q000502:physiology; D008159:Lumbar Vertebrae / Q000502:physiology*; D008279:Magnetic Resonance Imaging; D008297:Male; D012399:Rotation; D016474:Weight-Bearing / Q000502:physiology*