title: The impact of posture and prolonged cyclic compressive loading on vertebral joint mechanics.
abstract: METHODS
  An in vitro biomechanics investigation exposing porcine functional spinal units (FSUs) to submaximal cyclic or static compressive forces while in a flexed, neutral, or extended posture.
  
  
  OBJECTIVE
  To investigate the combined effect of cyclically applied compressive force (e.g., vibration) and postural deviation on intervertebral joint mechanics.
  
  
  BACKGROUND
  Independently, prolonged vibration exposure and non-neutral postures are known risk factors for development of low back pain and injury. However, there is limited basic scientific evidence to explain how the risk of low back injury from vibration exposure is modified by other mechanical factors. This work examined the influence of static postural deviation on vertebral joint height loss and compressive stiffness under cyclically applied compressive force.
  
  
  METHODS
  Forty-eight FSUs, consisting of 2 adjacent vertebrae, ligaments, and the intervening intervertebral disc were included in the study. Each specimen was randomized to 1 of 3 experimental posture conditions (neutral, flexed, or extended) and assigned to 1 of 2 loading protocols, consisting of (1) cyclic (1500 ± 1200 N applied at 5 Hz using a sinusoidal waveform, resulting in 0.2 g rms acceleration) or (2) 1500 N of static compressive force. RESULTS.: As expected, FSU height loss followed a typical first-order response in both the static and cyclic loading protocols, with the majority (~50%) of the loss occurring in the first 20 minutes of testing. A significant interaction between posture and loading protocol (P < 0.001) was noted in the magnitude of FSU height loss. Subsequent analysis of simple effects revealed significant differences between cyclic and static loading protocols in both a neutral (P = 0.016) and a flexed posture (P < 0.0001). No significant differences (P = 0.320) were noted between pre/postmeasurements of FSU compressive stiffness.
  
  
  CONCLUSIONS
  Posture is an important mechanical factor to consider when assessing the risk of injury from cyclic loading to the lumbar spine.
authors.affiliation: Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada.
authors.forename: Chad E
authors.initials: CE
authors.lastname: Gooyers
authors.forename: Robert D
authors.initials: RD
authors.lastname: McMillan
authors.forename: Samuel J
authors.initials: SJ
authors.lastname: Howarth
authors.forename: Jack P
authors.initials: JP
authors.lastname: Callaghan
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D019245:Compressive Strength / Q000502:physiology; D006801:Humans; D066298:In Vitro Techniques; D007403:Intervertebral Disc / Q000502:physiology* / Q000503:physiopathology; D017116:Low Back Pain / Q000209:etiology / Q000503:physiopathology; D011187:Posture / Q000502:physiology*; D018570:Risk Assessment; D012307:Risk Factors; D013124:Spinal Injuries / Q000209:etiology / Q000503:physiopathology; D013131:Spine / Q000502:physiology* / Q000503:physiopathology; D013314:Stress, Mechanical; D013552:Swine; D014732:Vibration / Q000009:adverse effects; D016474:Weight-Bearing / Q000502:physiology*