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The Impact of Posture and Prolonged Cyclic Compressive Loading on Vertebral Joint Mechanics

Gooyers, Chad E., MSc*; McMillan, Robert D., BSc*; Howarth, Samuel J., PhD; Callaghan, Jack P., PhD*

doi: 10.1097/BRS.0b013e318256f9e6
Basic Science
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Study Design. 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.

Summary of Background Data. 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.

Conclusion. Posture is an important mechanical factor to consider when assessing the risk of injury from cyclic loading to the lumbar spine.

When exposed to prolonged cyclic compressive loading, porcine vertebral joints experienced significantly greater height loss while in a flexed posture than in neutral or extended postures. These findings suggest that posture requires consideration as a mechanically influential factor when assessing the low back injury risk imposed from vibration.

*Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada

Department of Graduate Education & Research, Canadian Memorial Chiropractic College, Toronto, Ontario, Canada.

Address correspondence and reprint requests to Jack P. Callaghan, PhD, Department of Kinesiology, University of Waterloo, Burt Matthews Hall, Room 3122, 200 University Ave. West, Waterloo, Ontario, Canada N2L 6P2; E-mail: callagha@uwaterloo.ca

Acknowledgment date: October 28, 2011. First revision date: December 24, 2011. Second revision date: March 12, 2012. Acceptance date: March 19, 2012.

The manuscript submitted does not contain information about medical device(s)/drug(s).

Centre for Research Excellence for the Prevention of Musculoskeletal Disorders (CRE-MSD); Natural Science and Engineering Research Council of Canada (NSERC); and Canada Spine Biomechanics and Injury Prevention funds were received to support this work.

No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

© 2012 Lippincott Williams & Wilkins, Inc.