Disturbance and recovery of trunk stiffness and reflexive muscle responses following prolonged trunk flexion: Influences of flexion angle and duration

Brad Hendershot, Babak Bazrgari, Khoirul Muslim, Nima Toosizadeh, Maury A. Nussbaum, Michael L. Madigan

Research output: Contribution to journalArticlepeer-review

62 Scopus citations


Background: Experimental studies suggest that flexed working postures reduce passive support of the spine, which could represent a significant risk factor for the development of occupational low back disorders. Neuromuscular compensations to reduced passive stiffness include increases in baseline activity or reflexive activation of trunk muscles. Yet, alterations and recovery of the synergy between active and passive tissues following prolonged flexion in humans are currently unknown. Methods: Twelve healthy participants were exposed to all combinations of two trunk flexion durations (2 and 16 min) and three flexion angles (33, 66, and 100% of individual flexion-relaxation angle). Load relaxation was recorded throughout exposures, whereas trunk stiffness and reflexive behaviors of the lumbar extensor muscles were investigated during dynamic responses to sudden perturbations. Findings: The magnitude of load relaxation increased with increasing flexion angle. Trunk stiffness decreased and reflex gains increased following flexion exposures; for both outcomes, acute changes were larger following exposure to increasing flexion angle. Reflex gains remained elevated one hour after exposure to maximum flexion. Interpretation: Exposure to prolonged trunk flexion changed trunk stiffness and reflex behavior in patterns consistent with epidemiological evidence linking such exposure with the risk of occupational low back disorders. Observed increases in reflex gains, at least among healthy individuals, may be a compensation for decreases in passive trunk stiffness following acute exposure to flexed postures. It remains to be determined whether the neuromuscular system can similarly respond to accumulated disturbances in passive structures following exposure to repeated flexion tasks.

Original languageEnglish
Pages (from-to)250-256
Number of pages7
JournalClinical Biomechanics
Issue number3
StatePublished - Mar 2011

Bibliographical note

Funding Information:
This work was supported by Cooperative Agreement Number R01OH008504 from the Centers for Disease Control and Prevention (CDC) . Its contents are solely the responsibility of the authors and do not necessarily represent the official views of CDC. The authors thank Mr. Waldron and Mr. Vest for their assembly and maintenance of the testing apparatus, and Mr. Seung-Yeun Han for his assistance with data collection. The authors dedicate this work to Dr. Kevin P. Granata whose original ideas formed the foundation for the current investigation.


  • Load relaxation
  • Low back pain
  • Lumbar spine
  • Prolonged flexion
  • Reflex
  • Stiffness

ASJC Scopus subject areas

  • Biophysics
  • Orthopedics and Sports Medicine


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