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

Authors
Hendershot-B; Bazrgari-B; Muslim-K; Toosizadeh-N; Nussbaum-MA; Madigan-ML
Source
Clin Biomech 2011 Mar; 26(3):250-256
NIOSHTIC No.
20039332
Abstract
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.
Keywords
Biodynamics; Back-injuries; Biomechanical-modeling; Biomechanics; Body-burden; Body-mechanics; Ergonomics; Mathematical-models; Musculoskeletal-system; Musculoskeletal-system-disorders; Neuromuscular-function; Neuromuscular-system; Occupational-hazards; Physiological-response; Physiological-stress; Physiology; Posture; Repetitive-work; Risk-analysis; Risk-factors; Author Keywords: Low back pain; Lumbar spine; Load relaxation; Prolonged flexion; Stiffness; Reflex; Back-injuries
Contact
Maury A. Nussbaum, Wake Forest School of Biomedical Engineering and Sciences, Industrial and Systems Engineering, Virginia Tech, Blacksburg, VA 24061
CODEN
CLBIEW
Publication Date
20110301
Document Type
Journal Article
Email Address
nussbaum@vt.edu
Funding Type
Grant
Fiscal Year
2011
NTIS Accession No.
NTIS Price
Identifying No.
Grant-Number-R01-OH-008504
Issue of Publication
3
ISSN
0268-0033
Source Name
Clinical Biomechanics
State
VA
Performing Organization
Virginia Polytechnic Institute and State University
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