The stress-strain relationships of human spinal ligaments and  muscles indicate that the spinal musculature play a major role in spine  stability (1, 2). Low-loading rate, quasi-static assessments of  posteroanterior (PA) spinal stiffness have correlated increased PA spine  stiffness to voluntary contracture of the lumbar extensor muscles (3,  4), No study, however, has examined the contributions of lumbar extensor  muscle and high-loading rate dynamic PA spinal stiffness. The objective  of this study was to quantify PA dynamic spinal stiffness at rest and  during maximal voluntary contraction (MVC) efforts in patients with LBP.  Twenty-two consecutive patients with LBP underwent dynamic spinal  stiffness assessment in the prone resting position and during lumbar  extensor muscle MVC, A hand-held Activator Adjusting Instrument equipped  with an impedance head was used to deliver high-loading rate( < 0,1  msec) PA manipulative thrusts (450 N) to the L3 spinous process for  spinal stiffness assessment using a previously validated technique (5).  Surface, linear enveloped, electromyographic (sEMG) recordings were  obtained during the thrusts from electrodes (8 leads) located over the  L3 and L5 erector spinae and data was normalized to subject individual  MVC’s. The accelerance (peak acceleration/peak force, kg-1) or stiffness  index was calculated for each of the thrusts and compared for the  resting and active MVC trials using a 2-tailed, paired t-test. A  significantly increased spine stiffness index (8.36%) (P=0.012) was  found upon MVC trials compared to prone resting stiffness indices.  Lumbar spine extensor MVC contributes to increased PA lumbar spine  stiffness. These findings corroborate the findings of others and add  support to the significance of the trunk musculature in providing spinal  stability.

Reference: Christopher J. Colloca, D.C.1, Tony S. Keller, Ph.D. 2 Daryn E. Seltzer, D.C.3, Arlan w. Fuhr, D.C.1;  Muscular and Soft-Tissue Contributions of Dynamic Posteroanterior  Spinal Stiffness; Proceedings of the International Conference on Spinal Manipulation,  Bloomington, MN September 21-23,2000.

1 Postdoctoral & Related Professional Education  Department Faculty, Logan College of Chiropractic, St. Louis, MO, USA;  National Institute of Chiropractic Research, Phoenix, AZ, USA; Private  Practice of Chiropractic, Phoenix, AZ, USA. 2 Professor, Department of Mechanical Engineering  & Department of Orthopedics and Rehabilitation, The University of  Vermont, Burlington, VT, USA. 3 National Institute of Chiropractic Research, Phoenix, AZ, USA; Private Practice of Chiropractic, Phoenix, AZ, USA.

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