Transfer of loads between lumbar tissues during the flexion-relaxation phenomenon.
METHODS
This study used an anatomically detailed model of the lumbar tissues, driven from biologic signals of vertebral displacement and myoelectric signals, to estimate individual muscle and passive tissue force-time histories during the performance of the "flexion-relaxation" maneuver. Eight male university students performed three trials each of the "flexion-relaxation" maneuver with six pairs of surface myoelectric electrodes monitoring the right side of the trunk musculature, an electromagnetic device to record lumbar flexion, and videotape to record body segment displacement.


OBJECTIVE
To examine the loads on individual tissues during the transfer of moment support responsibility from predominantly active muscle to predominantly passive tissue.


BACKGROUND
No previous studies, to the authors' knowledge, have examined individual tissue loading during the flexion-relaxation maneuver.


RESULTS
Although most subjects were able to "relax" their lumbar extensors in full flexion, activity remained in the thoracic extensors and abdominals. Tissue load predictions suggested that while the lumbar extensor muscles were neurally "relaxed" (i.e., myoelectric silence), substantial elastic forces would assist the passive tissues in extensor moment support. On average, subjects sustained almost 3 kN in compressive load on the lumbar spine and about 755 N of anterior shear during full flexion with only 8 kg held in the hands.


CONCLUSIONS
The "relaxation" of lumbar extensor muscles appeared to occur only in an electrical sense because they generated substantial force elastically through stretching. Loading of the interspinous and supraspinous ligaments, in particular, was high relative to their failure tolerance.
McGill
S M
SM
Department of Kinesiology, Faculty of Applied Health Sciences, University of Waterloo, Ontario, Canada.
Kippers
V
V
D000328:Adult; D001696:Biomechanical Phenomena; D003198:Computer Simulation; D004576:Electromyography; D006801:Humans; D008159:Lumbar Vertebrae / Q000502:physiology*; D008161:Lumbosacral Region; D008297:Male; D008954:Models, Biological; D009119:Muscle Contraction / Q000502:physiology*; D009126:Muscle Relaxation / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D011187:Posture / Q000502:physiology; D016474:Weight-Bearing / Q000502:physiology