TRANSFER OF LOADS BETWEEN LUMBAR TISSUES DURING THE FLEXION-RELAXATION PHENOMENON

Study Design and Methods. This study used an anatomically detailed mod el of the lumbar tissues, driven from biologic signals of vertebral di splacement and myoelectric signals, to estimate individual muscle and passive tissue force-time histories during the performance of the ''fl exion-relaxation'' maneuver. Eight male university students performed three trials each of the ''flexion-relaxation'' maneuver with six pair s of surface myoelectric electrodes monitoring the right side of the t runk musculature, an electromagnetic device to record lumbar flexion, and videotape to record body segment displacement. Objectives. To exam ine the loads on individual tissues during the transfer of moment supp ort responsibility from predominantly active muscle to predominantly p assive tissue. Summary of Background Data. No previous studies, to the authors' knowledge, have examined individual tissue loading during th e flexion-relaxation maneuver. Results. Although most subjects were ab le to ''relax'' their lumbar extensors in full flexion, activity remai ned in the thoracic extensors and abdominals. Tissue load predictions suggested that while the lumbar extensor muscles were neurally ''relax ed'' (i.e., myoelectric silence), substantial elastic forces would ass ist the passive tissues in extensor moment support. On average, subjec ts sustained almost 3 kN in compressive load on the lumbar spine and a bout 755 N of anterior shear during full flexion with only 8 kg held i n the hands. Conclusions. The ''relaxation'' of lumbar extensor muscle s appeared to occur only in an electrical sense because they generated substantial force elastically through stretching. Loading of the inte rspinous and supraspinous ligaments, in particular, was high relative to their failure tolerance.