Biomechanics of increased exposure to lumbar injury caused by cyclic loading - Part 2. Recovery of reflexive muscular stability with rest

Study Design. Electromyographic responses from the lumbar multjfidus muscle of the cat were recorded in vivo during 50 minutes of cyclic loading follo wed by 2 hours of rest. Objective. To determine the rate of recovery of reflexive muscular stabiliz ing activity resulting from rest after viscoelastic laxity induced by 50 mi nutes of cyclic loading. Summary of Background Data. Muscular forces from agonists and antagonists w ere repeatedly shown to be the most significant stabilizing structures of t he lumbar spine. Reflexive muscular coactivation force from the multifidus muscle elicited by mechanoreceptors in the spinal viscoelastic structures w ere, however, shown to diminish drastically with the onset of laxity in the viscoelastic structures. Data describing the rate of recovery of reflexive muscular coactivation forces resulting from rest after cyclic loading were not found. Methods. Cyclic loading of the lumbar spine at 0.25 Hz was applied to L4-L5 for 50 minutes while electromyograms from the multifidus muscles of L1-L2 to L6-L7 were recorded, A rest period of up to 2 hours was given,during whi ch electromyographic responses and load were measured every 10 minutes to s ample recovery of laxity and reflexive muscular activity. Results. Load and electromyographic response demonstrated an exponential de crease during the 50 minutes of cyclic loading. The first 10 minutes of res t allowed a significant recovery in laxity and muscle activity, with additi onal slow recovery over the next 20 to 30 minutes. The electromyographic re sponse and load were increasing at an extremely slow rate thereafter. Overa ll, 2 hours of rest yielded only a 20% to 30% recovery in electromyographic response. Full recovery was never observed; A biexponential model was deve loped to predict loss and recovery of reflexive muscular activity and visco elastic tension with laxity. Conclusions. Laxity in the viscoelastic structures of the lumbar spine dese nsitizes the mechanoreceptors within and causes loss of reflexive stabilizi ng forces from the multifidus muscles. The first 10 minutes of rest after c yclic loading results in fast partial recovery of muscular activity. Howeve r, full recovery is not possible even with rest periods twice as long as th e loading period, placing the spine at an increased risk of instability, in jury, and pain.