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 followed by 2 hours of rest. Objective. To determine the rate of recovery of reflexive muscular stabilizing activity resulting from rest after viscoelastic laxity induced by 50 minutes of cyclic loading. Summary of Background Data. Muscular forces from agonists and antagonists were repeatedly shown to be the most significant stabilizing structures of the lumbar spine. Reflexive muscular coactivation force from the multifidus muscle elicited by mechanoreceptors in the spinal viscoelastic structures were, 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 which electromyographic responses and load were measured every 10 minutes to sample recovery of laxity and reflexive muscular activity. Results. Load and electromyographic response demonstrated an exponential decrease during the 50 minutes of cyclic loading. The first 10 minutes of rest allowed a significant recovery in laxity and muscle activity, with additional slow recovery over the next 20 to 30 minutes. The electromyographic response and load were increasing at an extremely slow rate thereafter. Overall, 2 hours of rest yielded only a 20% to 30% recovery in electromyographic response. Full recovery was never observed; A biexponential model was developed to predict loss and recovery of reflexive muscular activity and viscoelastic tension with laxity. Conclusions. Laxity in the viscoelastic structures of the lumbar spine desensitizes the mechanoreceptors within and causes loss of reflexive stabilizing forces from the multifidus muscles. The first 10 minutes of rest after cyclic loading results in fast partial recovery of muscular activity. However, full recovery is not possible even with rest periods twice as long as the loading period, placing the spine at an increased risk of instability, injury, and pain.