Study design: The electromyogram of the L1-L7 multifidus muscles of the in vivo cat were recorded while applying a prolonged steady displacement to the lumbar spine through the L4-L5 supraspinous ligament, simulating a moderate anterior flexion.
Objective: To demonstrate that tension-relaxation and laxity of the viscoelastic structures (ligaments, discs, and capsules) induced by prolonged static flexion of the spine results in loss of reflexive muscular stabilizing activity and in muscular disorders that may lead to or are associated with low back pain.
Summary of background: Epidemiologic data show that prolonged loading of the spine, such as in some occupational activities, can cause low back pain and muscle spasms. Direct experimental evidence linking prolonged loading to a decrease in spinal stability, low back pain, and muscle spasms was not found. It was hypothesized, however, that mechanoreceptors in the viscoelastic structures, when strained, reflexively activate the multifidus muscles to maintain intervertebral stability; that the reflexive muscular activity decreases with stress-relaxation and laxity in the viscoelastic structures; and that when severe strain and possible damage of the viscoelastic structures occurs with time, nociceptive receptors elicit spasms in the musculature and possible pain.
Methods: The lumbar spine of seven in vivo cat preparations was displaced through the L4-L5 supraspinous ligament into moderate flexion that was steadily maintained for 50 minutes while intramuscular electromyograms were recorded from each of the multifidus muscles of L1-L2 through L6-L7. Load and electromyogram were continuously monitored and recorded. Five additional preparations were used as controls, in which dissection and recordings were identical, but the lumbar flexion was excluded.
Results: Prolonged flexion of the lumbar spine resulted in initial reflexive electromyogram from the multifidus muscles that decreased to approximately 5% of its initial value as tension-relaxation began in the viscoelastic structures within the first 3 minutes, after which, random and unpredictable electromyogram discharges (i.e., spasms) of high amplitude were recorded from different levels. In some preparations the spasms were present in L1-L4, and in others in all the levels. In other preparations the spasms were recorded only at L5 and L6. The onset of the spasms was also unpredictable, because they were initiated in some cases within 2-3 minutes after the spine was loaded. In other cases, the spasms were observed anytime during the test period and up to 20 minutes after the load was removed. Spasms were also observed in the spinalis and longissimus muscles.
Conclusions: Prolonged flexion of the lumbar spine results in tension-relaxation and laxity of its viscoelastic structures, loss of reflexive muscular activity within 3 minutes and electromyogram spasms in the multifidus and other posterior muscles.