SPINE Volume 25, Number 22, pp 2916 –2924 ©2000, Lippincott Williams & Wilkins, Inc. Multifidus Spasms Elicited by Prolonged Lumbar Flexion Mathew Williams, BSc, Moshe Solomonow, PhD, MD (Hon), Bing He Zhou, EE, Richard V. Baratta, PhD, and Mitchel Harris, MD Study Design. The electromyogram of the L1–L7 mul- tifidus muscles of the in vivo cat were recorded while applying a prolonged steady displacement to the lumbar spine through the L4 –L5 supraspinous ligament, simulat- ing 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 asso- ciated 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 mus- cle spasms. Direct experimental evidence linking pro- longed loading to a decrease in spinal stability, low back pain, and muscle spasms was not found. It was hypoth- esized, however, that mechanoreceptors in the viscoelas- tic structures, when strained, reflexively activate the mul- tifidus muscles to maintain intervertebral stability; that the reflexive muscular activity decreases with stress-re- laxation and laxity in the viscoelastic structures; and that when severe strain and possible damage of the viscoelas- tic structures occurs with time, nociceptive receptors elicit spasms in the musculature and possible pain. Methods. The lumbar spine of seven in vivo cat prep- arations was displaced through the L4 –L5 supraspinous ligament into moderate flexion that was steadily main- tained for 50 minutes while intramuscular electromyo- grams 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 re- sulted in initial reflexive electromyogram from the multi- fidus muscles that decreased to approximately 5% of its initial value as tension-relaxation began in the viscoelas- tic structures within the first 3 minutes, after which, ran- dom 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. [Key words: electromyo- gram, ligament, low back pain, lumbar, multifidus, spasms spine] Spine 2000;25:2916 –2924 It is well established that the musculature associated with the lumbar spine is the primary structure responsible for its stability, whereas the passive viscoelastic structures (ligaments, discs, and capsules) function as secondary stabilizers. 5,7,9,16,21 The musculature and the viscoelastic tissues of the spine, however, function synergistically, so that the desired movement is accomplished while the sta- bility of the spine is preserved. 21 Recent evidence has shown that a spinal reflex arc exists from mechanorecep- tors in the ligaments, discs, and facet capsules to the lumbar multifidus and longissimus muscles in humans 28 and in fe- line 28,29 and porcine 13,14 models. Strain of the lumbar vis- coelastic structures was shown to excite the mechanore- ceptors within and reflexively contract the multifidus muscles of the distracted motion segment. Partial activa- tion of the multifidus muscles of two to three levels above and below was also observed. Such muscular forces pro- vide sufficient stiffness to prevent excessive displacement of several vertebrae relative to each other and the conse- quent possible injury. 28 More recently, Solomonow et al 27 and Gedalia et al 6 have shown that tension-relaxation and the associated laxity induced in the viscoelastic structures due to cyclic loading of the lumbar spine desensitize the mechanore- ceptors within. The resultant biexponential decrease of reflexive muscular activity of the multifidus leaves the spine without muscular protection against excessive in- tervertebral displacement. Adams et al 1 showed that creep in the viscoelastic structures was also present when the spine was subjected to steady load over time, as would be expected during prolonged anterior flexion while gardening or bricklay- ing, for example. They also observed that in human sub- jects, the musculature did not compensate for the lost stiffness (e.g., laxity) in the spine. The exact correlation between the load applied to the spine, the laxity induced, From the Bioengineering Laboratory, Department of Orthopaedic Sur- gery, Louisiana State University Medical Center, New Orleans, Loui- siana. This work was supported by Grant R01-0H-4709-01 from NIOSH. MW was a research medical student supported by the Department of Orthopaedic Surgery. Acknowledgment date: August 3, 1999. Acceptance date: January 13, 2000. Device status category: 1. Conflict of interest category: 14. 2916