Segmental Instability of the Lumbar Spine

of 13 Update Segmental Instability of the Lumbar Spine Key Words: Back, Pain, Spine, Trunk. Author Information Julie M Fritz, PT, ATC, is Assistant Professor, Department of Physical Therapy, School of Health and Rehabilitation Sciences, University of Pittsburgh, 6035 Forbes Tower, Pittsburgh, PA 15260 (USA) (jmfst46+@pitt.edu). Address all correspondence to Ms Fritz. Richard E Erhard, DC, PT, is Assistant Professor, Department of Physical Therapy, School of Health and Rehabilitation Sciences, University of Pittsburgh, and Director of Physical Therapy and Chiropractic Services, Spine Specialty Center, University of Pittsburgh Medical Center, Pittsburgh, Pa. Brian F Hagen, PT, is Clinical Assistant Professor, Department of Physical Therapy, School of Health and Rehabilitation Sciences, University of Pittsburgh, and Director of Outpatient Orthopaedic Rehabilitation, CORE Network, Pittsburgh, Pa. [Fritz JM, Erhard RE, Hagen BF. Segmental instability of the lumbar spine. Phys Ther. 1998;78:889-896.] Numerous clinical findings are proposed to be diagnostic of lumbar segmental instability. Low back pain (LBP) is a pervasive problem in modern societies, but identifying a specific pathoanatomical cause is not possible in a majority of cases. 1 Patients for whom a specific pathoanatomical diagnosis cannot be made are often described as having "mechanical" LBP. Many researchers and clinicians suggest that segmental instability of the lumbar spine is a possible pathomechanical mechanism underlying mechanical LBP. 1 3 Segmental instability of the lumbar spine, however, remains a controversial and poorly understood topic. 4 The purpose of this update is to review the current literature on segmental instability of the lumbar spine, with special emphasis on Panjabi's theory of the "neutral zone" and how that relates to physical therapy. Basic Biomechanics of the Lumbar Spine The lumbar spine, although often described as a single functional unit, is composed of 5 vertebrae forming what are called "motion segments" connected in series. 5 Each motion segment consists of 2 adjacent vertebral bodies and the connecting ligaments. 6 Spinal motion segments represent the smallest segments of the spine exhibiting biomechanical characteristics similar to those of the entire spine. 5 Translation and rotation can occur at each spinal motion segment. 5 Translation occurs when a shear force causes one vertebra to move parallel to the adjacent vertebra. Rotation is the spinning of one

of 13 vertebra about a stationary axis relative to the adjacent vertebra caused by a torque. Translation and rotation occur at each motion segment during lumbar spine movements in any of the cardinal body planes. 7 For example, lumbar flexion involves anterior translation and rotation, and lumbar extension involves posterior translation and rotation of each lumbar motion segment in the sagittal plane. 7,8 The maintenance of stability of the lumbar spine during movements, therefore, requires the coordinated movements of multiple motion segments, and a lack of stability may potentially occur at any lumbar segment in either translational or rotational movements, or both. 9 Definitions of Clinical Instability Segmental instability occurs when an applied force produces displacement of part of a motion segment exceeding that found in a normal spine. 10,11 Several researchers 5,12 14 have defined normal segmental motion in the lumbar spine. Most initial experiments were performed in vitro on cadaveric lumbar spines. Based on the results of these studies, the most commonly cited thresholds for segmental instability were (1) a sagittal-plane translation of at least 3 mm, or 9% of the vertebral body width, on either a flexion or extension radiograph, and (2) a sagittal-plane rotation of greater than 9 degrees for the lumbar motion segments between L1 and L5. 11 13 More recently, researchers 4,14 17 have studied the motion characteristics of the lumbar spinal segments in vivo by taking radiographs of subjects with the spine in neutral, flexed, and extended positions. These researchers 4,15,17 found large variability in the segmental motions of individuals without LBP. Hayes et al 17 found that 42% of subjects without LBP had at least one lumbar motion segment with at least 3 mm of sagittal-plane translation. Based on the results of in vivo studies, some authors 5,18 have recommended that the criteria for diagnosing segmental instability be increased to 4 to 4.5 mm, or 10% to 15% of vertebral body width, of sagittal-plane translation. Thresholds for rotational instability, according to White and Panjabi, 5 are greater than 15 degrees at L1 to L4, greater than 20 degrees at L4-5, and greater than 25 degrees at L5-S1. The range of motions observed in motion segments of persons with LBP illustrates the difficulty in defining segmental instability strictly in terms of increased joint laxity. The frequent disparity between joint laxity and the development of symptoms has been described in other joints. Snyder-Mackler et al 19 studied 20 patients with anterior cruciate ligament deficiencies and found no correlation between joint laxity and functional ability. They suggested that other factors, such as neuromuscular control, may influence the relationship between joint laxity and symptom development. A similar phenomenon may occur in the spine, with certain individuals unable to compensate for an excessive amount of joint laxity and with other individuals with equal amounts of laxity able to "cope" without substantial pain and disability. The rotational and translational movement criteria for diagnosing clinical instability have been established to identify patients requiring surgical fixation, but these criteria may not be adequate for detecting more subtle forms of segmental instability that can cause pain and disability. 20 Definitions of segmental instability based solely on clinical findings have been proposed. 21 23 Segmental instability has been defined as occurring in patients with low back problems whose clinical status is unstable, with symptoms fluctuating between mild and severe symptoms in response to even minor provocations. 22 Paris 23 defined instability as existing only when sudden aberrant motions such as a visible slip or catch are observed during active movements of the lumbar spine or when a change in the relative position of adjacent vertebrae is detected with palpation performed with the patient in a standing position versus palpation performed with the patient in a prone position. The validity of these clinical definitions has not been demonstrated. Frymoyer et al defined segmental instability as "a condition where there is loss of spinal stiffness, such that normally tolerated external loads will result in pain, deformity, or place neurological structures at risk." 24(p224)

of 13 A more recent theoretical premise of segmental instability using a "neutral zone" concept has been proposed by Panjabi. 25 The neutral zone concept is based on the observation that the load-displacement curve of the typical spinal motion segment is highly nonlinear, with high flexibility for motion occurring around the neutral position of the spine and with increased passive resistance to motion nearer to the end-ranges of spinal motion. 26 The total range of motion (ROM) of a spinal motion segment, therefore, may be divided into 2 zones: a neutral zone and an elastic zone. The neutral zone is the initial portion of the ROM during which spinal motion is produced against minimal internal resistance. The elastic portion of the ROM is the portion nearer to the end-range of movement that is produced against substantial internal resistance. 25 The neutral zone is the zone of high spinal flexibility, whereas movements occurring in the elastic zone encounter increased internal resistance to movement. An increase in the size of the neutral zone relative to the total ROM, therefore, increases the amount of laxity present and increases the demands on the stabilizing systems of the spine. 25 In vitro studies 25,27,28 suggest that an increase in the size of the neutral zone may be a better indicator of segmental instability than an increase in total ROM. Segmental instability, therefore, may be defined as a decrease in the capacity of the stabilizing system of the spine to maintain the spinal neutral zones within physiological limits so that there is no neurological deficit, no major deformity, and no incapacitating pain. 25 Although further research is needed, this definition of segmental instability may prove to be useful because it describes the quality of motion throughout the ROM instead of relying solely on total ROM values for diagnosis. Clinical methods for quantitatively assessing the size of the neutral zone have not been developed, but this definition, emphasizing the quality as opposed to the quantity of motion, appears to fit the clinical observation that many patients with suspected segmental instability have greater difficulty moving in the mid-ranges of spinal motion than at the end-ranges. In addition, because muscle activity can affect the size of the neutral zone, 29 the influence of factors such as muscle activity and neuromuscular control can be accounted for within this definition. The Stabilizing System of the Spine The stabilizing system of the spine must limit the excursion of spinal motion segments and maintain the proper ratio of neutral to elastic zone motion. 25 Panjabi 30 conceptualized the stabilizing system of the spine as consisting of 3 subsystems: (1) passive, (2) active, and (3) neural control. The functions of these 3 subsystems are interrelated, and reduced function of one subsystem may place increased demands on the other subsystems to maintain stability. 31,32 The Passive Subsystem The passive subsystem consists primarily of the vertebral bodies, zygapophyseal joints and joint capsules, spinal ligaments, and passive tension from the musculotendinous units. 30 The passive subsystem plays its most important stabilizing role in the elastic zone of spinal ROM (ie, near endrange). 33 The relative contributions of structures to segmental stability has been investigated by serially cutting the structures 32,34 and through mathematical modeling experiments. 33,35 The posterior ligaments of the spine (interspinous and supraspinous ligaments) along with the zygapophyseal joints and joint capsules and the intervertebral disks are the most important stabilizing structures when the spine moves into flexion. 35,36 End-range extension is stabilized primarily by the anterior longitudinal ligament, the anterior aspect of the annulus fibrosus, and the zygapophyseal joints. 32,34 Rotational movements of the lumbar spine are stabilized mostly by the intervertebral disks and the zygapophyseal joints. 37 Side-bending movements have not been studied extensively, but it appears that the

of 13 intertransverse ligaments may play an important role in segmental stability for movement occurring in the frontal plane. 33 In the neutral zone of ROM, the structures of the passive subsystem may function as force transducers, sensing changes in position and providing feedback to the neural control subsystem. 30,33,38 Evidence for this role is provided by anatomical observations of afferent nerve fibers capable of conveying proprioceptive information in most of the structures of the passive subsystem, including the intervertebral disks, the zygapophyseal joint capsules, and the interspinous and supraspinous ligaments. 31,38 Injury to the passive subsystem may have important implications for spinal stability. Intervertebral disk degeneration or disruption of the posterior ligaments of the spine may increase the size of the neutral zone, increasing the demands on the active and neural control subsystems to avoid the development of segmental instability. 25,29 The Active Subsystem The active subsystem of the spinal stabilizing system consists of the spinal muscles and tendons. The active and neural control subsystems are primarily responsible for spinal stability in the neutral zone, where passive resistance to movement is minimal. 30,34 In experiments performed with the musculature removed, the lumbar spine is known to be highly unstable at very low applied loads, attesting to the importance of muscle activity for spinal stability. 39 The relative importance of different muscle groups in providing stability for the lumbar spine has been a topic of much debate and research. 40 44 Differing roles have been suggested for the deeper, unisegmental muscles and the more superficial multisegmental muscles such as the abdominal and erector spinae muscles. 40 The unisegmental muscles of the lumbar spine, such as the intertransversarii and interspinalis muscles, are proposed to function primarily as force transducers, providing feedback on spinal position and movements to the neural control subsystem. 30 Evidence for this role is provided by the small size of these muscles, their close proximity to the center of rotation for spinal movements, and the high concentration of muscle spindles located in the smaller, unisegmental muscles of the body. 7,45 The larger, multisegmental muscles are responsible for producing and controlling movements of the lumbar spine. Lifting and rotational movements have been studied most extensively because these are tasks frequently performed by the lumbar spine. The lumbar erector spinae muscle group provides most of the extensor force required for most lifting tasks. 46 Rotation is produced primarily by the oblique abdominal muscles. 41 The oblique abdominals and the majority of the lumbar erector spinae muscle fibers lack direct attachment to the lumbar spinal motion segments and, therefore, are unable to exert forces directly on individual motion segments. The multifidus muscle is better suited for the purpose of segmental control. 47 This muscle originates from the spinous processes of the lumbar vertebrae and forms a series of repeating fascicles attaching to the inferior lumbar transverse processes, the ilium, and the sacrum. The multifidus muscle is proposed to function as a stabilizer during lifting and rotational movements of the lumbar spine. 47 Stability of the lumbar spine during movements in the frontal plane has not been studied extensively, but the quadratus lumborum muscle has been proposed to be the primary active stabilizer for these movements. 48 The role of the abdominal muscles in spinal stability has been the topic of much debate. The abdominals have been proposed to play an important role in generating extensor force during lifting tasks, either by increasing intra-abdominal pressure or by creating tension in the lumbodorsal fascia. 42,49 Research

of 13 indicates that the abdominal muscles are not capable of generating substantial extensor force through these mechanisms. 43,50 The abdominal muscles are primarily flexors and rotators of the lumbar spine. 41 The oblique abdominal and transversus abdominis muscles, with their more horizontal orientation, are thought to contribute to spinal stability by creating a rigid cylinder around the spine and by increasing the stiffness of the lumbar spine. 44,51 This theory is supported by studies demonstrating continuous activity of the transversus abdominis muscle throughout flexion and extension movements of the lumbar spine. 52 The Neural Control Subsystem The neural control subsystem is thought to receive input from structures in the passive and active subsystems in order to determine the specific requirements for maintaining spinal stability, then acting through the spinal musculature to stabilize the spine. 30,44,53 Dysfunction in the neural control system may place other spinal structures at risk for injury. 30 If proper functioning of the neural control system is not restored following an injury, the potential for reinjury may be heightened. 53 No evidence exists linking poor neuromuscular control with increased risk of an initial injury to the lumbar spine. Several studies 44,54 57 have shown that patients with LBP often have persistent deficits in neuromuscular control, indicating that recovery of proper function of the neural control subsystem is not automatic following an initial injury. Studies 54 56 have demonstrated increased postural sway and slower reaction times in patients with LBP when compared with subjects without LBP. Luoto et al 54 found that improvements in reaction time correlated with reduced disability in patients undergoing rehabilitation. These results support the hypotheses that neuromuscular control deficits often exist following lumbar spine injury and that reduction in these deficits correlates with improvements in functional status. The neural control system may play an important role in stabilizing the spine in anticipation of an applied load. Hodges and Richardson 44,57 reported that the transversus abdominis and multifidus muscle activity consistently precedes active extremity movement in subjects without LBP. This finding suggests that the neural control system normally anticipates the need for stabilization against the reactive forces from limb movements. In a study of patients with LBP, 44 the contraction of the transversus abdominis muscles was delayed, possibly indicating deficient neural control. Further research is needed to clarify the role of neural control in patients with LBP, but these preliminary findings indicate that enhancing neural control may be an important consideration in the prevention and rehabilitation of LBP. Diagnosing Spinal Instability In 1944, Knuttson 58 described a method for diagnosing segmental instability by taking lateral radiographs of the lumbar spine with the patient performing a maximum active extension while standing and a maximum active flexion while sitting. The amount of sagittal-plane translation and rotation were then calculated from the 2 radiographs and compared with criteria defining segmental instability. Flexion-extension radiographs have now become the standard by which segmental instability is diagnosed. 5,12,14 Variations in the exact technique used, large variability in the motion characteristics of individuals without LBP, and high false-positive rates using the established criteria, however, have caused many authors 17,27 to question the usefulness of flexion-extension radiographs. Friberg 59 described an alternative diagnostic method in which radiographs are taken during axial

of 13 compression while the patient supports a weight on the shoulders and then during traction while the patient hangs from a bar. Anteroposterior translation occurring between the 2 positions was reported by Friberg to be more accurate in detecting abnormal movement in patients suspected of having segmental instability, 59 but more recent work has questioned the accuracy of this technique. 60 Other authors 20 contend that a decrease in symptoms with bracing provides evidence of segmental instability. This technique is limited by the inability of most braces to effectively immobilize the spine, 61 and data have not been reported for a large group of patients. Olerud et al 62 used an external fixation device to achieve immobilization and reported that patients experiencing relief with this technique were more likely to achieve favorable results with spinal fusion. The invasiveness of this technique, however, precludes its use with most patients. Diagnosis of lumbar segmental instability may also be based on examination findings or the patient's history. Delitto et al 63 believe that confirmatory data for segmental instability include frequent recurrences of LBP precipitated by minimal perturbations, deformity (eg, lateral shift) in prior episodes of LBP, short-term relief from manipulation, a history of trauma, use of oral contraceptives, or an improvement of symptoms with the use of a brace in previous episodes of LBP. Some authors 23,64 contend that the presence of a "step-off" between the spinous processes of adjacent vertebrae felt with palpation or increased mobility with passive intervertebral motion testing are indicative of instability. The reliability of these techniques, however, has been questioned, and their validity has not been demonstrated. 65 Other authors 22,23 have emphasized that aberrant motions such as the "instability catch" occurring during active ROM testing of the motion indicate instability. The instability catch has been described by numerous authors 22,66,67 as a sudden acceleration or deceleration of movement, or a movement occurring outside of the primary plane of motion (eg, side-bending or rotation occurring during flexion), and is proposed to indicate segmental instability. Its presence, however, has never been related to symptoms or abnormal movements in diagnostic imaging studies. A diagnostic "gold standard" has not been identified for segmental instability. A criticism of current diagnostic imaging approaches is their inability to capture segmental spinal movements in the midranges of spinal motion, where aberrant motions such as the instability catch are most likely to be observed. 68 Flexion-extension radiographs taken at the end-range of movement will capture only the function of the passive stabilizing subsystem and fail to address the active and neural control subsystems. Because of the importance of motor control in spinal stability, some authors 20,69 believe that electromyographic techniques may hold more promise in diagnosing segmental instability, but clinically useful techniques have yet to be described. Treatment of Segmental Instability Patients with segmental instability resulting in severe disability may be considered candidates for lumbar spinal fusion. 18 The rate of lumbar spinal fusion is increasing rapidly in the United States, despite the fact that indications for the procedure are uncertain, costs and complication rates are higher than for other surgical procedures performed on the spine, and long-term outcomes are uncertain. 70 72 Fusion for segmental instability is usually reserved for patients with severe symptoms and radiographic evidence of excessive motion (greater than 4 mm translation or 10 o of rotation) who fail to respond to a trial of nonsurgical treatment. 18 The use of braces or corsets has been recommended in the treatment of persons with segmental

of 13 instability. 63,73 Spratt et al 73 reported on a treatment program consisting of patient education, extension exercises, and bracing to prevent flexion in a group of patients with radiographic evidence of segmental instability. The treatment program was effective in reducing pain. The design of the study, however, did not allow for an evaluation of the efficacy of bracing alone. Patient education may be an important component in the nonsurgical treatment of patients with segmental instability. Education should focus on avoiding end-range movements of the lumbar spine to avoid positions that may overload the passive stabilizing structures of the spine. Patients should be made aware that lifting even light loads from a position near the end-range spinal flexion can create potentially damaging forces in the passive stabilizing structures of the spine. 35,48 Patients also should be made aware of the importance of maintaining muscle strength and endurance, particularly in the muscles of the lumbar spine. Fatigue can adversely affect the ability of the spinal muscles to respond to imposed loads, 74 and general strengthening programs have been shown to be effective in patients with chronic LBP. 75 The physical therapy treatment for segmental instability often focuses on exercises designed to improve stability of the spine. Several muscle groups have been identified in the literature as potentially playing an important role in stabilizing the spine. 46,47,76 78 The lumbar erector spinae muscles are the primary source of extension torque for lifting tasks; therefore, strengthening this muscle group has been advocated. 46,79 Callaghan et al 79 studied erector spinae muscle activity and imposed loads on the lumbar spine during a variety of exercises thought to strengthen the back extensor muscles. Exercises performed with the patient in a quadruped position, including single-leg extension and contralateral arm and leg extension exercises, provided sufficient challenge to the erector spinae muscles without producing a high load on the lumbar spine. 79 Active trunk extension exercises performed with the patient in a prone position produced high levels of activity in the erector spinae muscles, but also imposed a substantial load on the lumbar spine, which may make these exercises a contraindication in the rehabilitation process. 79 The abdominal muscles, particularly the transversus abdominis and oblique abdominals, and the multifidus muscle have been proposed to play an important role in stabilizing the spine by cocontracting in anticipation of an applied load. 77,80 The multifidus muscle, because of its segmental attachments to the lumbar vertebrae, may be able to provide segmental control, particularly during lifting and rotational motions. 47 Exercises targeting these muscle groups, therefore, may be desirable. Richardson and Jull 78 described an exercise program that proposes to retrain the co-contraction pattern of the transversus abdominis and multifidus muscles. The exercise program is based on training the patient to draw in the abdominal wall while isometrically contracting the multifidus muscle. This cocontraction exercise is then performed in a variety of postures. 78 O'Sullivan et al 80 recently reported the results of a randomized trial comparing this exercise program with a program of general exercise (swimming, walking, gym exercises) in a group of patients with chronic LBP. The stabilization exercise group had less pain and functional disability following a 10-week treatment program than the general exercise group, a difference that was maintained at a 30-month follow-up. 80 Exercises proposed to address the abdominal muscles in an isolated manner usually involve some type of curl-up maneuver. McGill 76,81 found that dangerously high compressive and shear forces are imposed on the lumbar spine during both straight and bent-knee curl-up exercises. A horizontal sidesupport exercise has been recommended as an alternative to curl-ups for strengthening the abdominal muscles. 76 This exercise is performed with the patient side-lying and the upper body supported by the

of 13 elbow to create a side-bending of the spine. The patient then lifts the pelvis off the support surface to a position in line with the shoulders, eliminating the side-bending. This exercise provides a substantial challenge to the oblique abdominal muscles without imposing high compressive or shear loading forces on the lumbar spine. 76 In addition, the horizontal side-support exercise challenges the quadratus lumborum muscle, which may be an important spinal stabilizer for movements occurring in the frontal plane. 48 Exercises designed to challenge the neural control subsystem also have been recommended in the treatment of persons with segmental instability. 54 Effective exercises for achieving this outcome have not been identified. The use of unstable surfaces such as therapy balls have been recommended for this purpose. The efficacy of such an approach has not been investigated in the rehabilitation of patients with LBP, but similar approaches have been found to be effective in the treatment of patients with knee joint instability secondary to rupture of the anterior cruciate ligament. 82 Conclusion The concept of lumbar segmental instability is receiving increased interest from researchers and clinicians alike as a potential pathomechanical mechanism in patients with LBP. At present, much controversy exists regarding the proper definition of the condition, the best diagnostic methods, and the most efficacious treatment approaches. Further research is needed to clarify these issues. References 1 Nachemson AL. Advances in low-back pain. Clin Orthop. 1985;200:266 278. 2 Kaigle AM, Holm SH, Hansson TH. Experimental instability in the lumbar spine. Spine. 1995;20:421 430. 3 Panjabi MM. Low back pain and spinal instability. In: Weinstein JN, Gordon SL, eds. Low Back Pain: A Scientific and Clinical Overview. Rosemont, Ill: American Academy of Orthopaedic Surgeons; 1996:367 384. 4 Dvorak J, Panjabi MM, Chang DG, et al. Functional radiographic diagnosis of the lumbar spine: flexion-extension and lateral bending. Spine. 1991;16:562 571. 5 White AA, Panjabi MM. Clinical Biomechanics of the Spine. 2nd ed. Philadelphia, Pa: JB Lippincott Co; 1990:23 45. 6 Panjabi MM, White AA III. Basic biomechanics of the spine. Neurosurgery. 1980;7:76 93. 7 Bogduk N. Clinical Anatomy of the Lumbar Spine and Sacrum. 3rd ed. New York, NY: Churchill Livingstone Inc; 1997:67 79. 8 Pearcy M, Portek I, Shepherd J. Three-dimensional x-ray analysis of normal movement in the lumbar spine. Spine. 1984;9:294 297. 9 Frymoyer JW, Selby DK. Segmental instability: rationale for treatment. Spine. 1985;10:280 286. 10 Pope MH, Panjabi MM. Biomechanical definitions of spinal instability. Spine. 1985;10:255 256.

of 13 11 Morgan FP, King T. Primary instability of lumbar vertebrae as a common cause of low back pain. J Bone Joint Surg Br. 1957;39:6 21. 12 Posner I, White AA III, Edwards WT, Hayes WC. A biomechanical analysis of the clinical stability of the lumbar and lumbosacral spine. Spine. 1982;7:374 389. 13 Nachemson AL, Schultz AB, Berkson MH. Mechanical properties of human lumbar spine motion segments: influence of age, sex, disc level, and degeneration. Spine. 1979;4:1 8. 14 Dupuis PR, Yong-Hing K, Cassidy JD, Kirkaldy-Willis WH. Radiologic diagnosis of degenerative lumbar spinal instability. Spine. 1985;10:262 276. 15 Boden SD, Wiesel SW. Lumbosacral segmental motion in normal individuals: Have we been measuring instability properly? Spine. 1990;15:571 576. 16 Pitkanen M, Manninen H. Sidebending versus flexion extension radiographs in lumbar spinal instability. Clinical Radiography. 1994;49:109 114. 17 Hayes MA, Howard TC, Gruel CR, Kopta JA. Roentgenographic evaluation of lumbar spine flexionextension in asymptomatic individuals. Spine. 1989;14:327 331. 18 Sonntag VKH, Marciano FF. Is fusion indicated for lumbar spinal disorders? Spine. 1995;20 (suppl):138s 142S. 19 Snyder-Mackler L, Fitzgerald K, Bartolozzi AR III, Ciccotti MG. The relationship between passive joint laxity and functional outcome after anterior cruciate ligament surgery. Am J Sports Med. 1997;25:191 195. 20 Boden SD, Frymoyer JW. Segmental instability: overview and classification. In: Frymoyer JW, ed. The Adult Spine: Principles and Practice. 2nd ed. Philadelphia, Pa: Lippincott-Raven Publishers; 1997:2137 2155. 21 Farfan HF, Gracovetsky S. The nature of instability. Spine. 1984;9:714 719. 22 Kirkaldy-Willis WH, Farfan HF. Instability of the lumbar spine. Clin Orthop. 1982;165:110 123. 23 Paris SV. Physical signs of instability. Spine. 1985;10:277 279. 24 Frymoyer JW, Akeson W, Brandt K, et al. Clinical perspectives. In: Frymoyer JW, Gordon SL, eds. New Perspectives on Low Back Pain. Rosemont, Ill: American Academy of Orthopaedic Surgeons; 1989:217 248. 25 Panjabi MM. The stabilizing system of the spine, part II: neutral zone and instability hypothesis. J Spinal Disord. 1992;5:390 396. 26 Panjabi MM, Oxland TR, Yamamoto I, Crisco JJ. Mechanical behavior of the human lumbar and lumbosacral spine as shown by three-dimensional load-displacement curves. J Bone Joint Surg Am. 1994;76:413 424. 27 Panjabi MM, Lydon C, Vasavada A, et al. On the understanding of clinical instability. Spine.

of 13 1994;19:2642 2650. 28 Oxland TR, Panjabi MM. The onset and progression of spinal injury: a demonstration of neutral zone sensitivity. J Biomech. 1992;25:1165 1172. 29 Panjabi MM, Abumi K, Duranceau J, Oxland TR. Spinal stability and intersegmental muscle forces: a biomechanical model. Spine. 1989;14:194 199. 30 Panjabi MM. The stabilizing system of the spine, part I: function, dysfunction, adaptation, and enhancement. J Spinal Disord. 1992;5:383 389. 31 Indahl A, Kaigle AM, Reikeras O, Holm SH. Interaction between the porcine lumbar intervertebral disc, zygapophysial joints, and paraspinal muscles. Spine. 1997;22:2834 2840. 32 Haher TR, O'Brien M, Dryer JW, et al. The role of the lumbar facet joints in spinal stability: identification of alternative paths of loading. Spine. 1994;19:2667 2670. 33 Panjabi MM, Goel VK, Takata K. Physiologic strains in the lumbar spinal ligaments: an in vitro biomechanical study. Spine. 1982;7:192 203. 34 Sharma M, Langrana NA, Rodriguez J. Role of ligaments and facets in lumbar spinal stability. Spine. 1995;20:887 900. 35 McGill SM. Estimation of force and extensor moment contributions of the disc and ligaments at L4- L5. Spine. 1988;13:1395 1402. 36 Adams MA, Hutton MC, Stott JRR. The resistance to flexion of the lumbar intervertebral joint. Spine. 1980;5:245 253. 37 Farfan HF, Cossette JW, Robertson GH, et al. The effects of torsion on the lumbar intervertebral joints: the role of torsion in the production of disc degeneration. J Bone Joint Surg Am. 1970;52:468 497. 38 Jiang H, Russell G, Raso J, et al. The nature and distribution of human supraspinal and interspinal ligaments. Spine. 1995;20:869 876. 39 Nachemson A, Evans J. Some mechanical properties of the third human lumbar interlaminar ligament (ligamentum flavum). J Biomech. 1968;1:211 220. 40 Crisco JJ III, Panjabi MM. The intersegmental and multisegmental muscles of the lumbar spine: a biomechanical model comparing lateral stabilizing potential. Spine. 1991;16:793 799. 41 Macintosh JE, Pearcy MJ, Bogduk N. The axial torque of the lumbar back muscles: torsion strength of the back muscles. Aust N Z J Surg. 1993;63:205 212. 42 Gracovetsky S, Farfan HF, Helleur C. The abdominal mechanism. Spine. 1985;10:317 324. 43 Tesh KM, Dunn JS, Evans JH. The abdominal muscles and vertebral stability. Spine. 1987;12:501 508.

of 13 44 Hodges PW, Richardson A. Inefficient muscular stabilization of the lumbar spine associated with low back pain. Spine. 1996;21:2640 2650. 45 Peck D, Buxton DF, Nitz A. A comparison of spindle concentrations in large and small muscles acting in parallel combinations. J Morphol. 1984;180:243 252. 46 Bogduk N, Macintosh JE, Pearcy MJ. A universal model of the lumbar back muscles in the upright position. Spine. 1992;17:897 913. 47 Macintosh JE, Bogduk N. The biomechanics of the lumbar multifidus. Clinical Biomechanics. 1986;1:205 213. 48 McGill SM, Juker D, Kropf P. Quantitative intramuscular myoelectric activity of quadratus lumborum during a wide variety of tasks. Clinical Biomechanics. 1996;11:170 172. 49 Bartelink DL. The role of abdominal pressure in relieving the pressure on the lumbar intervertebral discs. J Bone Joint Surg Br. 1957;39:718 725. 50 McGill SM, Norman RW. Potential of lumbodorsal fascia forces to generate back extension moments during squat lifts. J Biomed Eng. 1988;10:312 318. 51 Gardner-Morse MG, Stokes IAF. The effects of abdominal muscle coactivation on lumbar spine stability. Spine. 1998;23:86 91. 52 Cresswell AG, Grundstrom H, Thorstensson A. Observations on intra-abdominal pressure and patterns of abdominal intra-muscular activity in man. Acta Physiol Scand. 1992;144:409 418. 53 Gardner-Morse MG, Stokes IAF, Laible JP. Role of muscles in lumbar spine stability in maximum extensor efforts. J Orthop Res. 1995;13:802 808. 54 Luoto S, Taimela S, Hurri H, et al. Psychomotor speed and postural control in chronic low back pain patients: a controlled follow-up study. Spine. 1996;21:2621 2627. 55 Luoto S, Hurri H, Alaranta H. Reaction times in patients with chronic low-back pain. Eur J Phys Med Rehabil. 1995;5:47 50. 56 Nies N, Sinnott PL. Variations in balance and body sway in middle-aged adults: subjects with healthy backs compared with subjects with low-back dysfunction. Spine. 1991;16:325 330. 57 Hodges PW, Richardson CA. Contraction of the abdominal muscles associated with movement of the lower limb. Phys Ther. 1997;77:132 142. 58 Knutsson F. The instability associated with disk degeneration in the lumbar spine. Acta Radiol. 1944;25:593 609. 59 Friberg O. Lumbar instability: a dynamic approach by traction-compression radiography. Spine. 1987;12:119 129. 60 Pitkanen M, Manninen HI, Lindgren KA, et al. Limited usefulness of traction-compression films in the radiographic diagnosis of lumbar spinal instability: comparison with flexion-extension films. Spine.

of 13 1997;22:193 197. 61 Fidler MW, Plasmans CM. The effect of four types of support on the segmental mobility of the lumbosacral spine. J Bone Joint Surg Am. 1983;65:943 947. 62 Olerud S, Sjostrom L, Karlstrom G, Hamberg M. Spontaneous effect of increased stability of the lower spine in cases of severe chronic back pain: the answer of an external transpeduncular fixation test. Clin Orthop. 1986;203:67 74. 63 Delitto A, Erhard RE, Bowling RW. A treatment-based classification approach to low back syndrome: identifying and staging patients for conservative treatment. Phys Ther. 1995;75:470 485. 64 Maitland GD. Vertebral Manipulation. 5th ed. Oxford, England: Butterworth Heinemann; 1986:74 76. 65 Maher CG, Adams R. Reliability of pain and stiffness assessments in clinical manual lumbar spine examination. Phys Ther. 1994;74:801 809. 66 Nachemson A. Lumbar spine instability: a critical update and symposium summary. Spine. 1985;10:290 291. 67 Ogon M, Bender BR, Hooper DM, et al. A dynamic approach to spinal instability, part II: hesitation and giving-way during interspinal motion. Spine. 1997;22:2859 2866. 68 Pope MH, Frymoyer JW, Krag MH. Diagnosing instability. Clin Orthop. 1992;279:60 67. 69 Ogon M, Bender BR, Hooper DM, et al. A dynamic approach to spinal instability, part I: sensitization of intersegmental motion profiles to motion direction and load condition by instability. Spine. 1997;22:2841 2858. 70 Deyo RA, Ciol MA, Cherkin DC, et al. Lumbar spinal fusion: a cohort study of complications, reoperations, and resource use in the Medicare population. Spine. 1993;18:1463 1470. 71 Katz JN. Lumbar spinal fusion: surgical rates, costs, and complications. Spine. 1995;20(suppl):78S 83S. 72 Turner JA, Ersek M, Herron L, et al. Patient outcomes after lumbar spinal fusions. JAMA. 1992;268:907 911. 73 Spratt KF, Weinstein JN, Lehmann TR, et al. Efficacy of flexion and extension treatments incorporating braces for the low-back pain patients with retrodisplacement, spondylolisthesis, or normal sagittal translation. Spine. 1993;18:1839 1849. 74 Wilder DG, Aleksiev AR, Magnusson ML, et al. Muscular response to sudden load: a tool to evaluate fatigue and rehabilitation. Spine. 1996;21:2628 2639. 75 van Tulder MW, Koes BW, Bouter LM. Conservative treatment of acute and chronic nonspecific low back pain: a systematic review of randomized controlled trials of the most common interventions. Spine. 1997;22:2128 2156.

of 13 76 McGill SM. Distribution of tissue loads in the low back during a variety of daily and rehabilitation tasks. J Rehabil Res Dev. 1997;34:448 458. 77 Hides JA, Richardson CA, Jull GA. Multifidus muscle recovery is not automatic after resolution of acute, first-episode low back pain. Spine. 1996;21:2763 2769. 78 Richardson CA, Jull GA. Muscle control-pain control: What exercises would you prescribe? Manual Therapy. 1995;1:2 10. 79 Callaghan JP, Gunning JL, McGill SM. The relationship between lumbar spine load and muscle activity during extensor exercises. Phys Ther. 1998;78:8 18. 80 O'Sullivan PB, Twomey LT, Allison GT. Evaluation of specific stabilizing exercise in the treatment of chronic low back pain with radiologic diagnosis of spondylolysis or spondylolisthesis. Spine. 1997;22:2959 2967. 81 McGill SM. The mechanics of torso flexion: sit-ups and standing dynamic flexion manoeuvres. Clinical Biomechanics. 1995;10:184 192. 82 Beard DJ, Dodd CAF, Trundle HR, et al. Proprioception enhancement for anterior cruciate ligament deficiency: a prospective randomised trial of two physiotherapy regimes. J Bone Joint Surg Br. 1994;76:654 659. Copyright 1998 by the American Physical Therapy Association. Requests for reprints should be directed to the corresponding author of the article. Students and other academic customers may receive permission to reprint copyrighted material from Physical Therapy by contacting the Copyright Clearance Center Inc, 222 Rosewood Dr, Danvers, MA 01923. Similar inquiries by all others should be made to the APTA Editorial Office, Attn: Physical Therapy.