• Share
  • Email
  • Embed
  • Like
  • Save
  • Private Content
The stabilizing system_of_the_spine_part_1
 

The stabilizing system_of_the_spine_part_1

on

  • 620 views

 

Statistics

Views

Total Views
620
Views on SlideShare
620
Embed Views
0

Actions

Likes
0
Downloads
28
Comments
0

0 Embeds 0

No embeds

Accessibility

Categories

Upload Details

Uploaded via as Adobe PDF

Usage Rights

© All Rights Reserved

Report content

Flagged as inappropriate Flag as inappropriate
Flag as inappropriate

Select your reason for flagging this presentation as inappropriate.

Cancel
  • Full Name Full Name Comment goes here.
    Are you sure you want to
    Your message goes here
    Processing…
Post Comment
Edit your comment

    The stabilizing system_of_the_spine_part_1 The stabilizing system_of_the_spine_part_1 Document Transcript

    • Reprinted from JOURNAL OF SPINAL DISORDERS & TECHNIQUESVol.5 No.4 August 1992Copyright @ L992 by Lippincott Williams & WilkinsPrinted in U.S.A. The Stabilizing System of the Spine. Part I. Function, Dysfunction, Adaptation, and Enhancement Manohar M. Panjabi Department of Orthopaedics and Rehabilitation, Yale University School of Medicine, New Haven, Connecticw U.S.A. Summary: Presented here is the conceptual basis for the assertion that the spinal stabilizing system consists ofthree subsystems. The vertebrae, discs, and ligaments constitute the passive subsystem. All muscles and tendons surrounding the spinal column that can apply forces to the spinal column constitute the active subsystem. The nerves and central nervous system comprise the neural subsystem, which determines the requirements for spinal stability by monitoring the various trans- ducer signals, and directs the active subsystem to provide the needed stability. A dysfunction of a component of any one of the subsystems may lead to one or more of the following three possibilities: (a) an immediate response from other subsys- tems to successfully compensate, (b) a long-term adaptation response of one or more subsystems, and (c) an injury to one or more components of any subsystem. It is conceptualized that the first response results in normal function, the second results in normal function but with an altered spinal stabilizing system, and the third leads to overall system dysfunction, producing, for example, low back pain. In situations where additional loads or complex postures are anticipated, the neural control unit may alter the muscle recruitment strategy, with the temporary goal of enhancing the spine stability beyond the normal requirements. Key Words: Spine stabilizing system-Spinal instability-Lumbar spine-Muscle function-Low back pain. Editors Comments of low back pain (38), it is not surprising that many of the present treatments are relatively ineffective. As co-editor of Journal of Spinal Disorders, I am Spinal instability is considered to be one of the im-delighted to ofer to our readership this elegant hy- portant causes oflow back pain but is poorly definedpothesis ofered by Panjabi. I encourage all ofyou to and not well understood (24). The basic concept ofread both the compelling article by Panjabi, and the spinal instability is that abnormally large interverte-articulate commentary by Krag. Such thoughtful dis- bral motions cause either compression and/or stretch-course will do much to enhance our understanding of ing of the inflamed neural elements or abnormal de-spinal stability. In addition, I hope that the areas of formations of ligaments, joint capsules, annularcontroversy will provide the impetusforfurther investi- fibers, and end-plates, which are known to have signif-gations. Manuscripts such as these will be published icant density ofnocioceptors (41). In both situations,on a periodic basis to offer the concepts, thoughts, and the abnormally large intervertebral motions may pro-ideas of recognized authorities who are involved in duce pain sensation.studying spinal disorders. Knutsson (19) was probably the first to propose a Dan M. Spengler, M. D. Low back pain is a well-recognized problem of the Address correspondence and reprint requests to Dr. M. M. Pan-lation and resulting in substantial social loss (2,I I,23, jabi, Department ofOrthopaedics and Rehabilitation, Yale Univer-37). Because the etiology is unknown for most types sity School of Medicine, New Haven, CT 06510, U.S.A. 383
    • 384 M. M, PANJABImechanical parameter as an indicator of spinal insta-bility: the retrodisplacement (anterior to posteriortranslation) of a vertebra observed on lateral radio-graphs while flexing the spine from the extended posi-tion. There is some recent evidence to support theseobservations of increased motion being related to lowback problems ( 12,20,40). Other studies have found amixed set of results. Decreased motion was found byPearcy et al. (34) and Dvorak et al. (8) in low backpain patients with degenerative changes in the spine.ln the same study, Dvorak et al. (8) reported increasedmotion in younger athletic patients with back pain. FlG, 1. The spinal stability system consists of three subsys- tems: passive spinal column, active spinal muscles, and neuralBoth hypo- and hypermobility of the spine as mea- control unit.sured by the range of motion without regard to thedirection of vertebral fiiovements, have been pro-posed by another hypothesis ofspinal instability (18). In addition to the abnormal magnitudes (larger or DESCRIPTION smaller than normal), the motion quality is another parameter. Abnormally large dispersion of the centers The basic biomechanical functions of the spinal of rotation during flexion, extension, and lateral system are (a) to allow movements between body parts, (b) to carry loads, and (c) to protect the spinal bending have been suggested as signs ofspinal instabil- ity, both in an in vitro model (35) and in low back cord and nerve roots (39). Mechanical stability of the pain patients (7). Seligman et al. (36), also using the spine is necessary to perform these functions and, concept of the center of rotation and experimental therefore, it is of fundamental significance to the hu- results of an in vitro study, suggested that the in- man body. First, the components of the spinal stabi- creased length of the path of the centers of rotation lizing system are presented, followed by descriptions during flexion/extension may be a predictor of spinal of its normal function, dysfunction, and enhanced instability. There may also be motion quality abnor- function. malities. Pearcy et al. (33) found coupled axial rota- The spinal stabilizing system is conceptualized as tions and lateral bending motions during flexion/ex- consisting of three subsystems (Fig. 1). The passive tension in low back pain patients as compared with musculoskeletal subsystem includes vertebrae, facet the control group. Similar observations concerning articulations, intervertebral discs, spinal ligaments, the coupled torques also have been made (32). and joint capsules, as well as the passive mechanical properties of the muscles. The active musculoskeletal Thus, there have been several attempts made in the past to relate the clinical problem oflow back pain to subsystem consists of the muscles and tendons an abnormality in intervertebral motion. Although surrounding the spinal column. The neural andfeed some useful information has been gathered, there are back subsysten consists of the various force and mo- contradictory observations and hypotheses. To make tion transducers, located in ligaments, tendons, and progress on this clinically important problem of spi- muscles, and the neural control centers These passive, active, and neural control subsystems, al- nal instability, new hypotheses must be developed, and, driven from the hypotheses, new types of motion though conceptually separate, are functionally information must be obtained. A better understand- interdependent. ing of the workings of the spinal stabilizing system may also be useful in this respect. Normal Function of the Spinal Stabilizing System In this part of a two-part article, the purpose is to present a group ofconcepts concerning the stabilizing The normal function of the stabilizing system is to system of the spine, including the normal function, provide sufficient stability to the spine to match the dysfunction, and adaptation/enhancement functions. instantaneously varying stability demands due to In the second part, the concept ofneutral zone is pre- changes in spinal posture, and static and dynamic sented. This kinematic parameter is hypothesized to loads. The three subsystems work togethil to achieve be a functional measure of the spinal stabilizing the goal as described in subsequent paragraphs andsystem. schematically shown in Fig. 2. J Spinal Disord Vol. 5, No. 4 1992
    • SPINE STABILIZING SYSTEM I 38s tension is measured and adjusted until the required stability is achieved. The requirements for the spinal stability and, therefore, the individual muscle ten- sions, are dependent on dynamic posture, that is, vari- ation of lever arms and inertial loads of different masses, and external loads. Dysfunction of the Spinal Stabilizing System Degradation of the spinal system may be due to injury, degeneration, and/or disease of any one of theFlG. 2. Functioning of the spinal stability system. The informa-tion from the (1) Passive Subsystem sets up specific (2) spinal subsystems (Fig. 3). The neural control subsystemstability requirements. Consequently, requirements for (3) individ- perceives these deficiencies, which may develop sud-ual muscle tensions are determined by the neuralcontrol unit. The denly or gradually, and attempts to compensate bymessage is sent to the (4) force generators. Feedback is providedby the (5) force monitors by comparing the (6) "achieved" and (3) initiating appropriate changes in the active subsys-"required" individual muscle tensions. tem. Although the necessary stability of the spine overall may be reestablished, the subsequent conse- quences may be deleterious to the individual compo- nents ofthe spinal system (e.g., accelerated degenera- The Passive ( Ligamentous) Subsystem tion of the various components of the spinal column, Components of the passive subsystem, (e.g., liga- muscle spasm, injury, and fatigue). Over time, thements) do not provide any significant stability to the consequences may be chronic dysfunction and pain.spine in the vicinity of the neutral position. It is to-ward the ends of the ranges of motion that the liga- Adaptation and Enhancement of thements develop reactive forces that resist spinal mo- Stabilizing Capacitytion. The passive components probably function inthe vicinity ofthe neutral position as transducers (sig- This ability of the spinal system to respond to dys-nal-producing devices) for measuring vertebral posi- function is one manifestation of its adaptability. Intions and motions, similar to those proposed for the addition, under circumstances of unusually demand-knee ligaments (3), and therefore are part of the ing loading conditions, there may be a functional re-neural control subsystem. Thus, this subsystem is pas- serve that can be called on to enhance spinal stabilitysive only in the sense that it by itselfdoes not generate beyond the normal level.or produce spinal motions, but it is dynamically ac-tive in monitoring the transducer signals. The Active (Musculotendenous) Subsystem The muscles and tendons of the active subsystemare the means through which the spinal system gener-ates forces and provides the required stability to thespine. The magnitude of the force generated in eachmuscle is measured by the force transducers built intothe tendons of the muscles. Therefore, this aspect ofthe tendons is part of the neural control subsystem. FlG.3. Dysfunction of the spinal stability system. (1) lnlury, de- generation and/or disease may decrease the (2) passive stability The Neural Control Subsystem and/or (3) active stability. (4) The neural control unit attempts to remedy the stability loss by increasing the stabilizing function of The neural subsystem receives information from the remaining spinal components: (5) passive and (6) active. Thisthe various transducers, determines specific require- may lead to (7) accelerated degeneration, abnormal muscle load- ing, and muscle fatigue. lf these changes cannot adequately com-ments for spinal stability, and causes the active sub- pensate for the stability loss, a (8) chronic dysfunction or pain maysystem to achieve the stability goal. Individual muscle develop. J Spinal Disord Vol. 5, No.4, 1992
    • 386 M. M. PANJABI DISCUSSION spinal loads. Thus, the deformations of soft tissues are capable of providing a comprehensive set of The main thrust of the proposed biomechanical signals from which stability requirements may beconcept of the stabilizing system of the spine is that determined.there are two musculoskeletal components and one In addition to the feedback provided by the liga-neural component. Under normal circumstances, ment deformation, instantaneous muscle tension iswithin the physiological ranges of spinal movements probably monitored by the muscle spindles and ten-and against normal spinal loads, the three subsystems don organs (9) and adjusted by the neural control unitare highly coordinated and optimized. Compensation in accordance with the requirements for stability.for dysfunction of the system, within certain limits, Thus, the normal function of the stabilizing system ofmay be provided by the system. If the dysfunction is the spine involves monitoring tissue deformationsbeyond these limits, then acute or chronic problems and selecting appropriate muscles and adjusting indi-may arise. In certain situations, the system may be vidual muscle tensions to accommodate changes inenhanced beyond the normal, if needed. physiological postures, spinal movements, and spinal loads. The spinal stabilizing system has been de- signed, developed, and optimized to achieve this goal. Normal Function The load-carrying capacity of the passive subsys- DYsfunctiontem, the so-called critical load of the spinal columnhas been determined by in vitro experiments (6,21). Any one or more of the subsystems may not func-They found that the spinal column specimens buck- tion appropriately, affecting the overall stability oftheled (became mechanically unstable) at a load of 20 N spinal system.(2 kg) and 90 N (9 kg), respectively, for the Tl-sacrumand L5-sacrum specimens. The normal loads on the The Passive SubsYstemspine provided by body mass alone in the standingposition (25) are many times larger, about two to The dysfunction of the passive subsystem may bethree times body weight (140-210 kg). Even bigger caused by mechanical injury such as overstretching ofloads may be expected under dynamic situations or the ligaments, development of tears and fissures in thefrom carrying external loads. This large load-carrying annulus, development of microfractures in the end-capacity is achieved by the participation of well-coor- plates, and extrusion of the disc material into the ver-dinated muscles surrounding the spinal column tebral bodies. The injury may result from overloadingThus, the importance of the active subsystem (the of a normal structure or normal loading of a weak-muscles) in providing the required stability is well ened structure. A structure may also be weakened byestablished. degeneration or disease. Degeneration of the disc is How does the spinal stabilizing system determine known to weaken it in resisting torsional loads (10)the forces needed from the muscles? What is the ini- There is some evidence to suggest that other multidi-tiating signal coming from the transducers located in rectional physical properties ofthe spine are also al-the passive subsystem? Probably it is the ligament de- tered by degeneration (29). In general, all these factorsformation (strain) and not the force (stress). This is decrease the load-bearing and stabilizing capacity ofbased on experimental observations from which we the passive subsystem. This may require compensa-know that all passive spines (cadaveric spine speci- tory changes in the active subsystem.mens devoid of musculature) exhibit measurable neu-tral zones, although the neutral zones are generally The Active SubsYstemlarger after degeneration (31) and injury Q7,28).Throughout the neutral zone, the reactive forces are The active musculoskeletal subsystem may developsmall. On the other hand, throughout the neutral deterioration ofits ability to receive and/or carry outzone, the deformations of the ligaments are large (3 1). the neural commands, to provide accurate feedbackThis leads to the hypothesis that the deformations in of muscle tension information to the neural controlthe ligaments provide a more useful feedback signal unit, or to produce coordinated and adequate musclethan do the forces for monitoring the requirements tensions; such deformation may result from disuse,for spinal stability. The stability requirements are also degeneration, disease, or injury. As a result, the stabi-dependent on the loads carried by the spine. Because lizing capacity of the spinal system may be decreasedthe ligaments deform under load, they can sense the This may compromise the capability of the system to J Spinal Disord Vol. 5, No. 4, 1992
    • SPINE STABILIZING SYSTEM I 387both provide compensatory help to the passive sub- tion in complex spinal motions involving lifting andsystem when needed, and to withstand unexpected twisting at the workplace.dynamic or abnormally large external loads. A neural control dysfunction may become chronic. This has been observed in studies conducted on pa- tients with spinal stenosis ( 14) and low back problems The Neural Subsystem (a). In the former study, the patients exhibited greater body sway after initiation of claudication. In the latter Dysfunction of the neural subsystem can also de- study, the low back patients, when challenged withvelop. To achieve the required stability at every in- the task of standing on an unbalanced platform withstance of time, the neural subsystem has the enor- eyes closed, had greater body sway compared with themously complex task of continuously and simulta- normal subjects performing the same task. One mayneously monitoring and adjusting the forces in each speculate that the control of the spinal stabilizing sys-ofthe muscles surrounding the spinal column. Instan- tem was permanently altered in the patients exam-taneous decisions must be made to redistribute the ined in both ofthese studies.muscle tensions, if there is a change in the postureand/or the external loads. The task is made much Adaptation and Enhancementmore complex if the posture and/or loads change dy-namically, requiring additional considerations for Either chronic dysfunction of components of anymasses, inertias, and accelerations involved. An appre- of the subsystems or increased functional demands onciation of the complexity of the task may be obtained them may lead to adaptive changes.by watching a sophisticated robot trying to walk ashort distance. The robot walks slowly, staggeringly, The Passive Subsystemand will easily topple if subjected to a sudden externalload, despite being controlled by high-performance, The muscular strength decreases in the later yearsstate-of-the-art computers. oflife. It has also been observed that the spinal col- One example of the kind of error that might occur umn stiffness increases in later years of life due tois that one or more muscles may fire in a manner that osteophyte formation and, possibly, facet hyper-is undesirable: too small or too large force and/or too trophy (10,15,18). The two phenomena may be re-early or too late firing. This may happen either due to lated; that is, with aging, the passive subsystem maythe faulty information transmitted from the spinal be attempting to compensate for the decreased stabi-system transducers or due to the fault ofthe control lizing ability of the active subsystem. In case theunit itself. Such an error may cause excessive muscle bodys own adaptive responses are not enough, thera-tension, resulting in soft tissue injury and pain. This peutic intervention in the form of surgical fusion andmay explain some of the instances of acute low back external bracing may be used as treatments designedpain initiations where negligible or maryinal loads are to enhance the spinal stability.involved (e.g., while picking up a piece of paper fromthe floor). Often such an incident may happen while The Active Subsystemperforming a complex maneuver (e.g., combined flex-ing, bending, and twisting), when the synchronizing A general increased muscle tone by training hascapability of the neural control subsystem may be ex- been shown to decrease the risk for development oftended to its maximum. Involvement of a heavy ex- low back problems (5). This may be explained on theternal load in such a case is not a requirement for basis ofenhanced stability ofthe spinal system in theproducing muscle injury and pain, but may further form of increased capacity to generate muscle ten-potentiate an injury. sion. Theoretical models of spinal stability have pre- In addition to damagingthe active subsystem, mus- dicted such an effect (i.e., increased spinal stabilitycle force errors might lead to overload of a passive due to increased muscle tension) (6,26). By using thestructure (e.g., disc). With the spine in an awkward contralateral knee as the control, a few clinical studiesposture, a single large overload has been shown experi- have documented the role of muscles in anterior cru-mentally to produce disc herniation ( l). One may also ciate ligament (ACl)-deficient knees. Hypertrophy ofexpect that an awkward maneuver that is repeated the involved side was found in patients who hadmany times (e.g., in work environment) would in- adopted best after the injury Q2). In another knee to occur. Kelsey et al.crease the chance for an error study, Giove et al. (13) found increased value of the(17) have documented increased risk for disc hernia- ratio of hamstring strength to quadricep strength to be J Spinal Disord Vol. 5, No.4, 1992
    • 388 M. M. PANJABIthe best indicator of successful rehabilitation of ACL- Strength fitness and subsequent back injuries in firefighters. "/deficient patients. Thus, strengthening of selective Occup Med 2l:269, 1979 6. Crisco JJ: The biomechanical stability of the human lumbarmuscle groups may compensate specific passive stabil- spine: experimental and theoretical investigations [Doctorality loss due to an injury. Dissertationl, New Haven, CT, Yale University, 1989 7. Dimnet J, Fischer LP, Gonon G, Carret JP: Radiographic stud- ies of lateral flexion in the lumbar spine. J Biomech I l:143- 150,1978 The Neural Subsystem 8. Dvorak J, Panjabi MM, Novotny JE, Chang DG, Grob D: Clinical validation of functional flexion-extension roentgeno- grams olthe lumbar spine. Spine 16:943-950, l99l Although there is no published work in spine litera- 9. Enoka RM: Neuromechanical Basis of Kinesiology, Cham-ture indicating enhancement of spinal stability by paign, IL, Human Kinetics Books 10. Farfan HF: Mechanical disorders of the low back, Philadelphia,changes in the neural control alone, certainly the pos- ka & Febigersibility exists. It is clear in many fields of endeavor 11. Frymoyer JW, Pope MH, Clements JH, et al.: Risk factors inthat training enhances the ability to perform complex low-back pain: an epidemiological study. J Bone Joint Surgmechanical tasks. It is hypothesizedthat if a specific 65A:213-218,1983 12. Gertzbein SD, Wolfson N, King G: The diagnosis of segmentalgroup of muscles responsible for a particular direc- instability in vivo by centrode length. Proceedings ofthe Inter-tional stability can be identified, then selectively and national Society for the Study of the Lumbar Spine, Miami,appropriately tensioning those muscles will enhance 1 988 13. Giove TP, Miller SJ, Kent BE, Sanford TL, Garrick JG: Non-the particular directional stability. Thus, on com- operative treatment of the lorn anterior cruciate ligament. "/mand from the control unit, spinal stability can be Bone Joint Surg 65A:184-192, 1983instantaneously increased. This strategy may be used t4. Hanai K, Ishii K, Nojiri H: Sway of the center of gravity in patients with spinal canal stenosis. Spine 13:1303-1307, 1988in situations where the application of external load to 15. Harris RI, Macnab I: Structural changes in the lumbar inter-the spinal system can be anticipated (e.g., dynamic vertebral discs. Their relationship to low back pain and sciat- ica. J Bone Joint Surg 368:304-322, 1954loads during weight-lifting or while catching a ball on 16. Ihara H, Nakayama A: Dynamic joint control training for kneethe football field). Again, there is some evidence in the ligament injuries. Am J Sports Med l4:309-315, 1986knee literature. Ihara and Nakayama (16) trained t7. Kelsey J, Githens P, White A, et al.: An epidemiologic study ofyoung female athletes with documented knee instabil- lifting and twisting on the job and risk for acute prolapsed lum- bar intervertebral disc. J Orthop Res 2:61-66, 1984ity by using unstable boards on which the athlete 18. Kirkaldy-Willis WH: Managing low back palz, New York,placed her foot and the therapist applied sudden dis- Churchill Livingstone, 1983turbance. After 3 months of such stability training, 19. Knutsson F: The instability associated with disk degeneration in the lumbar spine. Acta Radiol25:593-608, 1944there was a decrease in the muscle response time and 20. Lehman T, Brand R: Instability of the lower lumbar spine.resolution of knee instability problems. Proceedings of the International Society for the Study of the Lumbar Spine, Toronto, Canada, 1982 2l . Lucas DB, Bresler B: Stability of the ligamentous spine. Tech- Acknowledgment This work was supported by National nical Report esr. I I No. 40, Biomechanics Laboratory, Univer-Institutes of Health Grants AR3036l and AR39209. I thank sity of California at San Francisco, The LaboratoryRichard Brand, Joseph Crisco, Martin Krag, Thomas Ox- 22. McDaniel WJ, Dameron TB Jr: Untreated ruptures ofthe ante-land, Lourens Penning, and James Weinstein, who helped rior cruciate ligament. J Bone Joint Surg 62A:696-705, 1980sharpen my understanding of the problem, and have criti- 23. Morris A: Identifying workers at risk to back injury is not guesswork. Occup Health Safety 55:16-20, 1985cally read the manuscript and provided many valuable sug- 24. Nachemson A: Lumbar spine instability: a critical update andgestions. However, the responsibility for the ideas presented symposium summary. Spine 10:290-291, 1985is solely that of the author. 25. Nachemson A, Evans J: Some mechanical properties of the third human lumbar interlaminar ligament (ligamentum fla- vum). J Biomech l:2ll-220, 1968 26. Nolte LP, Panjabi M: Spinal stability and intersegmental mus- REFERENCES force-a mat hematical model, Kyoto, Japan, International cle Society lor the Study of the Lumbar Spine, 1989, p 80 l. Adams M, Hutton W: l98l Volvo Award in Basic Science. 27. Oxland TR, Panjabi MM: The onset and progression of spinal a hyperflexion injury. Spine Prolapsed intervertebral disc: instability: a demonstration of neutral zone sensitivity. "/ Bio- 7:184-r9r,1982 mech (in press) 2. Andersson GBJ, Pope MH, Frymoyer JW: Epidemiology. In: 28. Panjabi M, Abumi K, Duranceau J, Oxland T: Spinal stability Occupational low back pain, ed by MH Pope, JW Frymoyer, G and intersegmental muscle forces. A biomechanical model. Andersson, New York, Praeger, 1982, pp l0l-l 14 Spine 14:194-200, 1989 3. Brand RA: Knee ligaments: a new view. I Biomech Eng 29. Panjabi M, Goel V: Relationship between chronic instability 108:106-l 10, 1986 and disc degeneration. International Society for the Study of 4. Byl NN, Sinnqtt PL: Variations in balance and body sway in the Lumbar Spine, Toronto, Canada, 1982 middle-aged adults: subjects with healthy backs compared with 30. Panjabi MM, Duranceau JS, Oxland TR, Bowen CE: Multidi- subjects with low-back dysfunction. Spine 16:325-330, l99I rectional instabilities oftraumatic cervical spine injuries in a 5. Cady LD, BischoffDP, OConnel ER, Thomas PC, Allan JH: porcine model. ,Sprne 14:l l1l-l I15, 1989J Spinal Disord Vol.5, No.4, 1992
    • SPINE STABILIZING SYSTEM I 38931. Panjabi MM, Goel VK, Takata K: l98l Volvo Award in Bio- 36. Seligman J, Gertzbein S, Tile M, Kapasouri A: 1984 Volvo mechanics. Physiological strains in lumbar spinal ligaments. Award in Basic Science. Computer analysis of spinal segment An in vitro biomechanical study. Spine 7:192-203, 1982 motion in degenerative disc disease with and without axial32. Parnianpour M, Nordin M, Kahanovitz H, Frankel VH: The loading. Spine 9:566-57 3, 1984 triaxial coupling of torque generation of trunk muscles during 37. Spengler DM, Bigos SJ, Martin NA, et al.: Back injuries in isometric exertions and the effect of fatiguing isoinertial move- industry; a retrospective study. I. Overview and cost analysis. ments on the motor output and movem€nt pattems. 1988 Spine ll:241-245, 1986 Volvo Award in Biomechanics. Spine l3:982-992, 1988 38. White A, Gordon S: Synopsis: workshop on idiopathic low-33. Pearcy M, Portek I, Shepherd J: The effect oflow-back pain on back pain. Spine T :l4l-149, 1982 lumbar spinal movements measured by three-dimensional x- 39. White AA, Panjabi MMl Clinical biomechanics of the spine, ray analysis. Spine 10l.150-153, 1985 2nd ed, Philadelphia, JB Lippincott, 199034. Pearcy M, Shepherd J: Is there instability in spondylolisthesis? 40. Woody J, Lehman T, Weinstein J, Hayes M, Spratt K: The Spine lO:175-177, 1985 diagnosis ofsegmental instability in vivo by centrode length.35. Rofander SD: Mol ion of the lumbar spine with special reference Proceedings ofthe International Society for the Study ofthe to the stabilizing efect of posterior fusion, Goleborg, Sweden, Lumbar Spine, Miami, 1988 Department of Orthopaedic Surgery, University of Goteborg, 41. Wyke B: The neurological basis of thoracic spine pain. Riez- Tryckeri AB Litotyp, 1966 matol Phys Med 10:356, 1970 J Spinal Disord Vol.5, No.4, 1992