Experimental Aging Research

ISSN: 0361-073X (Print) 1096-4657 (Online) Journal homepage: http://www.tandfonline.com/loi/uear20

Biomechanical Aspects of Low-Back Pain in the Older Worker D. B. Chaffin & J. A. Ashton-Miller To cite this article: D. B. Chaffin & J. A. Ashton-Miller (1991) Biomechanical Aspects of Low-Back Pain in the Older Worker, Experimental Aging Research, 17:3, 177-187 To link to this article: http://dx.doi.org/10.1080/03610739108253896

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Expcnmrniul Aging Rrsrurcli, Volume 17, Number 3, 1991, ISSN 0734-0664 ri3 1991 Beech Hill Enterpri5es Inc.

Biomechanical Aspects of Low-Back Pain in the Older Worker

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D. B. CHAFFIN’ AND J. A. ASHTON-MILLER* The University of Michigun

This paper reviews 92 papers related to the biomechanics of low-back pain and potential risk factors associated with age and heavy manual work. T h e functional anatomy of the lumbar region is reviewed with special emphasis on tissue changes caused hy normal aging. A brief explanation of the biomechanics associated with load lifting, twisting, and bending tasks common to manual work in industry is given. It is argued that certain work conditions may present a special risk to an older individual’s low back. The need for further research to test hypothesized risk factors believed t o be associated with the older worker is indicated.

S

trehler has defined the aging process as “universal, decremental, progressive, and intrinsic” (cited in Goldman, 1970). We all incur functional and structural losses over time, particularly to our neuromusculoskeletal systems. These losses are innate to our genetic design and are not necessarily pathologic (Menard & Stanish, 1989). Indeed, many of the degenerative changes in the spine that we shall review in this paper should not be thought of as pathological, but rather as normal reductions i n physiological capacities which occur with time in a healthy organism. The contrast to this rather dismal state of nature (i.e.. normal age-related degenerative changes) is the reality that certain types of physical activity can either improve musculoskeletal function (i.e., provide a training stimuli) or create pathology (i.e., result in a broad array of inflammatory and degenerative over-use syndromes and injuries). The lower back is often cited as the part of the musculoskeletal system where both age-related tissue degeneration, injury, and pain are most prevalent. Whether the physiological changes and symptomatology are the result of normal aging of the lower back, or are the result of too much biomechanical stress over time is of immense importance when planning manual tasks to be performed by an aging workforce. This paper begins with brief reviews of the epidemiology of low-back pain (LBP) and functional anatomy of the lumbar spine. Some of the documented effects of age on the various tissues that comprise the lower back are

then presented, followed by a description of the biomechanical effects of common work-oriented physical activity on spinal structures. A description is given of various strategies used in industry to prevent low-back pain from developing. The paper ends with a set of research issues that must be resolved to reduce the risk of low-back pain in older workers.

Epidemiologv of Low-Back Pain Studies in Scandinavia, Israel, Netherlands, Japan, United States, and the United Kingdom, as summarized in Pope, Frymoyer, and Anderson (1984), indicate that low-back pain (LBP) is very prevalent and costly. When individuals under the age of 50 are asked if they currently are suffering LBP about 12% to 15% of the people sampled answer affirmatively. Lifetime prevalence of LBP is between 65% and 75% for individuals 60 yrs or older. About twice as many women as men, age 60 or older, report that they are currently suffering from low-back pain (Biering-Sorenson, 1982). A study in a large plant in New York reported that LBP was second only to upper respiratory illness in terms of medical absence (Rowe, 1969) and was more prevalent if the workers were engaged in heavy manual labor. Though most LBP is considered idiopathic (i.e., of unknown origin) epidemiological studies have indicated that heavy lifting, which engenders increased stresses on the low back, is associated with an %fold increase in the frequency rate of medically treated low-back pain

The authors wish t o acknowledge the assistance o f Pat Terrcll in preparing this manuscript. and the support of NIH grant ROI AR39599. ’ Professor. Departments o f Industrial and Operations Engineering and Environmental and Industrial Health, and Director, Center for Ergonomics. The University of Michigan, Ann Arbor, MI 48109-21 17. Research Scientist. Biomechanics Laboratory. Department o f Mechanical Engineering and Applied Mechanics, The University of Michigan. Ann Arbor. MI 48109-2125.

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(Chaffin ti Park, 1973). Also, frequent twisting and lifting of loads has been shown to increase the risk of LBP by Kelsey et al. (1984), Punnett (Punnett, Fine, Kcyserling, Herrin, & Chaffin, 1991), and Burton (Burton, Tillotson, & Troup, 1989) even when the load being lifted is relatively light. In general, it is conceded that low-back injury is the reason for two compensable injury medical claims per 100 workers in the United States each year (Kelsey & White, 1980). In a Finnish study (Biering-Sorenson, 1982), 63% of those suffering lowback pain had changed jobs because of it. Epidemiological studies indicating that the frequency and severity of low-back pain was greater in older workers, when performing the same physical tasks as younger cohorts, could not be found. Perhaps this is because of the difficulty of performing a study requiring both ;I careful matching ofjob task and worker attributes within ;in appropriate developmental design. Without such ;I study, however, only circumstantial evidence can he presented to indicate that age, when combined with certain types of biomechanical stress on the back, acts synergistically to raise the risk over that ascribed to cither attribute individually. Such circumstantial evidence relies on an understanding of the anatomy and biomechanics of the lower back, which is briefly presented in the following section. Functional Anatomy

The musculoskeletal system of the human trunk is comprised of the spine, rib-cage and pelvis together with the associated fascia, ligaments and musculature. In this review we shall concentrate on the lumbar region of the trunk, which includes all hard and soft connective tissues lying between the LI and Sl vertebral levels of the spine. The lumbar spine is chosen because it is the region most often injured in industrial work, compared to the thoracic or cervical regions, and because it is subjected to thc greatest mechanical stress when performing many different types of occupational tasks. The skeletal components of interest include the five lumbar vcrtebrae and sacrum which are interconnected by an intervertebral disc and six intervertebral ligaments. A spine motion segment or functional spinal unit is usually defined as two adjacent vertebrae with intact intervening soft tissues. Figure 1 depicts the major components of a motion segment. Bilateral zygoapophyscal or facet joints at each lumbar level help constrain intervcrtebral motions in certain directions, while allowing ;I few degrees of motion in other directions. The load-displacement behavior of a passive structure determines how much relative motion occurs under a given load. When stiffness (i.e., the ratio of load divided by displaccment) is high, the relative motion is small and vice vcrsa. For a flexible structure like the spine, stiffness us~iallyallows a reasonable range of motion (see for example. Hayes, Howard, Arnel, & Kopta. 1989). I t should not be so fexible, however, as to allow too much relative motion, or strain(s) in the disc and ligaments can

SUPFIASPINOUS

INTERSPINOUS

'FACET JOINT

FIGURE 1. A simplified lateral view of a spinal motion

segment showing internal disc structure in cut-away. reach magnitudes which can be deleterious, producing injury, disc protrusions (Harada & Nakahara.. 1089)7 instability, or even failure of the motion segment. In-vitro tests have documented that the overall static stiffnesses of lumbar motion segments generally arc about 700 N/mm in axial compression, 1-200 Nimm in anterior, posterior or lateral shear, 57-1 15 Nmirad i n flexion, extension and lateral bending and 390 frlm/rad in axial torsion (Berkson, Nachemson, & Schultz, 1079: Schultz, Warwick, Berkson, & Nachemson, 1079; Miller, Schultz, & Andersson, 1987). McGlashen, Miller, Schultz, and Andersson (1987) have found that the facet joints normally provide 50% of the resistance to motion in shear and bending and approximately 66% of the resistance to axial torsion. A consistent finding in all in-vitro studies has been the reporting of large interindividual variations in mechanical properties. Typically this variation is as large as the differences due to age or sex (Nachemson, Schultz, & Berkson, 1979). So, pre'diction of individual spine mechanical properties or functional capacity on the basis of age alone is fruitle this variation means that the same spine load can result in soft-tissue strain levels that can differ by up to an order of magnitude between similarly-aged individuals. Whether or not symptoms result depends on 21 largc number of anatomic and biologic factors. In-vivo tests of spinal range of motion can bc. measured by non-invasive methods (see Merritt, McLean, Erickson, & Offord, 1986), as well as by more accurate radiological techniques. Table 1 summarizes rad iological data for the lumbar region. The lumbar spine permits very little extension movement. Note that the lower levels differ from upper levels in that they are noticeably stiffer in lateral bending. Also, the lumbosacral joint (LS-S1) exhibits decreased flexion, but increased cxtcnsion relative to the rest of the lumbar spine.

Effects of Age on the Low-Back Structures Age affects the spine in two ways. Age can affect the geometry of structures such as the vertebral body, intervertebral disc and intervertebral ligaments, mainly through degenerative changes such as the development of osteophytes, calcification, etc. Secondly, it can affect the biomechanical properties of structures, sometimes

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TABLE 1

Average Range of Motion of the Healthy Lumbar Spine Motion Segments*

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Flexion/ Extension 8"/5" 10'13 12"Il" 13"/2" 9"/5"

Lateral Bending

Axial Torsion

6" 6" 6" 3" 1"

1" 1" 2" 2" 1"

*Data from Pearcy, Portek, & Shepard, 1984; Pearcy & Tibrewal, 1984.

altering their stiffness, strength, and energy dissipation to the point that the very integrity of these structures can be compromised under given loads. For example, the average decrements in cartilage, bone, and ligament strengths approach 30, 20 and 18%, respectively, between the third and eighth decades (Yamada, 1970). In the elderly, changes in spine configuration include loss of vertebral body height (Twomey & Taylor, 1987), increased thoracic kyphosis, and decreased lumbar lordosis (Milne & Lauder, 1974). These changes are partly caused by a loss in disc height which can occur with disc degeneration, but also by an increased curvature of the vertebral endplates (Ericksen, 1978; Twomey & Taylor, 1987), most likely due to osteoporotic changes in the cancellous bone of the vertebral bodies. Significant decreases have been found in lumbar ranges of motion with age (Fitzgerald, Wynveen, Rhealt, & Rothschild, 1983; Einkauf, Gohdes, Jensen, & Jewell, 1987; Korpi, Poussa, & Heliovaana, 1988). Vertebral Bodies

The vertebral body derives its considerable compressive strength from its trabecular bone core, not its cortical bone shell (Rockoff, Sweet, & Bleustein, 1969). Hansson, Roos, and Nachemson (1980) determined that the static compressive strength of a lumbar vertebra averaged about 4000 newtons (N) but showed a large variation between people. Also noted, has been an increase in compressive strength at the lower segments of the structure (i.e., increasing about 380 N per level from L1 to L4). Brinckmann, Biggeman, & Hilweg (1989) found that most of this increase is due to the increase in area of the vertebral body rather than an increase in trabecular bone density. They reported that bone density is surprisingly constant throughout the thoracic and lumbar spine of an individual. The fact that females' vertebrae had lower failure loads than males' was found primarily to be due to their smaller overall cross-sectional area. Brinckmann and co-workers have been able to estimate vertebral body compressive strength in-vivo to within 1000 N, using CT scanning techniques

to quantify the endplate load-bearing areas and bone densities. Osteoporosis is one of the better known changes that occur in the aging spine. It manifests itself as decreased trabecular bone density within the vertebral body. Osteoporosis is most common in post-menopausal women. However, in a study comparing men and women over 50 years, Finsen (1988) found the prevalence of back pain to be similar up to the 70-79 year group, whereafter it was higher in women. Those with excessive height loss (2 cm) or kyphosis, had a high prevalence of back pain, presumably due to advanced osteoporosis. The precise structures responsible for this pain remain unknown. Spondylolysis and spondylolisthesis are structural changes associated with the posterior elements of the vertebral body, in particular the laminae. These changes have been attributed to fatigue fractures of the laminae (Cyron & Hutton, 1978), due to repetitive loads on the facet joints caused by extreme and dynamic extension, torsion, and lateral bending. For example, spondylolysis is known to be four times more common in female gymnasts (Jackson, Wiltse, & Civincione, 1976) and in college athletes (Hoshina, 1980), than in the general population (Roche & Rowe, 1952). The endurance of the vertebral body to repetitive compression loads has been found to be surprisingly low. In vitro tests have shown that the probability of a fatigue failure in 5000 cycles of compression loads rises from 36% at a 30 - 40% compressive load level, to 92% at a 60-70% load level. Even 10 cycles at the latter load level produced an 8% probability of a vertebral endplate fracture (Brincknann, Biggeman, & Hilweg, 1988). Plane radiographs and CT scans can give useful information on changes occurring with age although such changes have not been shown to increase the symptomatology. Changes occurring in the anterior portion of the body include the development of osteophytes, loss of trabecular bone density, changes in vertebral body bone marrow (Modic, Steinberg, Ross, Masaryk, & Center, 1988), excessive bowing of the vertebral endplate, as well as crush fractures and endplate fractures (Harada & Nakahara, 1989; McFadden & Taylor, 1989). Posterior changes include stenosis or narrowing of the neural foramen, thickening of the ligamentum flavum (Twomey & Taylor, 1988), spondylolysis and spondylolisthesis laminae, arthrosis in the apophyseal joints, and erosion of the spinous processes. Stenosis, which can severely inhibit mobility, is probably caused by increased loading of the posterior elements as a result of decreased load-sharing by the disc due to degeneration.

Intewertebra 1 Disc The intervertebral disc, the largest avascular structure in the human body, is comprised of an outer annulus fibrosus surrounding the nucleus pulposus. The intervertebral disc acts as a flexible spacer between the adjacent semi-rigid vertebrae. The annulus consists of ten or

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more concentric lamellae, each about 1 mm thick, reinforccd with parallel collagen fibers. Adjacent lamelIne have fibers oriented at alternating angles to the long axis of the spine. Moving from the periphery, Type I collagen decreases while Type I1 increases (Eyre & Muir, 1976) and the inner lamellae become progressively niore indistinct as they merge with the nucleus pulposus. I n the young, the nucleus is a gelatinous substance consisting of a strongly hydrophilic proteoglycan gel enmeshed in a collagen matrix (Urban & McMullin, 1980). Nutrition of the disc occurs by diffusional mechanisms from blood vessels in the inner 40% of the vertebral endplate, and external to the annulus fibrosus (Urban, Holm, & Maronda, 1978). Disc degeneration normally starts in the second decade of life and continues steadily thereafter. This is evident macroscopically (Miller, Schmatz, & Schultz, I 9 X X ) ;IS well as in MR scans (DeCandido, Reinig, Dwyer, Thompson, & Ducker, 1988). Macroscopic degeneration begins in the nucleus and spreads to the annulus. By the fifth decade 97% of all lumbar discs show macroscopic degeneration, with changes in male discs occurring significantly earlier than in females (Miller et al., I9KX). Hult (1954) observed significantly more degenerative changes in the spines of workers involved in heavy labor than those performing lighter work. So, degenerative changes can be fostered by the placement of large loads on the spine. Various biophysical (Urban et al., 1978; Urban & McMullin, 1986, 1988) and magnetic resonance imaging (Modic et al., 1988) studies have shown that the proteoglycans gradually lose their ability to bind water and that the nucleus pulposus becomes progressively replaced by tibrocartilage with age. The mechanisms for this are unknown. Plane radiographs and CT scans are used to visualize the loss in disc height with age as well as the advent of osteophytes and ossified disc herniations. Disc degeneration, when part of a normal aging process, appears to be unrelated to pain or symptoms in itself. Surprisingly, disc degeneration by itself does not affect disc mechanical stiffnesses (Nachemson et al., 1970). The exception, of course, is when osteophytes bridge the disc space and cause dramatic decreases in motion t o an applied load (e.g., Moroncy, Schultz, Miller, & Andcrsson, 1988). Koeller, Muehlhaus, Meier, and Hartman ( 1986) found little effect of age on compres4ive stitfness of the disc but did find an increase in creep (rate of shrinkage under load) of elderly specimens. The resulting loss in disc height can lead to facet joint impingement (Yang & King, 1984) and secondary adaptationnl changes in the neural arch and facet joints (Gotfried, Bradford, & Oegema, 1986).

I n general, clinical and experimental evidence suggests that dcgencration in the facet joints lags behind t h a t of the disc (c.g., Gotfried et al., 1986). Currently, it

is surmised that disc degeneration leads to a temporary

decrease in disc stiffness which, in turn, results in larger loads being placed upon the apophyseal joints. ‘This may be a contributing factor to their degeneration. Degencrative changes in these joints parallel those in other synovial joints. Erosion of cartilage, geometric changes of the underlying bone, and changes in the form of the joint can occur (Twomey & Taylor, 1986), setting the stage for inflammatory reactions with increased activity and, possibly, resulting back pain. Spinal Ligaments Age related changes include calcification of these ligaments with concomitant changes in their elilsticity, residual deformation and energy dissipation of, for example, 7%, 28%, and 2296, respectively, betwcen the second and seventh decades in the anterior and pojterior spinal ligaments (Tkaczuk, 1968). Neural Stmctiires Little research has been conducted on aging of the neural structures that control low-back muscle function. Certainly, a loss of motor and sensory cells has been documented with age. This loss may account for slowing in tapping rates, diminished sensory levels and re action times (Katzman & Terry, 1983). In particular, it was found that the cervical muscle stretch reflex times. as measured by EMG onset after muscle jerk, decreased by approximately 9% in a group of subjects between 62 and 74 years of age compared to 18 to 24 year old subjects (Snyder, Chaffin, & Schutz, 1975). Similar results were shown by Spirduso (1980) for both simple and choice reaction times. Loss in vibration sense, and the ability to stand on one leg with the eyes closed was particularly acute with age according to Potvin (Potvin, Syndulko. Tourtellotte, Goldberg, Potvin, & Hansch, 1% I ). More research is needed into age related changes in nerves, nerve roots, and the cauda equina to increased pressure and decreased vascularization associated with stenosis and entrapment syndromes (c.g., Olmarker, Rydevik, Holm, & Bagge, 1989). Muscle Strength arid Endurance I t is well accepted that muscle strength and endurance are affected by both strenuous physical activity and age. Based on cross-sectional studies of large population groups, general muscle strength decreases after age 30, by 18% to 40% by age 65 (Shephard, 1978; Laubiich. 1976; Karvonen et al., 1980; Asmussen & Heeb@IINielsen, 1962). Presumably this decrease can be attributed to decrements in the number and size of muscle fibers, a loss in the number of active motor uiiits comprising skeletal muscles, as well as changes in the contractibility of muscle (Brooks & Faulkner, 1988). In a study of isometric lifting strengths demonstrated by workers, Chaffin, Herrin, and Keyserling (19‘78) found that torso strengths had decreased at age SO by an

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average of 30% compared to that of 20 year olds. Troup, Foreman, Baxter, and Brown (1987) showed that torso strengths do not decrease by age 50 years in active men, providing that people who have suffered from previous low-back pain are removed from groups being tested. In fact, in a healthy, active population, the decrease in muscle strength appears to be slight up to age 50, but then begins to accelerate, according to Larsson, Gimby, and Karlsson (1979). In a longitudinal study of 39 active men and 44 active women, with an average age of 55 years, torso strengths decreased between 9% and 10% for women, and between 16% to 22% for men during a 3.5 year follow-up period (NygPrd, Luopajarvi, & Ilmarinen, 1988). The authors argue that there may be a faster loss of trunk strength after age 50 than previously reported in cross-sectional studies. The ability to sustain or repeat submaximal contractions (i.e., a muscle’s endurance) does not appear to be as dependent on age as on habitual activity level, according to Simonsen (1971) and Larsson and Karlsson (1978). Biomechanical Aspects of Physical Work on Low-Back Pain Contrary to some popular notions, acts of manual lifting, stooping, and carrying of loads are not being replaced by automation and robotics. Few industries have the necessary organization, or the technology does not exist, to allow such displacement of workers. A report by the National Institute for Occupational Safety and Health in 1981 estimated that one-third of the workforce performs tasks in their jobs that require significant physical effort (NIOSH, 1981). In this same report, lifting is the single most cited cause of occupational low-back pain, with pushing and pulling objects, twisting, stooping, and slipping and falling also being cited. This section briefly presents some of the known biomechanical principles that define how physical exertions can cause excessive low-back stress.

Simple Load Lifting Biomechanics To understand the effects of load lifting and carrying on the low back, it is necessary to envision the musculoskeletal system as a set of articulated rigid links. Figure 2 displays such a linkage system developed to evaluate lifting in the sagittal plane. In this particular model, a static analysis is made, and hence it is referred to as a Static Sagittal Plane (SSP) biomechanical model by the researchers at The University of Michigan. In the SSP model, the mass and size of various body segments are determined from anthropometric data, and can be scaled to specific population strata. Postures demonstrated by individuals when lifting or pushing/ pulling on loads are recorded from videotapes or photographs of workers on the job, along with measurements of the weight and/or hand forces required by a task.

FIGURE 2. The static sagittal plane (SSP) biomechanical model, as derived by Chaffin and Park, 1973.

Newtonian mechanical principles are used to predict the moments (torques) at each joint that result from combinations of a person’s body weight, postures, and hand forces as measured during the performance of a particular job. The methods for performing this type of analysis along with details regarding the model are described by Chaffin and Anderson (1991). During the lifting of a load in front of the body the posterior lower back muscles must exert forces that are often 10 to 20 times larger than the load being lifted. The reason for this is that the load on the hands and body weight acting forward of the low back region cause large bending moments to exist (i.e.. the spinal column develops a strong tendency to flex forward). The posterior muscles of the low back (primarily the erector spinae muscles) act to resist the external flexion moment by contracting. Their contractile force (FMLl5(), however, often acts at a mechanical disadvantage by acting through much shorter moment arms (M) than the moment arms (H) and (B) for force on the hand (FH) and body (BW),

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respectively. ‘Therefore, the muscle force must be large to balance the bending moments caused by the external forces. Figure 3 displays the major components in such ;in analysis. When the muscle forces become large because of task requirements. they create a high compression (and on the spinal motion sometimes shear) force (FccjM,,) scgmcnts. As discussed earlier in this paper, this compression force is mainly resisted by the discs and vertebral bodies. If these disc compression forces become too large, t hc undcrlying trabecular bone and endplates of the vertebral bodies can develop small microfractures, which could result in accelerated degeneration of the disc by obstructing one of its major nutritional pathways. Figure 4 depicts the predicted spinal compression forces a t the LSISI disc when lifting various magnitudes of weight at different distances from the body. The National Institute for Occupational Safety and Health limits are also shown, based on the failure values discussed earlier in this paper. These limits depict compression forces wherein some cadaver motion segments failed (i.e., the Action Limit) and the force levels t h a t caused the majority to fail (Maximal Permissible Limit). From such an analysis one can see that for some individuals if a load is large, if it must be lifted from the floor, or if i t is lifted with a large H (horizontal) distance then the spinal compression forces become large enough to c;iusc microtrauma in the lumbar motion segments.

FIGURE 3.A simplified biomechanical model of the low back for evaluation of load lifting (from: Chaffin & Andersson,

1 984).

f

A

I

0

K m r o o # 0 4 9 0 ~ LOAD OW w o s “1

FIGURE 4. Effect of load and load location on disc compression forces compared to NlOSH disc failure limits (C:haffin& Andersson, 1984).

The recent work of Brinckmann et al. (1988), which was presented earlier in this paper, has further emp hasized the role of repetitive loading in increasing the probability of microtrauma in the lumbar motion sef,Imcnts. Their studies have indicated that under repetitive loads the vertebral body trabecular bone and/or endplates fail at even lower levels than depicted in the present NIOSH limits shown in Figure 4; especially if the loading is frequent, as can be the case in some assembly line work. warehouse operations, o r any other work that requires repeated bending and lifting of objects. Older borkers. especially the untrained who perform repetitive lifts, will be at increased risk for injury because of the abovementioned decrement in tissue properties. A further concern has been raised about twisting the torso. Farfan (1969) believes that the annular fibers of the discs are particularly susceptible to torsional strc recent studies are beginning to quantify annular strain levels under different loadings (e.g., Shirazi-Adl, Ahnied. & Shrisrasta, 1986; Stokes, 1987). Both electromyographic studies of torso muscle LKtions, as well as biomechanical modeling studies, have shown increased co-contraction of the muscles, and high disc compression force predictions if a load is lifted which is laterally offset from directly,jn front of the torso (Schultz, Andersson, Haderspeck, Ortengren, Norden, & Biijork, 1982; Schultz, Haderspek, Wanvick, b;: Porti110, 1983; Pope, Svensson, Andersson, Broman, & Zetter-

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berg, 1987; Bean, Chaffin, & Schultz, 1988). Thus, a load lifted directly in front of the body may be safe, but the same magnitude load may produce injury to spinal discs when twisting or laterally bending during the lift. This is confirmed by Kelsey and Hochberg (1988) who report a 3-fold risk of LBP in workers who frequently twist and lift 25 pounds. Finally, when lifts are performed rapidly, loads on the spine increase in proportion to the accelerations involved. For example, doubling the speed of lifting has been shown to increase low-back loads by at least 50% (Bush-Joseph, Schipplein, Anderson, & Andriacchi, 1988). This may result in a special risk to older workers who may not have the same muscle strength and response times as the younger worker. So, to minimize low-back stresses, loads should be lifted slowly and deliberately. Other Job Related Stressors ofthe Low Rack The handling of heavy loads is not the only source of mechanical injury to the low back. If one stumbles or loses traction while walking (i.e.. the foot begins to slip), a highly complex neurological system response is involved. If the resulting muscle contractions are fast enough and well coordinated, the lower back may not be overstressed. Yet Troup, Davies, and Manning (1988) have indicated that over 205% of serious low-back problems are caused by slips and falls. Given the effects of age in slowing both muscle contraction rates and response times as cited earlier, and its general effects on diminished sensory-motor coordination, it would appear that the combination of age and slippery walking conditions greatly increase the risk of low-back pain. It also must be realized that postures which require prolonged static exertions, even at extremely low lcvels of muscle activity, can cause muscle fatigue (i.e., loss of performance) and pain (Chaffin, 1973; Aaras, 1987). I n particular, seats in vehicles, in offices, and those associated with other plant furniture that are not ergonomically designed to support the lumbar region can cause increased muscle contraction states, disc compression forces, and discomfort (Anderson, Ortengren, Nachemson. & Elfstrom, 1974; Anderson & Ortengren, 1974; Yu, Keyerserling, & Chaffin, 1988). Further, epidemiological and physiological studies have shown that low frequency vibration (i.e., experienced while seated in such vehicles as trucks, earthmoving equipment, mining equipment, and helicopters) can cause increased torso muscle contractions, loss of disc height, and low-back pain (Pope, Wilder, & Frymoyer, 1980; Kelsey & Hochberg, 1988). Prevention of Low-Back Pain in Industry The major strategy utilized today to prevent low-back pain in industry relies on the following procedure: 1) Identifying job tasks that may be the source of low-back pain complaints, 2) evaluating objectively the source of

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the biomechanical stresses on the low back, and 3) designing (or redesigning) the tasks to reduce the stresses as much as is technically and economically feasible. In short, ergonomic principles and procedures are used to improve the physical work conditions to better accommodate the extreme variation that exists in performance capabilities of the workforce (Chaffin & Anderson, 1991; Pope, Frymoyer, Anderson, & Chaffin, 1991). Specifically, the following ergonomic guidelines need to be considered in the design of manual work for workers of any age:

I . Manual lifting of loads should be performed in an environment that: 1) Keeps the load close to the body, 2 ) does not require bending, stooping or twisting with the load, 3) provides adequate rest between repetitive lifting tasks, and 4) assures good foot traction and no tripping hazards. 2. Prolonged static torso postures should be avoided by providing a work environment which: I) Positions objects, controls, and displays in locations that minimize prolonged flexed, bending, or stooped postures, 2) has seats and workstations that adjust to individual anthropometry and that have good lumbar and arm supports, and 3) allows the use of different torso positions during the day. 3. Low frequency vibrations (e.g., common in driving tasks) should be minimized, if prolonged exposures exist, by using seats specifically designed to control such vibration and ones which provide good lumbar support. I n addition to designing the work environment in a way that minimizes harmful spinal stresses, it must be acknowledged that certain types of physical activity are needed to maintain the musculoskeletal system's ability to function. Indeed, the literature suggests that one of the greatest threats to health is not aging itself but continuous inactivity in the older individual (e.g., Troup & Videman, 1989). In the athlete, regular exercise has been estimated to retard the aging process by as much as 50%' (see review by Menard & Stanish, 1989). In men and women over SO years, a strong positive correlation has been found between moderate exercise and lumbar bone density (Michel, Block, & Fries, 1989). However, exercise may only have limited value in the prevention of osteoporosis (Block, Smith, Friedlander, & Genant, 1989). An extended period of relative inactivity can predispose a worker to injury when resuming normal activity levels. Brinckmann et al. (1989), for example, found that individuals who had been confined to bed for four or more weeks typically exhibited 1 kN, or 25%, lower average lumbar vertebral compressive strengths than active people before death. Similarly, with age, increasingly more time is required to adjust to a sudden increase in work load if over-use injuries are to be avoided.

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It is wcll known that frequent dynamic exercises that involvc a large number of muscles results in significant improvements in cardiopulmonary performance and endurance capability (i.e., fitness). Can certain types of cxercisc (i.e., hardening) programs reduce the effects of age on select spinal tissues‘? Certainly, muscle strength performance can be improved with regular exercise requiring individual muscles to be loaded at more than approximately 70% of their existing strengths. As an example, Beverly, Rider, Evans, & Smith (1989) showed muscle strength in elderly women could be significantly improved by a few seconds of daily activity. They found t h a t after six wccks o f squeezing a tennis ball as hard as possible, for as few as 30 seconds per day, wrist muscle strength increased by 15% and wrist bone mineral content increased by 3.4%. Six months later, inactivity had reversed these changes. Moritani and deVries ( 1980) found that 8 weeks of strength training increased rhe number o f active motor units in older (69 year old) subjects, rcsulting in a 20% strength gain. So, training can reverse some aging effects, while inactivity can enhance the changes. Though specific training programs to prevent lowback pain in older workers have not been well validated by controlled longitudinal studies, a good prevention program in industry should contain certain worker training elements that are both physiological and psychological in orientation. At a minimum, workers who have a history of low-back pain and/or have not performed jobs that require heavy or repetitive torso exertions, need to he carefully evaluated (Burton, Tillotson, & Troup. 11M9). This should include objective tests of strcngth and endurance that are based on specific job strcngth and endurance requirements (Chaffin et al., 1078; Kcyserling, Herrin, & Chaffin, 1980). Based on I hcse findings medically supervised exercise programs can be provided to increase musculoskeletal work capacities, if nectled, before the person is required to perform thc job. In addition, “phasing-in” the heavy work requirement over a couple of months, by using relief workers, or job rotations (ix., rotating workers between light and heavy duty jobs) may be needed, especially with individuals who have been off work with low-back injuries. Finally. any job training meant to prevent low-back pain must include instructions to workers regarding 1) the risks of low-back injury on the job, 2) a basic knowledge of body mechanics, 3 ) what specific motions to avoid (i.e.. jerking loads, twisting the torso rather than side stepping with the load, etc.), 4) how to plan specific ,job activities and use mechanical devices to best minimize back stresses, and 5 ) off-the-job injury prevention (‘l’sai,Bernacki, & Dowd, 1989). Summary and Research Recommendations

This paper has provided a brief overview of age and work et€ects on various functional capabilities of the lumbar spine. Clearly, advanced age does decrease the capability of the avcrage individual’s spine t o tolerate

certain types of physical stress. Yet large variations in individual physical performance capabilities and structural limits exist. Some of these attributes have bccn shown to depend on genetic endowment (e.g., size or shape of vertebral bodies) and some are dependent on habitual exercise levels (e.g., muscle strength and endurance, bone densities). In this context, it would appear that those workers who have been physically active during their careers and have not suffered froni serious low-back pain should be capable of continuing such activity without restrictions, though increased medical surveillance of their musculoskeletal system health also would be prudent. If, on the other hand, an older worker who has not been engaged in recent physically demanding work, and/or who reports having suffered from repeated or serious low-back pain, is applying for thc type of manual work which is known to produce large or repetitive back stresses then quantitative and objective job-related medical testing, work conditioning, and training should be considered as part of the employmcnt process. Unfortunately, the scientific basis for exact age rclated performance standards does not yet exist. Questions that need answering include risk assessment of future injury for an older worker who displiiys 1 ) diminished torso strength, endurance, and/or mobility, or 2) slowed response times, and/or 3) significant tactile and proprioceptive sensory loss? What is the change in risk level for older workers who become involved in low-back exercise programs before becoming employed in jobs that require significant exertions‘? What types of exercise programs are most effective in preventing lowback pain in older workers? How, and to what extent, is LBP risk accelerated by specific types of work rclated stressors due to lifting, twisting, and repetitive bending over many years? Morbidity in an aged worker m;3y be attributed to a single event, but the sequela is a life-span happening. These are a few of the very important research issues that must be addressed to control the rising costs and human suffering caused by a combination of age related changes and certain types of work related stresses on the low back. References

Aards, A. (1987). Postural load and the developmcr,it of musculoskeletal illness. Scandinavian Journul oJ’Rcliabilitation Medicine,, 18,s-35. Andersson, G., & Ortengren, R. (1974). Lumbar disc pressure and myoelectric back muscle activity during sitting. 11. Studies on an office chair. S‘candinui iun Journal of Rehabilitation Medicine, 3, 1 15-1 2 1. Andersson, G., Ortengren, R., Nachemson, A., & t l f strom, G. (1974). Lumbar disc pressure and myoelcctric back muscle activity during sitting, IV. Studies on a driver’s seat. Scundinavian Journal of Relznhilitnlion Medicine, 3, 128-133.

Downloaded by [The University Of Melbourne Libraries] at 00:09 12 December 2015

BIOMECHANICS O F LOW BACK PAIN

Asmussen, E., & Heeb@ll-Nielsen, K. (1962). Isometric muscle strength in relation to age in men and women. Ergonomics, 5, 167-169. Bean, J., Chaffin, D., & Schultz, A. (1988). Biomechanical model calculation of muscle contraction forces: A double linear programming method. Journal of Biomechanics, 21( l ) , 59-66. Berkson, M., Nachemson, A., & Schultz, A. (1979). Mechanical properties of human lumbar spine motion segments, Part 11: Responses in compression and shear: Influences of gross morphology. Journal of Biomechanical Engineering, 101,53- 57. Beverly, M., Rider, T., Evans, J., & Smith, R. (1989). Local bone mineral response to brief exercise that stresses the skeleton. British Medical Journal, 22, 233-235. Biering-Sorenson, F. (1982). Low back trouble in a general population of 30-. 40-, 50-, and 60-year old men and women. Study design, representativeness, and basic results. Danish Medical Bulletin, 29(6), 289-299. Block, J., Smith, R., Friedlander, A., & Genant, J. (1980). Preventing osteoporosis with exercise: A review with emphasis on methodology. Medical Hypotheses, 30, 9-19. Brinckmann, P., Biggeman, M., & Hilweg, D. (1988). Fatigue fracture of human lumbar vertebrae. Clinical Biomechanics, (Suppl I ) , 1, 1-23. Brinckmann, P., Biggeman, M., & Hilweg, D. (1989). Prediction of the compressive strength of human lumbar vertebrae. Clinical Biomechanics (Suppl. 2), 4, 1-27. Brooks, S., & Faulkner, J. (1988). Contractile properties of skeletal muscle from young adult and aged mice. Journal of Physiology (Lond.),404, 71-82. Burton, A., Tillotson, K., & Troup, J. (1989). Prediction of low-back trouble frequency in a working population. Spine, 14,939-946. Bush-Joseph, C., Schipplein, O., Anderson, G., & Andriacchi, T. (1988). Influence of dynamic factors on the lumbar spine moment in lifting. Ergonomics, 31, 21 1-216. Chaffin, D. (1973). Localized muscle fatigue-Definition and measurement. Journal of Occupational Medicine, 15(4), 346- 354. Chaffin, D., & Anderson, G. (1991). Occupational Biomechanics. N Y: W iley . Chaffin, D., & Park, K. (1973). A longitudinal study of low-back pain as associated with occupational weight lifting factors. American Industrial Hygiene Association Journal, 34,5 13- 525. Chaffin, D., Herrin, G., & Keyserling, W. M. (1978). Preemployment strength testing - An updated position. Journal of Occupational Medicine, 20(6), 403-408. Cyron, B., & Hutton, W. (1978). The fatigue strength of the lumbar neural order in spondylolysis. Journal of Bone and Joint Surgery, 60B, 234-238. DeCandido, P., Reinig, J., D y e r , A., Thompson, K., & Ducker, T. (1988). Magnetic resonance assessment of

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the distribution of lumbar spine disc degenerative changes. Journal of Spinal Disorders, 1,9-15. Einkauf, D., Gohdes, M., Jensen, G., & Jewell, M. (1987). Changes in spinal mobility with increasing age in women. Physical Therapy, 67,370-375. Ericksen, M. E. (1978). Aging of the lumbar spine 11. L1 and L2. American Journal of Physical Anthropology, 48, 24 1-246. Eyre, D., & Muir, H. (1976). Types I and 11 collagens in intervertebral disc. Biochemical Journal, 157, 267-270. Farfan, H. (1969). Effects of torsion o n the intervertebra1 joints. Canadian Journal of Surgery, 12,336-341. Finsen, V. (1988). Osteoporosis and back pain among the elderly. Acta Medica Scandinavica, 223,443-449. Fitzgerald, G., Wynveen, K., Rhealt, W., & Rothschild, B. (1983). Objective assessment with establishment of normal values for lumbar spine range of motion. Physical Therapy, 63,1776- 1781. Goldman, R. (1970). Speculations on vascular changes with age. Journal of the American Geriatrics Sociery, 18, 765-779. Gotfried, Y., Bradford, D., & Oegema, T. (1986). Facet joint changes after chemonucleolysis - induced disc space narrowing. Spine, 1I, 944-950. Hansson, T., Roos, B., Nachemson, A. (1980). The bone mineral content and ultimate compressive bone strength of lumbar vertebrae. Spine, 5, 16-55. Harada, Y. & Nakahara, S. (1989). A pathological study of lumbar disc herniation in the elderly. Spine, 14, 1020-1 024. Hayes, M., Howard, T., Arnel, C., & Kopta, J. (1989). Photographic evaluation of lumbar spine flexionextension in asymptomatic individuals. Spine, 14,327331. Hoshina, H. (1980). Spondylolysis in athletes. The Physician and Sportsmedicine,3,75-78. Hult, L. (1954). Cervical, dorsal and lumbar spinal syndromes. Acta Orthopaedica Scandinavica, I 7 (Suppl), 1- 102. Jackson, D., Wiltse, L., & Civincione, R. (1976). Spondylolysis in the female gymnast. Clinical Orthopaedics and Related Research, 1I7,68-73. Karvonen, M., Mainzer, J., Rohmert, W., Lowenthal, I . , Undentsch, K., Kupper, R., Gartner, K., & Rutenfranz, J. (1980). Occupational health studies of air transport workers 11. Muscle Strength of Air Transport Workers. International Archives of Occupational and Environmental Health, 47,233-244. Katzman, R., & Terry, R. (1983). The Neurology ofAging. Philadelphia: F. A. Davis. Kelsey, J., & Hochberg, M. (1988). Epidemiology of chronic musculoskeletal disorders. Annual Review of Public Health, 9, 379-401. Kelsey, J., & White, A. (1980). Epidemiology and impact on low- back pain. Spine, 5(2), 133-142. Kelsey, J., Githens, P., White, A., Walter, S., Southwick, W., Weil, U., Holford, T., Ostfeld, A., Calogero, J., & O'Connor, T. (1984). An epidemiologic study of lifting and twisting on the job and risk for acute

Downloaded by [The University Of Melbourne Libraries] at 00:09 12 December 2015

186

prolapsed lumbar intervertebral disc. Journal of Orthopaedic Research, 2 , 6 1-66. Keyserling, W. M., Herrin, G., & Chaffin, D. (1980). Isometric strength testing as a means of controlling medical incidents on strenuous jobs. Journal of Occupu fionul Medicine, 22(5),332-336. Koeller, W., Muehlhaus, S., Meier, W., & Hartmann, F. ( 1986). Biomechanical properties of human intervertebral discs subjected to axial dynamic compression influence of age and degeneration. Journal of BiomeLhutiics, 10, 807-816. M., & Heliovaana, M. (1988). Radioy of the lumbar spine and its relation to clinical back motion. Scandinavian Journal of Rehahilitution Medicine, 20, 71-76. Larsson, L., bi Karlsson, J . (1978). Isometric and dynamic endurance as a function of age and skeletal muscle characteristics. Acta Physiological Scandinav~ C U 104. . 129-136. Larsson, L., Gimby. G., & Karlsson, J. (1979). Muscle strength and speed of movement in relation to age and muscle morphology. Journal of Applied Physiology, 46, 4s 1-450. Laubach, L. (1976). Comparative muscular strength of men and women: A review of the literature. Aviation Space r i n d Environmental Medicine, 47,534-542. McFaddcn, K., & Taylor, J. (1989). End-plate lesions of the lunibar spine. Spine, 14,867-869. McGlashen, K., Miller, J., Schultz, A., & Anderson, G. (1987). Load displacement behavior of the human lumbosacral joint. Journal of Orthopuedic Research, 5, 488-496. Menard, D., & Stanish, W. (1989). The Aging Athlete. American Journal of Sports Medicine, 17, 187-196. Merritt, J., McLean, T., Erickson, R., & Offord, K. (1986). Measurement of trunk flexibility in normal subjects: Reproducibility of three clinical methods. Mayo Clinic Proceedings, 61, 192-197. Michel, B.. Block, D., & Fries, J. (1989). Weight-bearing cxercisc, overexercise, and lumbar bone density over age SO years. Archives oflntemal Medicine, 149, 23252329. Miller, J., Schmatz, C., & Schultz, A. (1988). Lumbar disc degeneration: A review of age, sex and level correlations in 600 autopsy specimens. Spine, 13, 173- 178. Miller, J., Schultz, A., & Anderson, G. (1987). Loaddisplacement behavior of sacroiliac joints. Journal of Orthopedic Research, 5 , 92-100. Milne, J., Kr Lauder, I . (1974). Age effects in kyphosis and lordosis in adults. Annals of Huntan Biology, 1, 327-337. Modic, M., Steinberg, P., Ross, J., Masaryk, T., & Ccntcr, J. (1988). Degenerative disk disease: Assessment of changes in vertebral body marrow with MR imaging. Kudiology, 166, 193- 199. Moritani, T., & devries, A. (1980). Potential for gross muscle hypertrophy in older men. Gerontology, 35(5), 671-682.

CHAFFIN/ASHTON-MILLER

Moroney, S. P., Schultz, A. B., Miller, J. A. A.. Kr Anderson, G. B. J. (1988). Load-displacement properties of lower cervical motion segments. .lorirnal o f Biomechanics, 21,769-779. Nachemson, A., Schultz, A., & Berkson, kl. (1979). Mechanical properties of human spine motion segments - Influence of age, sex, level and degeneration. Spine, 4 , 1-8. National Institute for Occupational Safety and Health. (1981). Work Practices Guide for Manuul Lifting (Tcchnical Report 81-122). Cincinnati, OH: NIOSH. NygPrd, C., Luopajarvi, T., & Ilmarinen, J (1988). Musculoskeletal capacity of middle-aged women and men in physical, mental and mixed occupations. European Journal of Applied Physiology and Occirparionril Physiology, 181- 188. Olmarker, K., Rydevik, B., Holm, S., bi Baggc, U. (1989). Effects of experimental graded compre on blood flow in spinal nerve roots. A vital microscopic study in porcine canda equina. Joirmril of Orthopaedic Research, 7,8 17-823. Pearcy, M., & Tibrewal, S. (1984). Axial rotation and lateral bending in the normal lumbar spine mcasured by three- dimensional radiography. Spine, 9, 582-587. Pearcy, M., Portek, J., & Shephard, J. (1984). Thrccdimensional x-ray analysis of normal measurement in the lumbar spine. Spine, 9,294-300. Pope, M., Frymoyer, J., & Anderson, G. (1984). 0 i ~ ~ pational Low-back Pain. NY: Praeger. Pope, M., Frymoyer, J., Anderson, G., & Chalfin. D. (1991). Occuputional Low-back fuin (2nd ed.). St Louis, MO: Mosby. Pope, M., Svensson, M., Anderson, G., Broman, H., Kr Zetterberg, C. (1987). The role of prerotation of the trunk in axial twisting efforts. Spine, 12( 10). 10411045. Pope, M., Wilder, D., & Frymoyer, J. (1980). Vibration as an aetiologic factor in low-back pain. Proceedings qf’ the Institution of Mechanical Engineers Confererrce on Low-hack Pain (Paper #C121/80). Inst. Mech. Engs. Potvin, A., Syndulko, K., Tourtellotte, W., Golberg, Z., Potvin, J. H., & Hansch, E. C. (1981). Quantitative evaluation of normal age related changes in neurologic function. In G. Maletta & F. Pirozzolo (Eds): Advances in neurogerontology: Behavioral assesr:rncwt and psychophamiacology (Vol. 2; pp. 13-57). New York: Praeger. Punnett, L., Fine, L., Keyserling, W., Herrin, Ci., L! Chaffin, D. (1991). A case-referent study of back disorders in automobile assembly workers: The hcalth effects of non- neutral trunk postures. Scandinc,~rlrin Joumal of Work, Environment & Health, 17, 337-346. Roche, M., & Rowe, G. (1952). The incidence of separate neural arch and coincident bone variations. Journal of Bone and Joint Surgery, 34A, 491-494. Rockoff, S., Sweet, E., & Bleustein, J. (1969). The relative contributions of trabecular and cortical bone

Downloaded by [The University Of Melbourne Libraries] at 00:09 12 December 2015

BIOMECHANICS OF LOW BACK PAIN

to the strength of human lumbar vertebrae. Calcified Tissue Research, 3, 163-175. Rowe, M. (1969). Low-back pain in industry: A position paper. Journal of Occupational Medicine, 11(4), 161169. Schultz, A., Anderson, G., Haderspeck, K., Ortengren, R., Nordin, M., & Bjork, R. (1982). Analysis and measurement of lumbar trunk loads in tasks involving bends and twists. Journal of Biomechanics, 15, 669675. Schultz, A., Haderspek, K., Warwick, D., & Portillo, D. (1983). Use of lumbar trunk muscles in isometric performance of mechanically complex standing tasks. Journal of Orthopaedic Research, I, 77-9 1. Schultz, A., Warwick, D., Berkson, M., Nachemson, A. (1979). Mechanical properties of human lumbar spine motion segments. Part I: Responses in flexion, extension, lateral bending and torsion. Journal of Biomechanics, 101, 46-5 2. Shephard, R. ( 1978). Human Physiological Working Capaci(y. Cambridge: Cambridge University Press. Shirazi-Adl, A., Ahmed, A., & Shrisrasta, S. (1986). Mechanical response of a lumbar motion segment in axial torque alone and compression. Journal of Orthopaedic Research, 11,914-927. Simonsen, E. (1971). Effect of age on work capacity and fatigue. In E. Simonsen (Ed.), Physiology of working capacity and fatigue (pp. 40W36). Springfield, IL: Charles C. Thomas. Snyder, R., Chaffin, D., & Schutz, R. (1975). Bioengineering study of basic physical measurements related to susceptibilityto hyperextension-hypegexioninjury (HS RI B1 pp. 75-76). Ann Arbor, MI: University of Michigan. Spirduso, W. (1980). Physical fitness, aging, and psychomotor speed: A review. Gerontology, 35(6), 85& 865. Stokes, I. (1987). Surface strain of intervertebral discs. Journal of Orthopaedic Research, 5,348-355. Tkaczuk, H. (1968). Tensile properties of human lumbar longitudinal ligaments. Acta Orthopaedica Scandinavica Supplementum, 115,l-69. Troup, J., & Videman, T. (1989). Inactivity and aetio-

187

pathogenesis of musculoskeletal disorders. Clinical Biomechanics, 4, 173-178. Troup, J., Davies. J., & Manning, D. (1988). A model for the investigation of back injuries and manual handling problems at work. Journal of Occupational Accidents, 10, 107-1 19. Troup, J., Foreman, T., Baxter, C., & Brown, D. (1987). The perception of back pain and the role of psychophysical tests of lifting capacity. Spine, 12, 645-657. Tsai, S., Bernacki, E., & Dowd, C. (1989). Incidence and cost of injury in an industrial population. Journal of Occupational Medicine, 31,781-784. Twomey, L., & Taylor, J. (1986). Age changes in lumbar zygoaphophyseal joints: Observations on structure and function. Spine, 11,739-745. Twomey, L., & Taylor, J. (1987). Age changes in lumbar vertebrae and intervertebral discs. Clinical Orthopaedics and Related Research, 224,97-104. Twomey, L., & Taylor, J. (1988). Age changes in the lumbar spinal and intervertebral canals. Paraplegia, 26,238-249. Urban, J., Holm, S., & Maronda, A. (1978). Diffusion of small solutes into the intervertebral disc: An in-vivo study. Biorheology, 15,203-223. Urban, J., & McMullin, J. (1986). Swelling pressure of the intervertebral disc: Influence of proteoglycan and collagen contents. Biorheology, 22, 145-157. Urban, J., & McMullin, J. (1988). Swelling pressure of the lumbar intervertebral disc: Influence of age, spinal level, composition, and degeneration. Spine, 13, 179187. Yamada, H. (1970). Ratios for age changes in the mechanical properties of human organs and tissues. In F. Gaynor Evans (Ed.) Strength of biological materials (pp. 255-282). Baltimore: Williams and Wilkins. Yang, K., & Kmg, A. (1984). Mechanism of facet load transmission as a hypothesis for low-back pain. Spine, 9,557-565. Yu, C., Keyserling, W. M., & Chaffin, D. (1988). Development of a work seat for industrial sewing operations: Results of a laboratory study. Ergonomics, 31(12), 1765-1786.