The objective of this section is to examine the evidence that there is a biomechanical pathway between physical occupational demands and the risk of suffering a low back disorder. Our assessment is made in relation to the conceptual model adopted in this report and specifically relative to the biomechanical pathways highlighted in Figure 1.2. This figure portrays a biomechanical pathway in terms of a relationship between loads imposed on a structure and the mechanical tolerance of the structure. This model also recognizes that both the loading characteristics and the tolerance levels can be influenced by physiological responses. In terms of the loading, the musculoskeletal system may be influenced by either adaptation to or intensification of the load. The tolerance may be mediated by pain responses or discomfort. Overall, if the loading of the structure exceeds the tolerance, then this situation can result in a disorder. Figure 1.2 accounts for the possibility that various influences may trigger this injury pathway and response. External loads, such as those associated with work, are expected to influence the biomechanical loading of the spine. This model also allows for the possibility that other factors may influence this load-tolerance-disorder pathway at different points in the pathway. It is important to realize that individual factors as well as organizational factors and social context can influence biomechanical loading and structure tolerance, as well as the risk of suffering a disorder; these issues are covered in other sections of this report. The objective of this section is to explore the evidence, in this context, that external loads can trigger the pathway to low back disorders. We examine exclusively the evidence that physical loading of the spine and supporting structures may result in low back pain. This contention is assessed via several approaches, including workplace observations of biomechanical factors relative to rates of low back pain reporting, biomechanical logic, pain pathways, and intervention research. Chapter 2 reviewed trends associated with types of work (job titles) and the reporting of low back disorders. These investigations identified warehousing, patient handling, and general materials handling jobs as associated with back pain at a higher rate than other types of occupations. Laboratory biomechanical analyses have shown that these types of activities can lead to greater loadings on the spine (Leskinen et al., 1983; Schultz et al., 1987; Zetterberg, Andersson, and Schultz, 1987; Cholewicki, McGill, and Norman, 1991; McGill, 1997; Marras and Davis, 1998; Chaffin, Anderson, and Martin, 1999; Granata and Marras, 1999; Marras et al., 1999a, 1999b; and Marras, Granata, et al., 1999), and thus jobs associated with these higher spine loading tasks are consistent with greater reporting of back injuries. This is consistent with the logic described in Figure 1.2. The panel reviewed the industrial observation literature for information relating biomechanical loading of the body and reports of low back disorder. For our assessment, the literature was screened with respect to biomechanical relevance. Whereas most epidemiologic studies are primarily concerned with methodological considerations, biomechanical assessments are primarily concerned that the information (exposure metric) assessed has biomechanical meaning. Hence, while many assessments of occupationally related low back disorder risk have occurred in the literature, many of these assessments have not used exposure metrics that would be considered relevant to a biomechanical assessment. Such a situation would mask or obscure any relationship with risk. For example, numerous studies have found that lifting heavy loads is associated with an increased risk of low back pain (Kelsey et al., 1984; Videman, Nurminen, and Troup, 1984; Bigos et al., 1986; Spengler et al., 1986; Battie et al., 1989; Riihimaki et al., 1989b; Burdorf, Govaert, and Elders, 1991; Bigos et al., 1992; Andersson, 1997; Bernard, 1997b). However, such gross categorical exposure metrics have little meaning in a biomechanical assessment. As discussed in a previous section, from a biomechanical perspective, a given external (to the body) load can impose either large or small loads on the spine (internal forces), depending on the load's mechanical advantage relative to the spine (Chaffin, Andersson, and Martin, 1999). Therefore, in order to understand biomechanical loading, specific quantifiable exposure metrics that are meaningful in a biomechanical context are necessary for the purposes of this review. Only then can one address the issue of how much exposure to a biomechanical variable is too much exposure. The literature was screened to identify biomechanically relevant, high-quality industrial surveillance studies. High-quality biomechanically related industrial surveillance studies consisted of studies that met the following criteria:
Several industrially based observational studies meeting these criteria have appeared in the literature and offer evidence that low back disorder is related to exposure to physical work parameters on the job. Chaffin and Park (1973) performed one of the first studies exploring this relationship. This study found that “the incidence rate of low back pain (was) correlated (monotonically) with higher lifting strength requirements as determined by assessment of both the location and magnitude of the load lifted” (Chaffin and Park, 1973:513). They concluded that load lifting could be considered potentially hazardous. It is important to note that this study suggested that not only was load magnitude significant in defining risk but also load location was important. This view is consistent with biomechanical logic, discussed later. This evaluation also reported an interesting relationship between frequency of exposure and lifts of different magnitude (relative to worker strength). This study suggested that exposure to moderate lifting frequencies appeared to be protective, whereas high or low rates of lifting were common in jobs with greater reports of back injury. A prospective study performed by Liles et al. (1984) observed job demands compared with worker's psychophysically defined strength capacity. The job demand definition considered load location relative to the worker, as well as frequency of lift and exposure time. Demands were considered for all tasks associated with a material handling job. This study identified the existence of a job demand relative to a worker strength threshold above which the risk of low back injury increased. This study found that there was a “job severity threshold above which incidence and severity dramatically increased” (Liles et al., 1984:690). Herrin and associates (1986) observed jobs over three years in five large industrial plants, where they evaluated 2,934 material handling tasks. They evaluated jobs using both a lifting strength ratio as well as estimates of back compression forces. A positive correlation between the lifting strength ratio and low back injury incidence rates was identified. They also found that musculoskeletal injuries were twice as likely for predicted spine compression forces that exceeded 6,800 N. The analyses also suggest that prediction of risk was best associated with the most stressful tasks (as opposed to indices that represent risk aggregation). Punnett and colleagues (1991) performed a case-control (case-referent) study of automobile assembly workers, in which risk of back pain associated with nonneutral working postures was evaluated. In this study, back pain cases over a 10-month period were studied, referents were randomly selected after review of medical records, interview, and examination, and job analyses were performed by analysts who were blinded to the case-referent status. Risk of low back pain was observed to increase as trunk flexion increased. Risk was also associated with trunk twisting or lateral bending. Finally, this study indicated that risk increased with exposure to multiple postures and increasing exposure time. Specifically, the study indicated that risk increased as the portion of the duty cycle spent in the most severe postures increased. Marras and colleagues (1993, 1995) biomechanically evaluated over 400 industrial jobs by observing 114 workplace and worker-related variables. Exposure to load moment (load magnitude × distance of load from spine) was found to be the single most powerful predictor of low back disorder reporting. This study has been the only study to examine trunk kinematics along with traditional biomechanical variables in the workplace. This study identified 16 trunk kinematic variables resulting in statistically significant odds ratios associated with risk of low back disorder reporting in the workplace. While none of the single variables was as strong a predictor as load moment, when load moment was combined with three kinematic variables (relating to the three dimensions of trunk motion) along with an exposure frequency measure, a strong multiple logistic regression model resulted that described reporting of back disorder well (O.R. = 10.7). This analysis indicated that risk was multivariate in nature, in that exposure to the combination of the five variables described reporting well. The model recognizes a trade-off between the variables. For example, a work situation that exposes a worker to low magnitude of load moment can still represent a high-risk situation if the other four variables in the model were of sufficient magnitude. This model has been recently validated in a prospective workplace intervention study (Marras et al., 2000a). When the results of this study are considered in conjunction with the Punnett study (1991), it is clear that work associated with activity performed in nonneutral postures increases the risk to the back. Furthermore, as the posture becomes more extreme or the trunk motion becomes more rapid, reporting of back disorder is greater. These results are meaningful from a biomechanical standpoint and suggest that risk of low back disorder is associated primarily with mechanical loading of the spine, as well as that when tasks involve greater three-dimensional loading, the association with risk becomes much stronger. Three-dimensional loading of the spine would be expected to affect the disc, ligaments, muscles, and other structures proximal to the spine. Norman and associates (1998) recently assessed cumulative biomechanical loading of the spine in automotive assembly workers. This observational study identified four independent factors for low back disorder reporting: integrated load moment (over a work shift), hand forces, peak shear force on the spine, and peak trunk velocity. This study showed that workers in the top 25 percent of loading exposure on all risk factors reported low back pain at a rate about six times greater than those in the bottom 25 percent of loading. Fathallah and associates (Fathallah, Marras, and Parnianpour, 1998b) evaluated a database of 126 workers and jobs to precisely quantify and assess the complex trunk motions of groups with varying degrees of low back disorder reporting. They found that groups with greater reporting rates exhibited complex trunk motion patterns involving high magnitudes of trunk combined velocities, especially at extreme sagittal flexion, whereas the low-risk groups did not exhibit any such patterns. This study showed that elevated levels of complex simultaneous velocity patterns along with key workplace factors (load moment and frequency) were unique to groups with increased low back disorder risk. Waters and colleagues (1999) evaluated the usefulness of the revised NIOSH lifting equation in an industrial observation study of 50 industrial jobs. The evaluation considered factors expected to be associated with spine loading, including load location measures. These measures defined an expected worker tolerance (identified by biomechanical, physiological, strength, or psychophysical limits) and were compared with the load lifted. The results of this study indicated that as the tolerance was exceeded, the odds of back pain reporting increased up to a point and then decreased. The findings from these studies are summarized in Table 6.4. Only two studies have estimated spinal load at work, and both have found a positive association between physical loading at work and low back pain reporting. The other studies are consistent with this finding. Even though these studies have not evaluated spinal loading directly, the exposure measures included were indirect indicators of spinal load. Load location or strength ratings are both indicators of the magnitude of the load imposed on the spine. All but one study found that one of these measures was significantly associated with back pain reporting. Most of the remaining exposure metrics (load location, kinematics, and three-dimensional analyses) are important from a biomechanical standpoint because they mediate the ability of the trunk's internal structures to support the external load. Therefore, as these metrics change, they can change the nature of the loading on the internal structures of the back. This assessment also shows that risk is multifactorial, in that risk is generally much better described when the analysis is three dimensional and more than one risk factor measure is considered. No high-quality biomechanical relevant industrial surveillance studies have been identified that contradict these results. Collectively, these studies demonstrated that when meaningful biomechanical assessments are performed at the workplace, strong associations between biomechanical factors and the risk of low back disorder reporting are evident. Several key components of biomechanical risk assessment can be derived from this review. First, all studies that have compared worker task demands with worker capacity have been able to identify thresholds above which reporting of low back disorder increases. Second, increased low back disorder reporting can be identified well when the location of the load relative to the body (load moment or load location) is quantified in some way. Nearly all studies have shown that these factors are closely associated with increased low back pain reports. Third, nearly all studies have shown that frequency of material handling is associated with increased reporting of low back pain. Fourth, many studies have shown that increased reporting of low back pain can be well characterized when the three-dimensional dynamic demands of the work are described, as opposed to static two-dimensional assessments. Finally, nearly all of the high-quality biomechanical assessments have demonstrated that risk is multidimensional, in that a synergy among risk factors appears to intensify increased reporting of low back pain. While many of these relationships are monotonically related to increased low back pain reports, some have identified associations that were nonmonotonic. Specifically, exposure at moderate levels of load and frequency of lifting appears to represent the lowest level of risk, whereas exposure at greater levels represents the greatest level of risk. Whereas many of the high-quality biomechanical studies explored different aspects of risk exposure, none of these studies provides evidence contradicting these key component findings. Biomechanical logic suggests that damage occurs to a structure when the imposed loading exceeds the structure's mechanical tolerance. In support of this, the high-quality biomechanical workplace observation studies demonstrate a positive correlation between increased biomechanical loading and increased risk for low back disorder at work. Currently, it is infeasible to directly monitor the spinal load of a worker performing a task in the workplace. Instead, biomechanical models are typically used to estimate loading. However, an understanding of the differences between methods of spine assessment can help place the findings of these different observational studies in perspective. Biomechanical models of spinal loading have evolved over the past several decades. The early models of spine loading made assumptions about which trunk muscles supported the external load during a lifting task (Chaffin and Baker, 1970; Chaffin et al., 1977). These models assumed that a single muscle vector could be used to summarize the load supporting (and spine loading) internal force that was required to counteract an external load lifted by a worker. These models assumed that lifts could be represented by a static lifting situation and that no coactivation occurred among the trunk musculature during lifting. All solutions to the model were unique in that workers with the same anthropometric characteristics performing the same task would be expected to yield the exact same spinal loads. The main focus of such models was assessment of spinal compression. These models could be employed in surveillance studies simply by videotaping a lifting task and measuring the weight of the object lifted. Such a model was employed in one of the surveillance studies described earlier (Herrin, Jaraiedi, and Anderson, 1986). Later models were expanded to the point at which they could account for the contribution of multiple internal muscles' reactions in response to the lifting of an external load. These models predicted compression forces as well as shear forces imposed on the spine. The first functional multiple muscle system model used for task assessment was developed by Schultz and Andersson (1981). This study demonstrated how loads handled outside the body could impose large spinal loads due to the coactivation of trunk muscles necessary to counteract this external load. This model represented a much more realistic situation. However, this modeling approach led to an indeterminant solution (since many muscles were represented in the model, a unique solution became difficult). Therefore, many subsequent modeling efforts attempted to determine which muscles would be active (Schultz et al., 1982b; Bean, Chaffin, and Schultz, 1988; Hughes and Chaffin, 1995). These efforts resulted in models that worked well for static loading situations but did not necessarily represent the more realistic, dynamic lifting situations well (Marras, King, and Joynt, 1984). Since prediction of muscle recruitment was difficult under realistic (complex) material handling conditions, later efforts attempted to monitor muscle activity directly using muscle activity as an input to multiple muscle models. These biologically assisted models typically employed electromyography (EMG) as the muscle activity monitor. These models were able to realistically model most dynamic three-dimensional lifting activities (McGill and Norman, 1985, 1986; Cholewicki, McGill, and Norman, 1991; Marras and Sommerich, 1991a, 1991b; Cholewicki and McGill, 1992; Cholewicki and McGill, 1994; Granata and Marras, 1993; 1995a; Marras and Granata, 1995, 1997a, 1997b). Available validation measures suggest that these models have good external as well as internal validity (Granata, Marras, and Davis, 1999; Marras, Granata, and Davis, 1999). Granata and Marras (1995a) demonstrated how miscalculations of spinal loading could occur unless realistic assessments of muscle recruitment could be determined. The disadvantage of these biologically assisted models is that they require EMG applications to the worker, which is often unrealistic at the workplace. The evolution of these models can have an impact on the interpretation of the work relatedness of mechanical loading of the spine. As indicated in the review of quantitative biomechanical surveillance studies, most spine loading estimates performed at the workplace employed two-dimensional, single-equivalent muscle models. Thus, one would expect that in these studies, the spinal compression was underestimated and shear force estimates would not be realistic. Given that these models are based on different modeling assumptions and vary greatly in their degree of comprehensiveness, it is not unexpected that some variability in reported findings would be apparent. Hence, when reviewing the status of risk-related evaluations, one must be vigilant in considering the analytical assumptions and tools used in reaching their conclusions. Given these limitations and the impracticality of monitoring EMG at the worksite, many tasks are simulated under laboratory conditions so that a better, more realistic, estimate of spine loading can be derived. A literature exists that has evaluated many work situations under such situations. In this section, we investigate whether the risk factor components identified in Table 6.4 can be associated with greater loading of the spine and back. It is indeed possible to evaluate several of the risk situations observed in Table 6.4 using quantitative biomechanical models. The assessment by Herrin et al. (Herrin, Jaraiedi, and Anderson, 1986) has applied a single-equivalent muscle model to work situations and found that compressive loads imposed on the spine of more than 6,800 N greatly increased risk. The assessment by Punnett and colleagues (1991) did include a biomechanical analysis of the loads lifted by the worker if the load exceeded 44.5 N. Using a three-dimensional biomechanical static model (Chaffin, Anderson, and Martin, 1999), compressive loads on the spine were evaluated as workers assumed various postures. Even though the risk analysis indicated that risk was associated with extreme flexion, lateral bending, and trunk twisting, the results of the biomechanical analysis indicated that “less than 3% of the analyzed postures resulted in peak compressive forces of 3,430 N (the point at which compressive forces are believed to cause damage)” (Punnett et al., 1991:344). It should be noted that the biomechanical model used for this assessment was a static “single-equivalent” muscle model. As noted earlier, since these types of models are unable to account for muscle coactivation, they often underestimate compression (Granata and Marras, 1995b). In addition, it is not clear from the paper that shear forces were analyzed. Given the nonneutral postures observed, one would expect that spinal shear forces would be more significant from a biomechanical standpoint than compressive loading. The field observations by Marras and colleagues (Marras et al., 1993, 1995, in press) identified moment, trunk flexion, trunk lateral velocity, trunk twisting velocity, and frequency of lifting as multivariate risk factors. These studies quantified the exposure levels at which each risk factor became safe or risky. Under controlled laboratory conditions, these authors employed biologically assisted models to assess the biomechanical significance of exposure to these “field documented” safe or risky exposure levels for all five risk factors. In a series of studies, they showed that exposure to higher load moments and forward flexion (Marras and Sommerich, 1991a, 1991b; Granata and Marras, 1993, 1995a), exposure to greater lateral trunk velocity (Marras and Granata, 1997b), exposure to greater twisting velocity (Marras and Granata, 1995), and exposure to higher repetitions (Marras and Granata, 1997a) were all similar in that at higher levels of exposure, increased cocontraction of the trunk musculature was observed. This higher level of coactivation was responsible for greater compressive spine loading. In addition, increases in both lateral and anterior-posterior shear were noted especially for the lateral bending and twisting risk factors. These analyses indicated that exposure to greater load moments, nonneutral postures, and trunk motion all resulted in a more complex recruitment of the trunk musculature that logically increased mechanical loading of the spine. Thus, these studies indicated that when more comprehensive, three-dimensional dynamic biomechanical models were employed, field observations of risk correlated well with biomechanical loadings (Granata and Marras, 1999). These analyses also relate well to the findings of Norman and associates (Norman et al., 1998). They employed a simplified two-dimensional quasi-dynamic model to analyze spinal loading. Even though this model was not three-dimensional and did not assess multiple trunk muscle recruitment, it was calibrated against a biologically assisted three-dimensional fully dynamic model (McGill and Norman, 1986, 1987). Both the field surveillance as well as the biomechanical interpretation of the risk factors in this study agree well with field surveillance and biomechanical interpretation of risk factors described earlier by Marras and colleagues. Hence, it is clear that unless sufficiently sensitive and robust biomechanical analyses are performed at the worksite, the relationship between factors associated with workplace observations of risk and biomechanical loading may not be apparent or this relationship may be underestimated. Related to this finding is the concept that for ergonomic interventions to be useful, the analysis must be sensitive enough to represent components of risk present in a particular job. For example, a prospective review of ergonomic interventions associated with 36 jobs with a history of back risk demonstrated that only one-third of the interventions sufficiently controlled low back disorder risk (Marras et al., in press). More in-depth analyses of these jobs indicated that workers responsible for ergonomic interventions often did not employ ergonomic assessment tools that were sensitive enough to identify the nature of the risk. This study showed that employment of more sensitive tools would have identified which assessments might have controlled for the biomechanically associated risks. Thus, this study shows that, often when ergonomic interventions are found to be ineffective, it is simply the case that the wrong intervention was selected, not that ergonomic interventions cannot be effective. Certain tasks or jobs have been associated with greater risk of low back disorder. These tasks include patient handling (Videman, Nurminen, and Troup, 1984; Jensen, 1987; Garg and Owen, 1992; Knibbe and Knibbe, 1996), material handling in distribution centers and ware-housing operations (Waters, Putz-Anderson, and Baron, 1998), and team lifting (Sharp et al., 1997). Several biomechanical evaluations of these jobs have been performed using some of the more robust models discussed above. A biologically assisted model was used to evaluate patient handling tasks (Marras et al., 1999a). An evaluation of spinal loading indicated that of the one-person and two-person patient handling techniques studied, none resulted in a spinal load that was within acceptable levels. Similar results were found using more traditional biomechanical assessments (Garg and Owen, 1992). Load handling has been studied from a biomechanical standpoint to a great extent, with numerous studies indicating that excessive loads could be imposed on the spine during lifting (Chaffin, 1979; Schultz and Andersson, 1981; Garg et al., 1983; Freivalds et al., 1984; McGill and Norman, 1985; Anderson, Chaffin, and Herrin, 1986; Chaffin, 1988; Cholewicki and McGill, 1992; Gallagher et al., 1994; Davis, Marras, and Waters, 1998; Fathallah et al., 1998a). Loading pallets in a distribution environment was studied recently (Marras, Granata, and Davis, 1999). This study is significant because it demonstrated that significant loading was not just a function of load magnitude but also a function of position of the load relative to the spine. Loads handled at low heights and at greater horizontal distances from the spine greatly increase the loading on the spine. This increased loading is due to two features. First, greater distance of the load from the spine increased the load moment, which required greater internal forces to counterbalance the external load. These increased internal forces resulted in greater spine loading in both compression and shear. These findings are consistent with the observations of the importance of load moment noted in Table 6.4. Second, lifting from low positions requires more of the body mass to extend beyond the base of support for the spine. This action also increases the moment imposed about the spine due to the weight of the torso and distance of its center of mass relative to the base of support for the spine. In addition, the supporting muscles must operate in a state of lengthened tension that is known to be one of the weakest positions of a muscle. Thus, risk is associated with greater loading of the spine as well as reduced muscular capacity of the trunk muscles. Finally, team lifting has been shown to severely alter the lifting kinematics and positions of workers (Marras et al., 1999b). This biomechanical analysis has shown that these constrained postures once again increase coactivation of the trunk musculature and result in increases in both compressive and shear loadings of the spine. If mechanical factors are responsible for low back pain reporting, then logic dictates that there should be evidence that mechanical stimulation of a structure should lead to the perception of low back pain. This section will examine the evidence that such a linkage or pathway exists between mechanical stimulation and low back pain. From a biomechanical stand-point, there are several structures that may lead to pain perception in the back when stimulated. There is evidence in the literature that both cellular and neural mechanisms can lead to pain. Both laboratory and anatomical investigations have shown that neurophysiological and neuroanatomical sources of back pain exist (Bogduk, 1995; Cavanaugh, 1995; Cavanaugh et al., 1997). Typically, these pathways to pain involve pressure on a structure that directly stimulates a pain receptor or triggers the release of pain-stimulating chemicals. Investigations have identified pain pathways for joint pain, pain of disc origin, longitudinal ligaments, and mechanisms for sciatica. In the case of facet pain, several mechanisms were identified including an extensive distribution of small nerve fibers and endings in the lumbar facet joint, nerves containing substance P, high-threshold mechanoreceptors in the facet joint capsule, and sensitization and excitation of nerves in the facet joint and surrounding muscle when the nerves were exposed to inflammatory or algesic chemicals (Dwyer, Aprill, and Bogduk, 1990; Ozaktay et al., 1995; Yamashita et al., 1996). Evidence for disc pain was also identified via an extensive distribution of small nerve fibers and free nerve endings in the superficial annulus of the disc and small fibers and free nerve endings in the adjacent longitudinal ligaments (Bogduk, 1991, 1995; Cavanaugh, Kallakuri, and Ozaktay, 1995; Kallakuri, Cavanaugh, and Blagoev, 1998). Several studies have also shown how sciatic pain can be associated with mechanical stimulation of spine structures. Moderate pressure on the dorsal root ganglia resulted in vigorous and long-lasting excitatory discharges that would explain sciatica. In addition, sciatica could be explained by excitation of dorsal root fibers when the ganglia were exposed to the nucleus pulposus. Excitation and loss of nerve function in nerve roots exposed to phospholipase A2 could also explain sciatica (Cavanaugh et al., 1997; Chen et al., 1997; Ozaktay, Kallakuri, and Cavanaugh, 1998). Finally, the sacroiliac joint has also been shown to be a significant, yet poorly understood source of low back pain (Schwarzer, Aprill, and Bogduk, 1995). Hence, these studies clearly show that there is a logical and well demonstrated rationale to expect that mechanical stimulation of the spinal structures can lead to low back pain perception and reporting. How these relate operationally to clinical syndromes is less certain. Biomechanical logic dictates that loads imposed on a structure must exceed a mechanical tolerance limit for damage to occur. In this section we examine the load tolerances associated with different spinal structures that have been shown to be sensitive to pain, in an attempt to determine whether the levels at which the spinal structures are loaded in the workplace can be expected to exceed the tolerances of those structures. In general, the issue of cumulative trauma is significant for low back pain causality in the workplace. Lotz and colleagues (Lotz et al., 1998) have demonstrated that compressive loading of the disc does indeed lead to degeneration and that the pattern of response is consistent with a dose-response relationship that is central to the idea of cumulative trauma. The literature is divided as to the pain pathway associated with trabecular fractures of the vertebral bodies. Some researchers believe that damage to the vertebral endplate can lead to back problems in workers, whereas others have questioned the existence of this pathway. Those supporting this pathway believe that health of the vertebral body endplate is essential for proper mechanical functioning of the spine. Damage to the endplate nutrient supply has been found to result in damage to the disc and disruption of spinal function (Moore, 2000). This event is capable of initiating a cascading series of events that can lead to low back pain (Brinkmann, 1985; Siddall and Cousins, 1997a, 1997b; Kirkaldy-Willis, 1998). The tolerance of the vertebral endplate has been studied in several investigations. Studies have shown that the endplate is the first structure to be injured when the spine is loaded (Brinkmann, Biggemann, and Hilweg, 1988; Calahan and McGill, in press). The tolerance of the endplate has been observed to decrease by 30-50 percent with exposure to repetitive loading (Brinkmann, Biggemann, and Hilweg, 1988). This pattern is consistent with the evidence that the disc is sensitive to cumulative trauma exposure. The endplate is also damaged by anterior-posterior shear loading (Calahan and McGill, in press). Several biomechanical studies have demonstrated that the tolerances of specific spinal structures can be exceeded by work tasks. Evidence of activity-related damage may also be suggested by the presence of Schmorls nodes. Some research (but not all) suggests that Schmorls nodes are healed trabecular fractures (Vernon-Roberts and Pirie, 1973) and linked to trauma (Vernon-Roberts and Pirie, 1973; Kornberg, 1988). Significant evidence exists that endplate tolerance is dependent on the position of the spine when the structure is loaded. Fully flexed positions of the spine have been shown to greatly reduce loading tolerance (Adams and Hutton, 1982; Gunning and McGill, in press). Thus, proper biomechanical assessments of low back risk at work can be performed only when the posture of the trunk is considered. The industrial surveillance efforts of Punnett et al. (1991) and Marras et al. (1993, 1995) show that risk of low back disorder increases as trunk postures during work deviate from an upright posture. Shear forces applied to the spine have also been shown to decrease the tolerance of the disc structure, especially when the spine is in a flexed position (Cripton et al., 1985; Miller et al., 1986; McGill, 1997). These findings are consistent with the field surveillance observations of Norman et al. (1998) as well as spine loading observations (McGill and Norman, 1985, 1986; Granata and Marras, 1993, 1995a). Finally, age and gender have been identified as individual factors that affect the biomechanical tolerance limits of the endplate. Jagger and colleagues (Jager, Luttman, and Laurig, 1991) have demonstrated through cadaver studies that increasing age as well as gender can affect the strength tolerance of the endplate. All of the industrial surveillance studies shown in Table 6.4 indicate that load location (known to affect trunk posture), observed trunk posture, or both are associated with an increased risk of low back pain at work. Furthermore, the review of the spine loading literature has also indicated that handling loads with the trunk moving in nonneutral postures increases muscle coactivation and the resultant spine loading (Marras and Sommerich, 1991a, 1991b; Granata and Marras, 1993, 1995a, 1995b; Marras and Granata, 1995, 1997b). Loading the spine in these deviated postures decreases the tolerance of the spine structures. Hence, the pattern or risk in the workplace, spine structure loading, and endplate tolerance reductions are all consistent with a situation that would indicate that certain work conditions are related to an increased biomechanical risk for low back disorder. The disc itself is subject to direct damage with sufficient loading. Herniation may occur when under compression and when the spine is positioned in an excessively flexed posture (Adams and Hutton, 1982). Also, repeated flexion under moderate compressive loading has produced repeated disc herniations in laboratory studies (Calahan and McGill, in press). Anterior-posterior shear forces have been shown to produce avulsion of the lateral annulus (Yingling and McGill, in press). Torsion tolerance of the disc is low and occurs at a mere 88 Nm in an intact disc and as low as 54 Nm in the damaged disc (Farfan et al., 1970; Adams and Hutton, 1981). Fatallah and colleagues have shown that such loads are common in jobs associated with greater rates of low back disorder reporting (Fathallah, Marras, and Parnianpour, 1998a, 1998b). Complex spinal postures including hyperflexion with lateral bending and twisting can also produce disc herniation (Adams and Hutton, 1985; Gordon et al., 1991). This observation is consistent with industrial surveillance studies indicating increased risk associated with complex working postures, as laboratory investigations of spinal loading while tasks are performed in these complex postures, both by Fathallah, Marras, and Parnianpour (1998a, 1998b). These investigators have also implicated load rate via trunk velocity in complex working postures as playing a significant role in risk. Evidence exists that biomechanical tolerance to risk factors associated with material handling might also be modulated as a function of the time of day when the lifting is performed. Snook and colleagues (1998) showed that flexion early in the morning is associated with greater risk of pain. Fathallah, Marras, and Wright (1995) showed similar results and concluded that risk of injury was also greater early in the day when disc hydration was at a high level. Hence, the literature suggests a temporal component of risk associated with the time of day of the biomechanical exposure. The cancellous bone of the vertebral body is damaged when exposed to compressive loading (Fyhrie and Schaffler, 1994). This event often occurs along with disc herniation and annular delamination (Gunning and McGill, in press). Damage to the bone appears to be part of the cascading series of events associated with low back pain (Brinckmann, 1985; Siddall and Cousins, 1997a, 1997b; Kirkaldy-Willis, 1998). Ligament tolerances are affected by the load rate (Noyes, De Lucas, and Torvik, 1994). Thus, this could explain the increased risk associated with bending motions (velocity) that have been observed in surveillance studies (Fathallah et al., 1998a, 1998b). The architecture of the interspinous ligaments can create anterior shear forces on the spine when it is flexed in a forward bending posture (Heylings, 1978). This finding is consistent with the more recent three-dimensional field observations of risk (Punnett et al., 1991; Marras et al. 1993, 1995; Norman et al., 1998). In vitro studies of passive tissue tolerance have identified 60 Nm as the point at which damage begins to occur (Adams and Dolan, 1995). This is consistent with the field observations of Marras et al. (1993, 1995), who found that exposure to external load moments of 73.6 Nm was associated with high risk of occupationally related low back pain reporting. Similarly, Norman and colleagues (1998) reported nearly 30 percent greater load moment exposure in jobs associated with risk of low back pain. The mean moment exposure for the low back pain cases was 182 Nm of total load moment (due to the load lifted plus body segment weights). Spine curvature has also been shown to affect the loading and tolerance of the spinal structures. Recent work has shown that when spinal curvature is maintained during bending, the extensor muscles support the shear forces of the torso. However, if the spine is flexed during bending and posterior ligaments are flexed, then significant shear can be imposed on the ligaments (McGill and Norman, 1987; Potvin, McGill, and Norman, 1991; McGill and Kippers, 1994). Cripton and colleagues (1985) found that the shear tolerance (2000-2800 N) of the spine can be easily exceeded when the spine is in full flexion. There also appears to be a strong temporal component to ligament status recovery. Ligaments appear to require long periods of time to regain structural integrity, and compensatory muscle activities are recruited (Solomonow et al., 1998; Stubbs et al., 1998; Gedalia et al., 1999; Solomonow et al., 2000; Wang et al., 2000). The time needed for recovery can easily exceed the typical work-rest cycles observed in industry. The facet joints can fail in response to shear loading. A tolerance has been estimated at 2,000 N of loading (Cripton et al., 1985). Lateral shear forces have been shown to increase rapidly as lateral trunk velocity increases (Marras and Granata, 1997b), especially at the levels of lateral velocity that have been associated with high-risk jobs (Marras et al., 1993). Torsion can also cause the facet joints to fail (Adams and Hutton, 1981). More rapid twisting motions have been associated with high-risk jobs and laboratory investigations have explained how increases in twisting motion can lead to increases in spine loading in compression as well as shear (McGill, 1991; Marras and Granata, 1995). As with most tolerance limits of the spine, the posture of the spine affects the overall loading of the spine significantly (Marras and Granata, 1995). Loading of the specific structure depends greatly on specific posture and curvature of the spine. Load sharing occurs between the apophyseal joints and the disc (Adams and Dolan, 1995). Thus, spinal posture and the nature of the spine loading dictate whether damage will occur to the facet joints or the disc. It has been well established that tissues adapt and remodel in response to load. Adaptation in response to load has been identified for bone (Carter, 1985), the ligaments (Woo, Gomez, and Akeson, 1985), the disc (Porter, Adams, and Hutton, 1989), and the vertebrae (Brinkmann, Biggeman, and Hilweg, 1989a, 1989b). Adaptation suggests that there is good rationale for the higher risk observed in response to high-risk jobs demanding high spinal loading as well as very low levels of spinal loading (Chaffin and Park, 1973; Videman, Nurminen, and Troup, 1990). The lowest level of risk has been observed at moderate levels of loading. Thus, there appears to be an ideal zone of loading that minimizes risk. Above that level, tolerances are easily exceeded; below that level, adaptation does not occur. This is consistent with epidemiologic findings as well as the adaptation literature. A body of literature exists that attempts to explain how psychosocial factors may be related to the risk of low back disorder. While reviews have implicated psychosocial factors as associated with risk (Bongers et al., 1993; Burton et al., 1995) and some have dismissed the role of biomechanical factors (Bigos et al., 1986), few studies have properly evaluated biomechanical exposure along with psychosocial exposure in these assessments. A recent study by Davis and Heaney (2000) has shown that no studies have been able to adequately assess both dimensions of risk. A recent biomechanical study (Marras et al., 2000) has shown that psychosocial stress does have the capacity to influence biomechanical loading. This laboratory study has demonstrated how individual factors such as personality can interact with perception of psychosocial stress to increase trunk muscle coactivation and subsequent spine loading. Hence, it appears that psychosocial stress may influence risk through a biomechanical pathway. Collectively, this review has shown that there is a strong biomechanical relationship between risk of low back disorder reports and exposure to physical loading in the workplace. The epidemiologic evidence has shown that risk can be identified when ergonomic evaluations properly consider: (1) worker capacity in relation to job demands, (2) the load location and weight magnitude relative to the worker, (3) temporal aspects of the work, (4) three-dimensional movements while the worker is lifting, and (5) exposure to multiple risk factors simultaneously. The biomechanical literature that has evaluated the loading of the spine structures in response to these field-identified risk factors has shown that there are identifiable changes in the recruitment pattern of the muscles and subsequent increases in spine structure loading associated with greater exposure to these risk factors. The literature has also identified pain pathways associated with increased loading of the structures. Finally, our review of the literature has shown that the loading of these spinal structures can lead to structural damage that can precipitate the pain response pathway. While there are certainly individual factors that put a person at risk for back pain, overall this body of literature indicates that back pain can be related to levels of excessive mechanical loading of the spine that can be expected in the workplace. The literature also indicates that appropriate reduction of work exposure can decrease the risk of low back disorder. Studies that have not been able to identify this linkage typically have used assessment techniques that were either not appropriate or insufficiently sensitive for proper biomechanical assessment at the workplace. Hence, it is clear, from a biomechanical perspective, that exposure to excessive amounts of physical loading can increase the risk of low back disorder. |
A force of acts on a body for 2 s and produce a displacement of the average power is
Pos Terkait
Copyright © 2024 kemunculan Inc.
