Whole-Body Vibration Exposure and Occupational Work-Hardening

There are some 7 million workers in the United States who are exposed to occupational whole-body vibration (WBV). These include operators of trucks, buses, locomotives, lift trucks, helicopters, heavy equipment, and farm vehicles, as well as workers in plants containing vibrating heavy machinery. Over the past 25 years, numerous worldwide epidemiology studies of various worker populations have linked WBV exposure to lumbar spine pain and degenerative/herniated disc diseases. The Achilles' heel of human vulnerability appears to be in the 4- to 8-Hz frequency band when vertical (head-to-toe) vibration impinges on the body; the body trunk responds to this vibration by uncontrollably internally amplifying and thus exacerbating the effects of the impinging vibration. Consequently, the lumbar spine stretches the most in this frequency range (the seated person's natural frequency ranges from 4.5 to 5.5 Hz), thus further exacerbating the situation. At resonance, a small amount of impinging vibration acceleration can produce an undesirable and negative human response, as compared with other vibration frequencies that require higher acceleration levels to produce the same order of response. The practical reality of the foregoing and of the workplace is that many trucks, buses, and other sources of WBV also produce these vibration frequencies, which impinge on workers' bodies and thus are potentially very harmful.1,2 Background In an effort to protect millions of workers from the physical agent of vibration, whole-body exposure guidelines and standards such as the American Conference of Governmental Industrial Hygienists threshold limit value for WBV, the American National Standards Institute Standard S3.18, and the International Standards Organization (ISO) Standard 26313-5 and the European Union directive for WBV6 were established and have been in use, some for as long as 20 years. WBV engineering controls, such as the development of "air-ride seats" and floating cabs in vehicles have also been used for many years. Epidemiologic work7 indicates significant association of the low back complaint with exposures to both industrial and nonindustrial (particularly vehicular) vibration. It is a significant problem not only in terms of pain and lost productivity, but also in terms of societal cost.8 The Centers for Disease Control9 concurred that back pain and vertebrogenic pain may be associated with WBV. Germany has ascribed an occupational disease number (number 2110) to vibration-induced spinal disorders.10 Heliovaara11 studied the apparent ill effects of man in the machine age, and Kelsey et al12 found an increased incidence of herniated discs with long-term exposure to riding in automobiles and trucks. Bongers and Boshuizen13 and Bovenzi and Zadini14 recommended against relaxing current vibration-exposure guidelines, based on their meta-analysis and epidemiologic work, respectively. Behrens et al15 confirmed the high incidence of low back trouble in trucking and heavy equipment operation. Various industrial workers exposed to vibration, including shipyard workers, sheet metal workers, miners, and those workers who use vibrating hand tools, have demonstrated abnormal changes in the musculoskeletal, cardiovascular, and gastrointestinal systems.16 Gearhart17 found that helicopter vibration can cause changes in the musculoskeletal system. Investigators have suspected or have found associations between truck or tractor vibration environments and early or unusual degenerative changes in the spine.18-20 In a study of 3920 patients,21 it was determined that the complaint of low back pain was more common in individuals exposed to vibration, eg, truck and tractor driving and heavy construction equipment operation. In the New York City subway system, lateral vibration was associated with a high prevalence of low back pain.22 This corroborates the effects on the spine found in other railroad environments, attributed to vertical and horizontal vibration.23,24 Seidel and Heide25 reviewed 185 articles on vibration and concluded that people who sit in a vibrating environment that is close to or exceeds the ISO exposure limit5 place their musculoskeletal system at risk. In all, their review covered 43,000 workers exposed to WBV and 24,000 control subjects. The data "indicate an increased health risk of the spine and peripheral nervous system after intense, long-term whole body vibration." The authors assessed each article for control groups, statistical analyses, the profession exposed, and the organ system studied. The effects of the following variables were classified: worker orientation, age, sex, combined effects, controls, subjective complaints, trunk muscles, various musculoskeletal conditions, biomechanical factors, biochemical factors, exposure duration, and body type. Their compilation indicated that certain vibration exposures are a health risk for the back and that the ISO 2631/15 exposure limit should be changed to a more conservative level. Hulshof and van Zanten26 also reviewed studies of spine disorders that resulted from vibration. They rated the studies in terms of quality of exposure data, health-effects data, and methodology. No study rated highly on all three criteria. Few of the epidemiological studies were prospective, and none measured vibration exposure limits according to the recommendations set out by ISO 2631/1.5 Determining the vibration exposure limit would be problematic in a laboratory-based study. Although it would be possible under the informed-consent concept, it is unlikely that subjects would volunteer to being shaken until low back problems were produced. An epidemiologic approach was taken by other research groups. 14,27-29 Bovenzi and Zadini14 followed up a group of bus drivers. Bovenzi and Betta27 followed up a group of agricultural tractor drivers. Both studies correlated low back diagnoses to vibration exposure levels and concluded that the ISO 2631/15 vibration guidelines were not conservative enough, corroborating the findings of Bongers and Boshuizen13 in their meta-analysis on low back pain related to WBV exposure at work. A more recent study27 recommended following the Commission of European Communities'30 A(8) action level of 0.5 ms-2 for WBV. Magnusson et al28,29 studied bus and truck drivers both in Goteborg, Sweden, and in Vermont and found a significant correlation between low back pain and history of WBV exposure. These were important studies because they followed the guidelines set out by Hulshof and van Zanten26 and represent the beginning of the development of firm guidelines for reducing WBV in the workplace. These studies begin to clarify the exposure-response relationship needed for setting exposure guidelines.1-2,31 Using data collected for the 1988 Occupational Health Supplement to the National Health Interview Survey,32,33 Behrens et al15 reported that the highest prevalence of injury-related back pain and the overall prevalence of back pain as a result of repeated activities at work were associated with WBV work conditions. Whole-Body Vibration and Work-Hardening We have become keenly aware of the proliferating use of simulated WBV exposure as part of the therapeutic "work hardening" modality of returning previously back-injured workers to their jobs. Many of these workers include truck and bus drivers, heavy equipment operators, etc, for whom WBV may well have been significantly involved in their initial medical problems. Although technological advances and economics have changed the scope of work, it is evident that the work-hardening programs of today have their roots in the work cure of the early 1900s.34 On the other hand, human motor behavior continues to evolve and, therefore, may never be fully understood.35 Purposeful activity is viewed as a necessary prerequisite for the formation of coordinative structures, which are the bases for skilled movement.36 Occupational therapy must face and confront its roles in relation to the field of motor control; a health care profession involved with performance and motor control issues cannot afford to ignore the rapid advances in neuroscience and the parallel developments in the field of motor control.35 Occupational therapy treatment approaches that are based on neurophysiological principles are fairly well accepted, and there is much potential for the development of treatments that will facilitate recovery of function.35 There are two main approaches to motor-skill acquisition as a theoretical treatment of the learning and relearning of motor skills in rehabilitation of physical dysfunction: (1) traditional approaches to motor learning focus primarily on the actor, and (2) the event approach treats the actor and the environment as inseparable in the acquisition of skills.36 In occupational therapy, clients must learn or relearn motor skills through the use of activities, and the processes that underlie motor learning should guide therapy.37 Three major factors that affect motor learning are environmental conditions, cognitive processes, and movement organization.37 The processes underlying skill acquisition also depend on the nature of the stage of the learner, type of task, feedback, practice, and facilitation of skill acquisition.38 Examination of the clinical implications of these factors, however, results in contradictory predictions regarding optimal practice conditions for motor skills. Proteau et al39 showed that the basis of movement learning is a sensorimotor representation that consists of integrated information from central processes and sensory feedback derived from previous experiences on the movement task. The authors developed the idea that learning is specific to the conditions that prevail during skill acquisition and that this has implications for the ideas of the generalized motor program and schema theory.39 If the criterion skill is identical with that experienced in practice, the practice should be tailored to be as specific as possible to the actual task conditions that will be ultimately experienced.40 The limitation of this rule is that it requires some time for the motor control to be involved. Therefore, for activities or external impacts that have too short a latency, a different strategy should be applied. For the extremity muscles, it has been found that after the load is added, several modifications to the background electromyography (EMG) activity represent the compensation for the added load: (1) monosynaptic stretch reflex (M1 reflex) with a latency of 30 to 50 ms,40 (2) functional polysynaptic stretch reflex (long loop reflex, or M2 reflex) with a latency of 50 to 80 ms40 or 90 ms,41 (3) triggered reactions with a latency of 80 to 120 ms,40 (4) proprioceptive visual and vestibular responses triggered by changes in posture and taking approximately 150 ms,42 and (5) voluntary reaction time (M3 response) with latency of 120 to 180 ms.40 Aleksiev et al43,44 and Magnusson et al44 found that patients with chronic low back pain are exposed to several aggravating factors during sudden load: (1) the time from load onset to the first detectable erector spinae EMG response was significantly slower in low back pain patients vs healthy subjects, thus allowing the load to stress the spinal structures longer, (2) the EMG reaction magnitude was lower, a probable inability of the muscular system to protect the spine adequately from sudden load, and (3) the ground reaction latencies to sudden load were shorter and the forces and moments were larger, thus allowing the larger forces to propagate faster through the multijoint system. It seems more appropriate in work-hardening programs involving persons exposed to WBV to use motor transfer principles. The goal is to apply the learning to some variation in the skill or context, ie, to generalize to novel situations.40 The premise of work-hardening is that if an individual practices a task, he or she can remind and retune the associated pool of motoneurons to perform it in a better manner. This greatly facilitates one's return to a functional status. The technique works because with practice, opposing muscles-alternately acting as agonists and antagonists-optimize their reciprocal inhibition and hence the sequence in which they alternately turn on and off. With practice, the control system retunes its model of the task so it can perform in an optimal, anticipatory fashion. Muscles fire under control of the model of the task with some monitoring of the task conditions. The method also works because it assumes that the task being practiced does not change faster than the rate at which the muscles can accommodate it. In addition to the system-response characteristics, the seated human exhibits a natural frequency and can resonate when vertically vibrating. WBV studies have established that the seated human has a resonant frequency in the range of 4.5 to 5.5 Hz, close to those frequencies produced in common working and vehicular environments. A structure vibrating at its resonant frequency is more likely to fail.45-48 In vivo WBV studies have established the motion characteristics of the lumbar region during vibration exposure, which include maximal stretching of the posterior aspect of the intervertebral disc.49-52 In addition, in a dynamic, sinusoidal vibration environment, erector spinae muscles respond increasingly out of phase with increasing sinusoidal frequency.53,54 This, as well as shock loading, would create a condition in which the discs are loaded asymmetrically for a brief period before the additional load of the stabilizing effect of the trunk muscles is applied. With a superimposed flexion resulting from a seated position, accompanied by axial torque and lateral bend because of operating controls, the posterolateral region of the disc is at risk of mechanical damage or failure.55,56 Discussion and Conclusions One of the questions central to this discussion is: Is maintaining a supported, seated posture a motor skill? If it were a motor skill, the musculoskeletal control system would contain a model that could be modified and trained for use in purposeful action whose demands could be anticipated. It could be modified, if the task to be accomplished is slow enough that the individual can anticipate and accommodate slight variations in the normal performance of the task. But we know from studies of seated subjects' responses to WBV and unexpected sudden load application that the body is unable to anticipate rapidly applied loads and therefore can only react to them. Once the task demands change faster than the muscles can respond, then the way the neuromuscular control system adapts to an environment no longer works. The literature confirms that commonly experienced jolt/vibration environments contain components that occur faster than the human neuromuscular control system can accommodate. Hence the muscles can only react, and those reactions place unnecessary and deleterious loads on the intervertebral disc. That reaction, coupled with the mechanically extreme nature of the flexed sitting position, puts the intervertebral disc at risk of failure.2 Back muscles respond out of phase with a vibratory stimulus. The delay is based on a musculoskeletal response characteristic over which we have no conscious control. Even if we could voluntarily respond to a jolt/vibration stimulus, it would still serve only to add load to the spine. For these reasons, it is important to isolate the spine from jolt/vibration conditions, especially in the mechanically extreme, seated posture. Although work hardening is appropriate under conditions in which the muscles can control and optimize the loading of the hard and soft tissues, it is not appropriate for the complex, seated, jolt/vibration environment. One's time and money would be better spent diagnosing and treating the seated, jolt/vibration workplace with the aim of providing optimal back support and maximal attenuation of jolt/vibration. We have devoted many years of our collective professional lives to the health and safety consequences of occupational vibration exposure and to its control and etiology; we know of no situation in which WBV exposure can be deemed therapeutic, either alone or together with other modalities. Justifiably, we therefore must question and voice our collective concern and caution to the occupational medicine community when using WBV as part of a therapeutic modality regime such as work-hardening. D. E. Wasserman, MSEE, MBA D. G. Wilder, PhD M. H. Pope, DrMedSc M. Magnusson, DrMedSc A. R. Aleksiev, MD, PhD J. F. Wasserman, PhD