As many as 85% of adults experience lower back pain that interferes with their work or recreational activity and up to 25% of the people between the ages of 30 to 50 years report low back symptoms when surveyed . Of all lower back patients, 90% recover within six weeks irrespective of the type of treatment received . The remaining 10% who continue to have problems after three months or longer account for 80% of disability costs . Webster and Snook  estimated that lower back pain in 1989 incurred at least $11.4 billion in direct workers' compensation costs. Frymoyer and Cats-Baril  estimated that direct medical costs of back pain in the U.S. for 1990 exceeded $24 billion, and when indirect costs predominately associated with workers' compensation claims were added, the total cost was estimated to range from $50 billion to $100 billion. One U.S. workers' compensation insurance company incurred costs for lower back pain of about $1 billion per year, whereas the total cost for carpal tunnel syndrome in 1989 was $49 million . Hence, it can be concluded that despite an increasing public attention to cumulative trauma disorders (CTDs) of the upper extremities, occupational low back disorders account for the most significant industrial musculoskeletal disorders (MSDs).
The prevention of low back pain is nearly impossible due to its prevalence. However, occupational safety and ergonomic principles correctly dictate that one should reduce the physical risk factors by worker selection, training, and administrative and engineering controls in order to diminish the risk of severe low back injuries due to overexertions or repetitive cumulative trauma disorder of the low back [6,7]. The fundamental inability to determine "How much of a risk factor is too much?" has been one
The Ohio State University of the most critical hindrances toward developing an ergonomics guideline for safe and productive manual material-handling tasks.
Industrial low back disorder (LBD) is a complex multifactorial problem. A full understanding of it can only be gained by considering the personal and environmental risk factors which include both the biomechanical and psychosocial factors; the latter have been identified in the literature in the form of predictors or exacerbators of musculoskeletal disorders . However, careful review  of this literature indicates that the results are inconclusive while the following factors are identified to be of significance: monotonous work, high perceived workload, time pressure, low control on the job, and lack of social support. As for the former factors, the results of epidemiological studies have associated six occupational factors with low back pain symptoms. These are (1) physically heavy work, (2) static work postures, (3) frequent bending and twisting, (4) lifting and sudden forceful incidents, (5) repetitive work, and (6) exposure to vibration . In a large retrospective survey, lifting or bending episodes accounted for 33% of all work-related causes of back pain . Troup et al.  have identified the combination of lifting with lateral bending or twisting as a frequent cause of back injury in the workplace.
Parnianpour et al. , in their study of the fatiguing dynamic movement of the trunk against a set resistance, were the first to report on the combined analysis of triaxial motor output and movement patterns. They showed that during fatiguing trunk flexion and extension, there were significant reductions in the velocity, range of motion, and total angular excursion in the intended (sagittal) plane of motion, and a significant increase in the range of motion and total angular excursion in the accessory (coronal and transverse) planes. The presence of more unintended motion in the accessory planes indicates a loss of coordination and more injury-prone loading conditions for the spine. Numerous studies have demonstrated that soft tissues subjected to repetitive loading show creep and stress relaxation behavior because of their viscoelastic properties . Since the internal stability of the spine is maintained by its passive and active structures, there is an even greater need for muscular control in maintaining a given level of spinal stability after repetitive movements. Hence, the presence of repetitive dynamic trunk exertions increases the risk by adversely affecting the performance of the neuromusculoskeletal system (i.e., diminished control and coordination, reduction in magnitude and rate of tension generation in the muscles, and the reduction in the stiffness of spinal tissues).
Videman et al. , based on their prospective cohort study among 5649 nurses, strongly suggested that job-related factors rather than personal characteristics were the major predictors of back disorders among nurses. Bigos et al. , in the "Boeing" study, showed that manual handling tasks and falls were associated with 63% and 10% of low back compensation cases, respectively. Burdorf  reviewed 81 original papers concerning the LBD in occupational groups and concluded that very few studies provided quantitative measures of the exposures. Punnet et al.  showed increased odds ratios of low back disorders (determined from injury records and physical exams) for exposure to awkward postures of the trunk in an industrial setting. The tasks with severe trunk flexion greater than 10% of cycle time had an odds ratio (OR) of 8.9. Marras et al.  extended the analysis to include the dynamic components of the trunk motion. It was shown that the mean peak sagittal trunk velocity and acceleration were 49°/sec and 280°/sec2, respectively, while the maximum peak in the database exceeded 200°/sec and 1300°/sec2. Furthermore, asymmetric dynamic lifting tasks were found to be more the norm than the exception . The identified risk factors were: lift rates, maximum moment, peak sagittal trunk flexion, and lateral and twisting velocities.
The inability of classical injury models or overexertion phenomena to describe the majority of industrial low back disorders has motivated epidemiologists and biomechanists to search for alternative paradigms. Hansson  proposed a biomechanical loading injury model to describe the possible mechanisms for the occurrence of low-back injuries which we have further modified (Fig. 1.1). Biomechanical loads leading to tissue damage can be from overloading (single application of load surpassing the tissue tolerance), repetitive submaximal loading, and prolonged static loading. Repetitive loading, even below the yield stress of the material, may impose microdamage to the structure, depending upon the magnitude, duration, and frequency of the loading. Due to stress relaxation, the resistance of the material will diminish in prolonged loading, and alternative load paths may predispose the spine to higher
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