A new trend in the study of low back pain in workplaces

In 1996, the number of low back pain (LBP) cases totaled 5,162 in Japan, accounting for 60% of the total number of officially recognized cases of occupational diseases. In recent decades, the number of LBP cases, however, has been gradually decreasing. The rate of this decrease is slower in non-manufacturing industries than in other sectors, while the prevalence rate of LBP has tended to increase among workers in tertiary industries in 1995 and 1996. Epidemiological studies have clarified that workplace factors of LBP include not only the handling of heavy materials, but also unnatural postures, sudden and unexpected motions, and individual worker's characteristics. It was, therefore, suggested that comprehensive countermeasures to prevent LBP be undertaken on the basis of work environmental control, work management practice, and health care. The Labor Ministry of Japan issued guidelines for the prevention of LBP in 1994, in which various factors in the work environment, the importance of readiness for motion, standardization of work procedures, and exercise before work were newly outlined. In addition to the psychophysical and biomechanical models of LBP so far reported, established findings of physiological studies on muscle tone and postural reflexes have been conceptually introduced into the guidelines. Such physiological findings are essential for the mechanistic elucidation of work-related LBP and the preparation of its countermeasures, as LBP can also be caused by sudden and unexpected motions as well as various environmental factors. Physical and mental readiness to cope with changes in voluntary motions is a prerequisite for the prevention of LBP in daily work, which constitutes time-sequential changes in posture and motion under various environmental conditions. This paper critically reviews the workplace factors of LBP, its models for evaluating the workload on the body, and environmental risk factors with reference to the neural control of muscle contraction underlying the voluntary motion of workers.