This work studied in vivo, the effects of common loading environmental on the mechanical response of the seated human. The eventual goal of this work is to evaluate and control the occupational health hazard low back pain, a musculoskeletal injury, by establishing an envelope of loading conditions which should not be exceeded if the spine is to avoid mechanical damage. Proceeding from prior in vitro findings of short-column buckling in the lumbar spine following vibration exposure, this proposal evaluated how the supporting trunk musculature responded to an unexpected load application after a 40minute vibration or static load intervention [seated vertical vibration, seated lateral vibration, seated lateral and vertical vibration or sitting still (as a control)]. This simulated the sudden and unexpected shift of an object in the hands of the car or truck driver who has driven for 40 minutes at an exposure level consistent with the 8-hour, Fatigue, Diseased Proficiency (FDP) level of the ISO 2631 (1985) guideline on human exposure to whole-body vibration. Normal walking (as a break) for five minutes, prior to an unexpected load application, was also tested to determine if it would serve as a reasonable control measure. This would allow lumbar discs to return, via creep behavior, to the upright posture orientation where the facets are more firmly engaged. Separate forty-minute exposures to vertical (5 hertz) and side-to-side (2 hertz) sinusoidal vibration, at the International Standard Organization 8-hour, FDP limit, significantly increased the erector spinae total reaction time and peak muscle EMG activity in response to an unexpected load. In other words, the muscles take more time to respond and generate greater predicted forces following those relatively short (when compared to an 8-hour day) vibration exposure durations. This work questions whether the ISO 8 hour FDP vertical vibration limit is conservative enough to protect the disc from mechanical damage. Walking for five minutes is beneficial because it tends to reset the system. The effect of walking on subject who had been vibrated from side-to-side showed a trend of decreasing reaction time and peak EMG activity in response to an unexpected load, but it did not reach the p<.05 level of significance. Walking after vibrating up-and-down tended to shorten the back muscles' reaction time and significantly p<.005 decreased the peak muscle EMG activity, hence decreasing the predicted forces generated and applied to the intervertebral disc. The results show that one should walk around for five minutes after exposure to vertical and also probably horizontal whole-body vibration prior to placing oneself in a condition to be exposed to an unexpected load.
University of Vermont, Burlington, Vermont; Iowa Spine Research Center, Biomedical Engineering Depatment, University of Iowa, Iowa City, Iowa