Thursday, July 21, 2005

Digital Vibration Threshold - Maybe an early sign of risk

Validation studies of risk assessment tools in ergonomics reliably rank jobs within the limits of statistical power and a limited time window. The goal is identifying a level of stressors which x percent of the exposed population can tolerate for 30 years. or some shorter career.

The obstacle is that workstations [jobs] change within a year, workers move from job to job, injuries and pain experienced in an observed job may come from previous jobs, previously injured workers may be in lower stressor jobs, etc.

Probably the complaints of newly exposed workers is the most reliable measure of long term risk. But workers notoriously under report physical problems. This is because a worker is usually in the job which the worker likes better than any other job they can get, under a union contract or management dictated assignment system. A complaint will gain the worker nothing at all, or may gain a restriction into a job the worker didn't want.

The other problem with symptoms or complaints is the meat vs gristle issue. What hurts is muscle [meat], which has lots of innervation compared to connective tissue [gristle]. Muscle recovers from over exertion by building more strength and endurance; discomfort is reduced by conditioning. The muscle is strong enough to pull on the connective tissue with enough force to damage that tissue. As the connective tissue becomes inflamed, it squeezes or otherwise irritates a nerve, which causes pain or numbness. Which comes first may be determined by where the pain receptor neurons a located in the nerve bundle.

Maybe digital vibration threshold is an early early indicator of impact on the nerves, a response to sub-acute exposures which will predict damage from chronic exposure. In addition, DVT might be a continuous variable, easier to manage for exposure response.

DVT in response to vibration would not be interesting. DVT in response to "awkward postures" may be considered the increased muscle force and connective tissue impact of that force caused by the awkward posture applied to the same external force.

BrooklynDodger notes that correlations with certain risk factors may disappear in a particular cohort if there is no or little variation in that risk factor. For example, if repetition is about the same in all the jobs, then repetition disappears as a risk factor in the cohort.

Ergonomics. 2005 Jan;48(1):66-77.

Digital vibration threshold testing and ergonomic stressors in automobile manufacturing workers: a cross-sectional assessment.

Gold JE, Punnett L, Cherniack M, Wegman DH.

Department of Work Environment, University of Massachusetts, Lowell, MA 01854, USA.

... Reduced sensitivity to vibration in the fingertips (a function of nerve integrity) has been found in those exposed to segmental vibration, to hand force, and in office workers. The objective of this study was to determine whether an association exists between digital vibration thresholds (VTs) and exposure to ergonomic stressors in automobile manufacturing. Interviews and physical examinations were conducted in a cross-sectional survey of workers (n = 1174). ... VTs were separately associated with hand force, vibration as felt through the floor (whole body vibration), and with an index of multiple exposures in both tool users and non-tool users. Additional associations with contact stress and awkward upper extremity postures were found in tool users. Segmental vibration was not associated with VTs. ...

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