Saturday, May 28, 2005

Ergonomics - Who's the Expert? Ask the person doing the job.

The current problem in ergonomics for manufacturing jobs is that most jobs are rated "moderate" by risk assessment methods. Most MSD injuries come from these moderate jobs, and appear randomly across this stratum. Is this because the risk assessment tools are not distinguishing risk? Or, do all the jobs have to be abated in no particular order. This U of M Center for Ergonomics may shed some light.

Eight analysts observed 32 subjects and estimated force as a percent of subjects' maximum voluntary contraction (% MVC). High force exertions (> 67% MVC) were underestimated and low force exertions (<34%> were overestimated. Because forceful exertions often lack conspicuous, visual cues that indicate the magnitude of the exertion, it is recommended that the individuals who actually perform the activity provide feedback when evaluating force."

BrooklynDodger thinks the problem is observing hand and wrist posture. Ergonomic risk is understood by the force, frequency, posture, duration paradigm. In this listing "force" is external force applied on objects by the worker. But the damage is done by the internal force applied by the muscle to some body structure. The force the muscle must apply internally is a function of the external force and the joint posture. Posture of the back and the shoulder are relatively easy to observe: these body parts are large and don't move that fast. Not so the hand and wrist.

The Dodger expects these observers were not biased by management wanting the jobs to pass the analysis. They weren't consultants fearing loss of the contract if they flunk too many jobs, or flunk jobs where the workers aren't complaining. They weren't biased by knowing there had been an injury and wanting to flunk the job. They likely were biased by not wanting to be too far off the evaluations by their peers. Which means rating the job really bad when most observers rated it really good, or the opposite. This would have biased observations to the mean, as observed.

Now that science - at least one study which should be replicated - suggests that the worker doing the job knows best, how to implement?

J Occup Rehabil. 2004 Dec;14(4):281-94.

Observational assessment of forceful exertion and the perceived force demands of daily activities.

Marshall MM, Armstrong TJ.
Department of Industrial and Systems Engineering, Rochester Institute of Technology, Rochester, New York 14623, USA.

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