Get a grip on ergonomic handle design

By: 
February 07, 2000

Elastomer Systems

Take a walk down the aisles of any hardware or housewares store, and you can't help but notice the growing popularity of thermoplastic elastomer (TPE) grips. TPE's tactile feel, matte appearance, moldability, and heat resistance have already made it the "soft-touch" material of choice for power and hand tools, appliances, kitchen tools, and other products. And thanks to the tactile characteristics of TPE, these soft-touch grips are invariably described as "ergonomic." Yet more often than not, there's no attempt to quantify this term--as if all ergonomic designs and materials are created equal.

The lower the muscle activity-which is expressed in as a percent of the test subject's maximum contraction capability or MVC-the more efficient a material from an ergonomic standpoint. One of the Santoprene TPE grades came in with the lowest mean muscle activity, while the bare metal knob chalked up the highest values. The muscles studied were the forearm extensor muscles, and two muscles in the hand-the adductor pollicis (AP) and the Interossei/lumbvricale (LUM).

Of course, competing designs are rarely equal, and the number of soft-touch materials available to the design engineer appears to grow almost daily. For these reasons, we believe that design engineers would benefit from an objective measurement system for grip ergonomics, one based on quantifiable measures such as 15% less muscle activity or 40% reduction in stressful wrist postures.

"Feeling tone" ratings try to capture subjective notions of what makes a good grip material. Ratings were based on questionnaires filled out by study participants prior to testing, after the torque tests, and after the pinch tests. In each case a TPE, one of two Santoprene grades, scored the highest mean rating.

In what could serve as a model for future ergonomic assessments from a materials perspective, Ergonomic Technologies Corp. (Oyster Bay, NY) recently evaluated how well bare metal, polypropylene, and various Santoprene rubber TPE grades performed in two simulated grip designs: a knob and a sheet. The evaluation determined the effectiveness of each material in terms of the muscle effort exerted by 20 test subjects as they first twisted, or applied torque force, to the knob and later held the sheets in a pinch grip while counteracting a known weight. Our guiding principle: The more ergonomically effective the material, the lower the muscle effort required for a given operation.

Measuring ergonomics. For the torque test, the test subjects used a fist-like "power grip" to grasp the knobs while attempting to register a maximum torque on the tester. For the pinch test, the subjects held each sheet, fitted with5- and 10-lb weights, in a pinch grip with their arms hanging straight down for four seconds. All the tests were "randomized" both in the initial selection of the subjects and in the order in which the various grip samples were presented during testing.

To get a handle on muscle effort, the study relied on a technique called surface electromyography (EMG), which measures electrical activity on the skin surface as the exertion of an underlying muscle group triggers biochemical changes. Ergonomics Technology reassured those muscle sites responsible for

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