Designing for the Life Span Segment 4
Designing for the Life Span Segment 4 
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[D]
Testing the deflection in a chair with a suspension system.
Use the clenched fist to test the full deflection of a suspension system. Position the fist directly over the center of the seat cushion... Plunge the fist into the cushion into the seat cushion until resistance stops the deflection. There should be no "bottoming"in the suspension system.
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After examining the mechanical design of the chair, the chair should be set back upright. Push the fist down into the middle of the chair with increasing force. A good spring system will permit deflection and also progressively resist deflection - but never give the feeling of a hard spot where the deflection stops. If that feeling is established, the chair will always be uncomfortable in some period of time. Also examine for what is called "ramming," a rise on the leading edge of the cushion that prevents pelvic rotation. The cause of most chair inflicted decubitus ulcers - or bed sores - result from unchecked pelvic rotation placing the tissue on the buttocks in shear causing the sores. The rise or "ramming" of the chair positions the pelvis properly and prevents this scrubbing.
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