No literature related to the study of sitting forces on chairs sat on by people who weighed over 136 kg was found. The Business Institutional Furniture Manufactures Association needs force data for development of performance test standards to test chairs for users who weigh up to 181 kg. 20 participants who weighed from 136 to 186 kg completed 6 tasks on an instrumented chair in the sequence of sitting down, remaining seated and rising. Effects of sitting motion, armrest use and seat cushion thickness on vertical sitting forces and centre-of-force were investigated. Results indicated hard sitting down yielded the highest sitting force of 213% in terms of participants' body weights. Armrest use affected sitting forces of normal sitting down, but not of rising and hard sitting down. Cushion thickness affected sitting forces of normal and hard sitting down and shifting, but not of rising, static seating or stretching backward situations. Practitioner Summary: Results of the sitting force and centre-of-force data obtained for this research can help furniture manufacturers develop new product performance test standards for creating reliable engineering design and manufacturing quality and durable products to meet a niche market need.

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Sinuous springs are the most commonly used method of seat support in upholstered furniture. The dynamics of load transference by these springs to the furniture frame members has not been thoroughly studied. Rational strength design of sinuous-type upholstered furniture frames, especially their seating support systems, requires information concerning the magnitudes and directions of spring loads on spring supporting rails. These values are important for frame designers to be able to adjust member sizes and materials and select appropriate fasteners. Presently, furniture design largely depends on trial and error. A load cell was conceived, designed, and verified which can determine horizontal and vertical components of a spring load. Preliminary results showed that the average initial spring tension on each load cell, with no external loading applied to the springs, was 46 lb. (205 N) horizontal. For some situations, the horizontal load components actually decrease when a person comes to rest on the seat. Vertical and horizontal loading is greatest on the center three springs of the seat which used five, equally spaced springs in this paper.