Journal of Biomechanics 31 (1998) 263 — 271 A kinematic and kinetic analysis of the sit-to-stand transfer using an ejector chair: implications for elderly rheumatoid arthritic patients Bridget J. Munro*, Julie R. Steele, Guy M. Bashford, Melinda Ryan, Nicole Britten Department of Biomedical Science, University of Wollongong; Institute of Rehabilitation and Allied Health, Wollongong, NSW 2522, Australia Illawarra Regional Hospital, Institute of Rehabilitation and Allied Health, Wollongong, NSW 2500, Australia Received in final form 9 September 1997 Abstract Twelve elderly female rheumatoid arthritis patients (mean age"65.5$8.6 yr) were assessed rising from an instrumented Eser Ejector chair under four conditions: high seat (540 mm), low seat (450 mm), with and without the ejector mechanism operating. Sagittal plane motion, ground reaction forces, and vertical chair arm rest forces were recorded during each trial with the signals synchronised at initial subject head movement. When rising from a high seat, subjects displayed significantly (p(0.05) greater time to seat off; greater trunk, knee and ankle angles at seat off; increased ankle angular displacement; decreased knee angular displacement; and decreased total net and normalised arm rest forces compared to rising from a low seat. When rising using the ejector mechanism, time to seat off and trunk and knee angle at seat off significantly increased, whereas trunk and knee angular displacement, and total net and normalised arm rest forces significantly decreased compared to rising unassisted. Regardless of seat height or ejector mechanism use, there were no significant differences in the peak, or time to peak horizontal velocity of the subjects’ total body centre of mass, or net knee and ankle moments. It was concluded that increased seat height and use of the ejector mechanism facilitated sit-to-stand transfers performed by elderly female rheumatoid arthritic patients. However, using the ejector chair may be preferred by these patients compared to merely raising seat height because it does not necessitate the use of a footstool, a possible obstacle contributing to falls.  1998 Elsevier Science Ltd. All rights reserved. Keywords: Ejector chair; Seat height; Rheumatoid arthritis; Elderly 1. Introduction Rising from a chair is a common activity of daily living. However, many elderly people, in particular, those with rheumatoid arthritis (RA), often have difficulty rising from a chair. Consequently, these individuals have prob- lems living independently and risk institutionalisation. The ability of the RA patient to rise from a chair may be restricted by pain, stiffness, reduced joint range of motion, muscle weakness, and incapacitation due to joint deformity (Scott, 1978). Questionnaires completed by 379 elderly patients, 48.5% of whom were afflicted with RA, found that 42% of all patients questioned experienced difficulty when rising from a chair (Munton et al., 1984). Therefore, modified chairs which facilitate the rising pro- cess may benefit the elderly RA patient. * Corresponding author. The process of rising can be facilitated by raising seat height and/or providing chair arm rests for assistance (Burdett et al., 1985; Ellis et al., 1984; Wheeler et al., 1985; Wretenberg et al., 1993). Both increased seat height and use of chair arm rests have been found to reduce net moments and joint loading forces acting about the knee in healthy subjects during rising (Arborelius et al., 1992; Wretenberg et al., 1993). It is suggested that reductions in joint moments and loading forces are associated with less damage to the joint structures (Fleckenstein et al., 1988). A third-chair modification to assist in rising is use of a mechanical device which imparts a lifting force to assist the sit-to-stand (STS) transfer (Bashford et al., 1994; Ellis et al., 1979; Wretenberg, et al., 1993). Again, knee mo- ments and joint loading forces have been reported to decrease when subjects rose using a motorised chair compared to rising unassisted (Ellis et al., 1979). The Eser Ejector chair, which uses a spring mechanism to assist 0021-9290/98/$19.00  1998 Elsevier Science Ltd. All rights reserved. PII S0021-9290(97)00130-9