Review papers such as by (Kerr et al., 1991; Janssen et al., 2002) categorized studies ac-cording to chair related, subject related or strategy related experiments. Here, with regard to this thesis, chair related influences on STS movement are explained as has been well doc-umented in (Janssen et al., 2002): “The literature indicated that the chair has an influence on the performance of the STS movement (e.g. the height of the seat can make an STS movement impossible). Most research has been focused on the height of the seat, and few studies tried to clarify the influence of the armrest position, use of armrests, or the type of chair on the STS movement.” (Janssen et al., 2002, p. 868ff.)
Seat height
“Lowering the height of the seat makes the STS movement more demanding or even un-successful according to the literature we reviewed (Munton et al., 1981; Schenkman et al., 1990; Munro et al., 1998; Hughes et al., 1996; Burdett et al., 1985). The minimum height for successful rising for elderly people (community-dwelling and nursing home residents 64-105 years of age) with chair rise difficulties appears to be 120% of lower leg length. A lower seat apparently leads to increased angular velocity of the hip in order to stand and to more repo-sitioning of the feet (also called the “stabilization strategy”). In young subjects (25-36 years of age) without impairments, lowering the seat of the chair from 115% to 65% of knee height results in an increase in trunk flexion angular velocity of almost 100% in order to stand. A lower seat has been shown to increase trunk, knee and ankle angular displacement. Chang-ing the seat height affects the maximum moment needed at the hip and knee. Differences for hip and knee moments can be as large as 50% to 60%, with seat height can result in changing biomechanical demands (e.g. the need to move the body’s center of mass over a large distance) or in an altered strategy (e.g. “stabilization strategy”, due to the imposed biomechanical demands by a different foot, trunk, or arm position).” (Janssen et al., 2002, p.
874)
Armrests
“Issues related to the armrests use include positioning of the hands on the armrests, height of the armrests, and the moments exerted. There is no research on the relationship among the height of the armrests, seat height, hand positioning, and their cumulative effect on per-formance of the STS movement.
Using armrests, according to the articles we reviewed, results in lower moments at knee and hip; at the hip, a reduction of about 50% of the extension moment needed to perform the STS movement has been calculated. (Burdett et al., 1985) found no influence of the use of arms on joint angles in subjects without impairments (24-41 years of age). In a study by Alexander et al., young and old subjects without impairments used a hand bar positioned in front of them to perform the STS movement. They found no differences in body segment rotations in the young subjects (19-31 years of age). A difference in trunk rotation was observed in the old subjects (63-86 years of age), although this movement was analyzed only at the moment of maximum anterior head displacement.” (Janssen et al., 2002, p. 874)
Chairtype
“We found only three studies on the influence of specially designed chairs. Different types of chairs designed to “ease” the STS movement were studied. (Wheeler et al., 1985) suggested a negative influence of seat posterior slant because of tilting the body’s center of mass farther backward. use of an ejector mechanism lowered vertical impulses applied to the armrests by 47% in patients with arthritis, but no differences were found for knee and ankle moments.”
(Janssen et al., 2002, p. 874) Backrests
“We found no experimental studies concerning the influence of backrests of STS movement.
In only eight studies was a chair with a backrest used. When a backrest was used, it was to standardize the STS movement starting position. The influence of trunk position has been studied; however, this influence cannot necessarily be related to backrest use or backrest position, because the trunk position studied was not comparable to the trunk position using a backrest.” (Janssen et al., 2002, p. 874)
Bio-Kinematic Design of Individualized Lift-Assist Chairs for the Support of Sit-to-Stand Movement 29
5. Individualized Computational Bio-Kinematic Design Procedure
In this chapter a computational design procedure is described that calculates the dimen-sions of a lift-assist mechanism to fit to the dimendimen-sions of the user with respect to a limited workspace in which the mechanism must fit inside. An overview of the process is seen in Fig. 18. At first the biomechanical structure of the human is analyzed and different kinematic models that aim to represent the kinematics of the human are compared. This is followed by forming individualized task positions with respect to a kinematic structure of the LAD. The third step consists of finite position synthesis given the task positions of the user as well as the workspace restrictions of the chair. If a solution exists, a kinematic analysis of the total structure, i.e. LAD and user combined, is carried out in the final step.
Biomechanical parameters of user and definition of the workspace of the chair
Definition of individualized task positions
Finite Position Synthesis given Workspace Restrictions
Kinematic Analysis
Solution exist? Yes, No
Mechanism that offers best Solution is selected
Figure 18Overview of the computational bio-kinematic design procedure