A study of the relationship between sit-to-stand activity and seat orientation

Transcription

1 A study of the relationship between sit-to-stand activity and seat orientation Chikamune Wada a, Takahito Oda a, Yoshiyuki Tomiyama a and Shuichi Ino b a Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Kitakyushu, Fukuoka, JAPAN; b National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki, JAPAN 1. Introduction Sit-to-stand (STS) activity from a chair is an important motion in daily life. However, some elderly or physically disabled people experience difficulty in standing because of a deterioration in physical function. Although nursing is necessary for such people, the burden on nursing staff is heavy, especially in the toilet. There are some commercial products to assist with STS activity in toilets. However, these products are problematic because they are large in size and not controlled according to the residual ability of the user. Therefore, to solve these problems, we would like to develop a new device to help STS activity in toilets by reconsidering the ascent/descent mechanism of the seat and actuator. Moreover, no optimal seat movement were shown, although there were some researches about seat orientation during the STS activity [Kawagoe 2000, Takahashi 2012]. In this paper, we investigated the relationship between ease of STS activity and seat orientation during STS activity to improve the design of the seat mechanism. 2. Method 2.1 Measurement device We developed a chair with an adjustable seat orientation (shown in Figure 1). The device could change seat position and tilt, moving it 0.2 [m] in a forward-backward direction, 0.35 [m] in an upward direction, and 30 [degrees] in tilt. Figure 1. Seat orientations of the adjustable chair. To evaluate the ease of STS performance, we measured changes in posture, muscle activity, and pressure during STS activity. A three dimensional motion capture system and electromyography (EMG) were used to measure posture and muscle activities. A pressure measurement device was developed to measure pressure change. This measurement device consisted of two plates. One plate, set under both feet, had 12 pressure sensors to measure the pressure distribution of the foot (shown In Figure 2). Another plate, set under the buttocks, had 10 sensors (shown In Figure 3). All data were input into a computer. The sampling rate was 100 [Hz] for the motion capture system and our pressure measurement device. EMG data were sampled at 1 [khz]; however, data were re-sampled at 100 [Hz] to match the sampling rates of the other data. 1

2 (a) Sensor position (b) Appearance Figure 2. The pressure data measurement device for foot. (a) Sensor position (b) Appearance Figure 3. The pressure data measurement device for buttocks. 2.2 Experimental procedure Five non-disabled young subjects were asked to stand up from the seat-orientation-adjustable chair when seat height and seat tilt were changed. Before the experiment, seat height was determined so that the knee angle was 75 [degrees] and the thigh became horizontal. This seat height was set to 0 [m] in height. Horizontal seat tilt was set to 0 [degrees]. In the experiment, four kinds of seat heights [0, 0.05, 0.1, and 0.15 (m)] and five seat tilts [0, 10, 20, 30, and 40 (degrees)] were prepared. The subject repeated the STS activity five times for each experimental condition. 2.3 Estimation of centre of gravity (COG) position STS activity from a chair has two phases: the flexion phase, in which the trunk is tilted to move the COG forward onto a base of support on the feet (defined as BOSf), and the extension phase, in which the trunk is moved upward to move the COG upward after the flexion phase. We hypothesized that STS activity was easily performed if the COG position was close to the BOSf during the flexion phase. Then, we evaluated the COG position by developing a human body-linkage model consisting of three body parts: the trunk, the thigh, and the shank (shown in Figure 4). The COG position and the mass of each part is shown in Table 1. The COG position was calculated from the model using motion capture data. Table 1. The parameters of the human body model. Position of COG [%] Mass ratio (%) LG m LG m 2 20 LG m

3 Figure 4. The three-linkage human body model.. 3. Results 3.1 Results of the EMG EMG results of the tibialis anterior muscle (TA), the rectus femoris muscle (RF), the biceps femoris muscle (BF), and the gluteus maximus muscle (GMA) are shown in Figure 5. Figure 5 shows the results for one subject as an example. The vertical axis shows the percentage of maximal voluntary contraction (%MVC) per second, while the horizontal axis shows the angle of seat tilt. The legend indicates the height of the seat. No difference among experimental conditions was found for the TA. The RF %MVC value decreased by > 20 [degrees]. The BF %MVC value decreased as the seat ascended. No trend was revealed for the GMA because of variation in the data. From all results of five subjects, no statistical significant was obtained but it was found that %MVC value decreased as the seat ascended in the TA and RF. (a) EMG of TA (b) EMG of RF 3

4 (c) EMG of BF (d) EMG of GMA Figure 5. EMG in the relationship between height and tilt angle of the seat (one subject). Figure 6 shows the EMG of the RF immediately after the subject sat on the chair before starting STS activity. The EMG increased as the tilt angle was increased. This tendency was same for all subjects. Figure 6. EMG of the RF when sitting on a chair (one subject). 3.2 Results for the COG position Figure 7 shows the calculated COG position which was projected on the horizontal plane. The vertical axis shows the COG position. Zero and minus values show the heel position and backward direction from the heel, respectively. The horizontal axis and legend are the same as in Figure 5. The COG moved to the heel as the tilt angle increased. 4

5 Figure 7. COG position in the backward-forward direction (one subject). 4. Discussion Based on the results shown in Figure 5, we thought that increasing seat height would decrease muscle activity. We also thought that increasing tilt angle would enhance the ease of STS activity because it would bring the COG position closer to the BOSf. However, as indicated in Figure 6, muscle activity increased as tilt angle increased when sitting on a chair for one subject. This tendency was obtained for all subjects. We then deduced that the desirable way of moving the seat to support STS activity was to tilt the seat just before the buttocks left it, after increasing seat height. Based on the results shown in Figure 5, we thought that increasing seat height would decrease muscle activity. We also thought that increasing tilt angle would enhance the ease of STS activity because it would bring the COG position closer to the BOSf. However, as indicated in Figure 6, muscle activity increased as tilt angle increased when sitting on a chair. We then deduced that the desirable way of moving the seat to support STS activity was to tilt the seat just before the buttocks left it, after increasing seat height. Acknowledgements Part of this research was financially supported by the Grant-in-Aid for Scientific Research (A, # ) of the Japan Society for the Promotion of Science. References Kawagoe, S., Tajima N., and Chosa E Biomechanical analysis of effects of foot placement with varying chair height on the motion of standing up. J.Orthop.Sci, 5, Takahashi M. and Nanba R Analysis and improvement of standing-up action for elderly people from a chair using a cushion-type support tool. Computer Technology and application, 3,