Five healthy young male subjects (age 26 ± 3 years, height 1.74 ± 0.04 m, mass 73.8 ± 3.4 kg) participated in this experiment with informed consent. None of them had any known musculoskeletal or neurological disorders. This project was performed under the approval of the ethics committee of the University of Tokyo.
To obtain lower extremity joint kinematics during STS movements, 3D coordinates of the landmark points of the subject's body were acquired using a 3D optical motion capture system with 7 cameras at 200 Hz (Hawk Digital System, Motion Analysis Corporation, Santa Rosa, CA, USA). Seven reflective markers were placed on the subject's body (the right acrominon, sacroiliac joint, right and left anterior superior iliac spines, right epicondylus lateralis, right malleolus lateralis and the distal end of the fifth metatarsal). All raw coordinates data were smoothed using a fourth-order butterworth low-pass digital filter. The cut off frequency (7 Hz) was determined with a residual analysis [13 ]. The hip joint center position was calculated from the sacroiliac joint, right and left anterior superior iliac spines and right epicondylus lateralis [14 (link)]. Joint angles were calculated from those coordinate data. The joint angles were defined as shown in Fig.1b . The chair height was set at 0.40 m, since the Japan Industrial Standard and British Standards Institute recommend 0.40 m as the standard chair height (JIS S 1011 and JIS S 1015) and the toilet pedestal height [15 (link)], respectively.
The subject's arms were folded across the chest. The subjects wore corsets for the neck (COLLAR KEEPER me, Nippon Sigmax, Tokyo, Japan) and back (MAXBELT CH, Nippon Sigmax, Tokyo, Japan) to prevent the head-arm-trunk (HAT) segment from bending except at the hip joints. (The HAT segment was assumed to be a rigid body.) STS movement was initiated from a squat posture in which the subject's buttocks lightly touched the chair. In this study, joint moment development during the STS task was the focus of analysis. In the sitting phase, since the body is supported by the chair, the load imposed on the lower limb is small. In preceding studies, Schenkman et al. (1990), Kotake et al. (1993) and Kralj et al. (1990) reported that the joint moments reach the maximum after the buttocks lose contact with the chair [16 (link)-18 (link)]. Therefore, the STS movement was simplified and only the rising phase was analyzed. This design was appropriate for the purpose of this study. Each subject was instructed to perform a total of 50 STS movements using various speeds and movement patterns without countermovement or arm support. The initial posture and feet position of the subjects were not restricted. The movements in which a countermovement was generated were excluded from further analyses. A joint movement greater than 3 degrees in the opposite direction was regarded as a countermovement and rejected. Since the number of successful trials that were compliant with the instruction ranged between 17 and 44 among the subjects, 17 trials per subject were used for further analysis (for those subjects who had performed a greater number of successful trials, 17 trials were randomly selected). The number of the trials for each subject was restricted to 17 to avoid the possibility that the kinematics of certain subjects influence the final results more than others. As a result, 85 trials (5 subjects, 17 trials per subject) were adopted in total. To translate the raw movement data into a format suitable for use in the next step, the joint angle time series data were normalized about the movement time and the range of change in the joint angle between the initial posture and the standing posture (Fig.1a ). The start and finish time were determined based on the joint angle deviation (3 deg) with respect to the stationary initial and final joint angles, respectively. The normalized data were fitted with 8th-order polynomial equations that produced average residual error from the experimental data of less than 1%. This fitting was used in order to adjust the time scale of hip, knee and ankle joint kinematic data at the next computation step.
To obtain lower extremity joint kinematics during STS movements, 3D coordinates of the landmark points of the subject's body were acquired using a 3D optical motion capture system with 7 cameras at 200 Hz (Hawk Digital System, Motion Analysis Corporation, Santa Rosa, CA, USA). Seven reflective markers were placed on the subject's body (the right acrominon, sacroiliac joint, right and left anterior superior iliac spines, right epicondylus lateralis, right malleolus lateralis and the distal end of the fifth metatarsal). All raw coordinates data were smoothed using a fourth-order butterworth low-pass digital filter. The cut off frequency (7 Hz) was determined with a residual analysis [13 ]. The hip joint center position was calculated from the sacroiliac joint, right and left anterior superior iliac spines and right epicondylus lateralis [14 (link)]. Joint angles were calculated from those coordinate data. The joint angles were defined as shown in Fig.
The subject's arms were folded across the chest. The subjects wore corsets for the neck (COLLAR KEEPER me, Nippon Sigmax, Tokyo, Japan) and back (MAXBELT CH, Nippon Sigmax, Tokyo, Japan) to prevent the head-arm-trunk (HAT) segment from bending except at the hip joints. (The HAT segment was assumed to be a rigid body.) STS movement was initiated from a squat posture in which the subject's buttocks lightly touched the chair. In this study, joint moment development during the STS task was the focus of analysis. In the sitting phase, since the body is supported by the chair, the load imposed on the lower limb is small. In preceding studies, Schenkman et al. (1990), Kotake et al. (1993) and Kralj et al. (1990) reported that the joint moments reach the maximum after the buttocks lose contact with the chair [16 (link)-18 (link)]. Therefore, the STS movement was simplified and only the rising phase was analyzed. This design was appropriate for the purpose of this study. Each subject was instructed to perform a total of 50 STS movements using various speeds and movement patterns without countermovement or arm support. The initial posture and feet position of the subjects were not restricted. The movements in which a countermovement was generated were excluded from further analyses. A joint movement greater than 3 degrees in the opposite direction was regarded as a countermovement and rejected. Since the number of successful trials that were compliant with the instruction ranged between 17 and 44 among the subjects, 17 trials per subject were used for further analysis (for those subjects who had performed a greater number of successful trials, 17 trials were randomly selected). The number of the trials for each subject was restricted to 17 to avoid the possibility that the kinematics of certain subjects influence the final results more than others. As a result, 85 trials (5 subjects, 17 trials per subject) were adopted in total. To translate the raw movement data into a format suitable for use in the next step, the joint angle time series data were normalized about the movement time and the range of change in the joint angle between the initial posture and the standing posture (Fig.
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