Papers by Author: Won Hak Cho

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Authors: Hyeon Ki Choi, Si Yeol Kim, Won Hak Cho
Abstract: We investigated the relationship between kinematic and kinetic characteristics of foot joints resisting ground reaction force (GRF). Passive elastic characteristics of joint were obtained from the experiment using three cameras and one force plate. The relationship between joint angle and moment was mathematically modeled by using least square method. The calculated ranges of motion were 7o for TM joint, 4o for TT joint and 20o for MP joint. With the model that relates joint angle and plantar pressure, we could get the kinematic data of the joints which are not available from conventional motion analysis. The model can be used not only for biomechanical analysis which simulates gait but also for the clinical evaluations.
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Authors: Chang Sung Seok, Jae Sil Park, Hyung Ick Kim, Young Min Lee, Won Hak Cho, Weon Keun Song
Abstract: The fatigue characteristic of a material or a structure is derived from fatigue tests of standard specimens. However, many researches have reported that test results of standard specimens are very different from those of real structures or components. One reason for this difference is the constraint effects according to the geometrical difference. Therefore, to calculate more accurate fatigue life, the constraint effect must be considered by comparing test results of standard specimens with those of real structures or components. Another reason for this difference is the surface condition. All surfaces of a standard specimen are polished to obtain similar conditions in a fatigue test. However, in a piping system, surface conditions of components are different from each other and very different from that of a standard specimen. Because fatigue life is effected by a surface condition, to evaluate the fatigue life of a piping system, fatigue tests must be conducted with a specimen extracted from a pipe with the same surface condition. The objective of this paper is to evaluate the fatigue characteristic of a real waterworks pipe by conducting fatigue tests with standard specimens and non-standard specimens of base metal and weld metal. Standard fatigue specimens and non-standard specimens were extracted from a steel pipe used in waterworks. Also, fatigue tests of pipes used in water service were carried out and then compared with those of standard specimens and non-standard specimens. From these results the relation between the S-N diagram of a specimen and that of a pipe specimen was evaluated.
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Authors: Hyeon Ki Choi, Jae Hoon Jeong, Sung Ho Hwang, Hyeon Chang Choi, Won Hak Cho
Abstract: We recognized EMG signal patterns of lower limb muscles by using neural networks and performed feature evaluation during the recovery of postural balance of human body. Surface electrodes were attached to lower limb and EMG signals were collected during the balance recovery process from a perturbation without permitting compensatory stepping. A waist pulling system was used to apply transient perturbations in five horizontal directions. The EMG signals of fifty repetitions of five motions were analyzed for ten subjects. Twenty features were extracted from EMG signals of one event. Feature evaluation was also performed by using DB (Davies-Bouldin) index. By using neural networks, EMG signals were classified into five categories, such as forward perturbation, backward perturbation, lateral perturbation and two oblique perturbations. As results, motions were recognized with mean success rates of 75 percent. With the neural networks classifier of this study, the EMG patterns of lower limb muscles during the recovery of postural balance can be classified with high accuracy of recognition.
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Authors: Hyeon Ki Choi, Won Hak Cho
Abstract: This study addressed the effect of balance control problems on the high-heeled women. The specific purposes of this study are to quantify the displacements and velocities of center-of-pressure (COP) of a body during waist pulling perturbation and to compare the differences between the bare-feet and the high-heeled. Another purpose of the study is to identify the effects of a high-heeled posture on electromyography (EMG) activities and muscle fatigue. We used a waist pulling system which has three different magnitudes to sway the subjects. The COP displacement of a high-heeled posture was about twice as much as that of bare-feet posture. Also the COP velocity of a high-heeled posture became about twice as much as that of bare-feet posture. Muscle fatigue could be identified by the shift of the median frequency (MF) of the EMG power spectrum toward lower frequencies. Median frequency of the EMG power spectrum from tibialis anterior was reduced more rapidly during high-heeled situation than during bare-feet situation. COP kinematics and muscle fatigue analysis in postural balance researches are considered to provide useful information in understanding the balance control mechanism of women’s high-heeled posture.
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Authors: Hyeon Ki Choi, Min Jwa Seo, Ja Choon Koo, Hyeon Chang Choi, Won Hak Cho
Abstract: We assessed the effects of muscle forces on ankle joint kinetics during postural balance control of human boy. Nine male subjects (mean age of 25.8 yrs) participated in the experiment. An ankle joint model assumed ball and socket joint was used, which was capable of three dimensional rotations. A six-camera VICON system was used for motion analysis. Waist pulling system and force platform were adopted for forward sway and GRF (ground reaction force) measurement. We used linear optimization programs to calculate the variation of muscle forces and angular displacements of shank and foot segments. With the experimental data and linear programs, we could calculate joint reaction forces, and bone-on-bone forces. The results presented in this study give us the insights to understand the roles of lower limb muscles during postural balance control and ankle injury mechanism.
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Authors: Won Hak Cho, Hyeon Ki Choi
Abstract: High-heeled women have been identified with balance control problems. The purposes of this study were to objectively quantify the displacements and velocities of center-of-pressure (COP) of body during waist pulling and to compare the differences between barefooted and high-heeled situations. We used a waist pulling system which has three different magnitudes to sway the subjects. We found that the kinematic information of barefooted and high-heeled women’s COP is very important in understanding the mechanism of postural balance control of women in every-day life. In the high-heeled’s case, the displacement of COP increases by 9% (1kg), 33% (2kg), 44% (3kg) as against bare footed. Also the velocity of COP grows two times than that of the bare footed. COP analysis in postural balance study of high-heeled women is considered useful in development of the safety systems that prevent high-heeled women from falling.
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