• Korean Journal of Applied Biomechanics
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• v.24 no.1
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• pp.19-26
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• 2014

This study investigated foot pressure patterns between experienced skiers and intermediate skiers during alpine skiing. Five experienced skiers and five intermediate skiers participated in this study. Foot pressure measurement system was used to measure vertical ground reaction force (vGRF) and contact area under the six plantar regions. Each participant was asked to perform basic parallel turns and carved turns on a $18^{\circ}$ groomed slope. Each right turn was divided into the initiation phase, the steering phase 1 and 2. For the initiation phase of the basic parallel turns, significantly greater contact area was found on the LRF and RRF of the intermediate skiers (p<.05) and significantly greater vGRF was found on the LRF of the intermediate skiers (p<.05). Also significantly greater vGRF and contact area were found on the LRF and RRF of the intermediate skiers at the steering phase 1 (p<.05) and on the LRF of the intermediate skiers at the steering phase 2 (p<.05). For the carved turns, significantly greater vGRF and contact area were found on the LRF and RRF of the intermediate skiers at all three phase (p<.05). On the other hand, significantly greater vGRF was found on the RFF of the experienced skiers at the steering phase 1 (p<.05). Also significantly greater vGRF and contact area were found on the RMF of the experienced skiers at the steering phase 2 (p<.05). In order to increase performance, we suggest that the intermediate skiers should be unweighted at the initiation phase and shift the body weight to the forefoot of the outer foot at the steering phase 1. Also, the outer ski should be loaded more than the both skis at the steering phase 1 and 2.

• Physical Therapy Korea
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• v.13 no.3
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• pp.1-9
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• 2006

The purpose of this study was to assess the influence of spine orthosis and sit-to-stand motor strategies on ground reaction force (GRF) and lower extremity muscle activity. Twenty healthy adult men participated, and subjects randomly performed sit-to-stand motions in three different conditions: Momentum-transfer strategy (MTS); MTS with spine orthosis; and zero-momentum strategy (ZMS) with spine orthosis. GRF data, onset time, and muscle activity were determined and compared using force plate and electromyography. Data were statistically analyzed by the SPSS version 13.0. One-way repeated analysis of variance (ANOVA) was used to determine the statistical significance, and least significant difference was used as a post hoc test. The level of significance was .05. The results of this study were as follows: 1. Peak GRF and relative time to peak GRF were not significantly different in the three different conditions (p>.05). 2. Onset time of four muscles, tibialis anterior, gastrocnemius, biceps femoris and rectus femoris, in the three different conditions were significantly different (p<.05). 3. The tibialis anterior and rectus femoris muscle activity before hip-off and tibialis anterior, gastrocnemius, and rectus fermoris muscle activity after hip-off were significantly different in the three different conditions (p<.05).

• Korean Journal of Applied Biomechanics
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• v.25 no.1
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• pp.85-94
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• 2015

Purpose : The purpose of this study was to investigate the influence on the ground reaction force parameters according to wearing positions of backpack for during stair ascending and descending. Methods : Participants selected as subject were consisted of young female(n=10) and performed stairs walks(ascending and descending) with 2 types of wearing position(front of trunk[FT], rear of trunk[RT]). Passive(Fz 1) and active(Fz 2) forces of the vertical GRF were determined from time function and frequency domain. Also shear forces(Fx, Fy 1, Fy 2), dynamic postural stability index(MLSI, APSI, VSI, DPSI), loading rate and center of pressure (${\Delta}COPx$, ${\Delta}COPy$, COP area) were calculated from time function and frequency domain. Results : Fx, Fy 1, Fy 2, and Fz 1 in GRF didn't show significant differences statistically according to the wearing positions of backpack(p>.05), but stair descending showed higher forces than that of stair ascending. Particularly, Fz 2 of stair ascending showed higher forces than that of stair descending(p<.001), RT types showed higher than that of FT types(p<.05). MLSI, APSI, VSI, and DPSI of stair descending showed the increased stability index than that of stair ascending(p<.05), MLSI of RT types showed the decreased stability index than that of FT types(p<.05). Loading rate didn't show significant differences statistically according to the wearing positions of backpack(p>.05), but stair descending showed higher loading rate than that of stair ascending(p<.001). Also, ${\Delta}COPx$ in stair descending showed the increased movement than that of stair ascending(p<.05). Conclusions : A backpack of 10 kg(10 kg(ratio of body weights $17.61{\pm}1.17%$) showed significantly change GRF parameters according to wearing positions during stair ascending and descending. If possible, we suggest that the dynamic stability, in case of stairs walking with a smaller weights can be further improved.

• Korean Journal of Applied Biomechanics
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• v.18 no.1
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• pp.169-178
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• 2008

The goals of this study were to provide data of 3 dimensional ground reaction force(GRF) parameters during stair ascent and descent on three different stair runs and to investigate variability and asymmetry index of them. 10 healthy adults participated in this study and performed 7 different types of gait with 10 trials each. After data analysis, following results were found. Firstly, stair run did not affect on the pattern of GRF parameters, coefficient of variation and asymmetry index. Secondly, a significant different GRF pattern was found between level walking and stair walking. Especially, ascending stair walking has only large Fz1 and small Fz3 while level walking and descending stair walking have a "M" shape connected by Fz1, Fz2 and Fz3. Thirdly, only vertical GRF parameters of stair walking revealed acceptable coefficient of variation and asymmetry index.

• Korean Journal of Applied Biomechanics
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• v.21 no.3
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• pp.289-296
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• 2011

The purpose of this study was to quantify the biomechanical characteristics of the ground reaction force(GRF) during the Taekwondo's Apkubi, one of the basic movement in Taekwondo and the walking. The GRF profiles under the stance foot of Apkubi movement and walking were directly measured in sample of 20 healthy older persons. In the anterior-posterior and vertical direction, the GRF of the Apkubi movement reached to the peak braking force at 10% of the normalized stance time percent and the peak driving force at 90% of stance time, but that of the walking reached to the peak braking force at 20% of stance time and the peak driving force at 80% of stance time. In vertical force, the GRF of the walking showed two peak values, but that of the Apkubi movement seemed three peak values. Moreover the first peak vertical force was significantly(t=6.085, p<.001) greater in the walking(about 1.8 times of body weight) than the Apkubi(about 1.4 times of body weight). The walking velocity was affected significantly(over p<.05) by the braking impulse, the peak braking force and the first peak vertical force. Futhermore the peak braking force in the Apkubi showed a significant effect on the Apkubi's stride length(p<.01). So, we concluded that the braking force after the right touch down, the stance foot on the ground contributed to move the leg forward.

• Journal of Biomedical Engineering Research
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• v.32 no.1
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• pp.61-73
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• 2011

The purpose of this study was to investigate changes in the center of pressure (COP), ground reaction force (GRF) and joint angles of elderly people and young people while stair-descent. The participants in this experiment were 5 elderly people and 5 young people, each of which was asked to descend stairs of three different heights (8 cm, 16 cm, and 32 cm). As they climbed down the stairs, they received vibration stimulation on the lower limb. The change of COP, GRF and joint angles were analyzed during the standing phase. COP decreased as the Achilles tendon and tibialis anterior tendon were vibrated. Vertical GRF increased as the Achilles tendon was vibrated, and the joint angle differed according to vibration stimulation conditions. These results mean that ankle joint, knee joint and hip joint were influenced by the vibrations on the lower limb as the participants descended the stairs. It was concluded that the vibration stimulation on the lower limb allowed the participants to efficiently climb down the stairs.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.6
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• pp.659-665
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• 2010

This paper proposes a control algorithm for quadruped robots moving on irregularly sloped uneven surfaces. Since the body balance of a quadruped robot is controlled by the forces acting on its feet during touchdown, the ground reaction force (GRF) is controlled for stable running. The desired GRF for each foot is generated on the basis of the desired galloping pattern; this GRF is then compared with the actual contact force. The difference between the two forces is used to modify the foot trajectory. The desired force is realized by considering a combination of the rate change of the angular and linear momenta at flight. Then, the amplitude of the GRF to be applied at each foot in order to achieve the desired linear and angular momenta is determined by fuzzy logic. Dynamic simulations of galloping motion were performed using RecurDyn; these simulations show that the proposed control method can be used to achieve stable galloping for a quadruped robot on irregularly sloped uneven surfaces.

• Physical Therapy Korea
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• v.15 no.1
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• pp.1-11
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• 2008

The purpose of this study was to assess the influence of two shoe size conditions on foot pressure, ground reaction force (GRF), and lower extremity muscle fatigue. Seven healthy men participated. They randomly performed walking and running in two different conditions: proper shoe size and 10 mm greater than proper shoe size. Peak foot pressure, and vertical, anterior and mediolateral force components were recorded with the Parotec system and Kisler force platform. To assess fatigue, the participants performed treadmill running for twenty-five minutes twice, each time wearing a different shoe size. Surface electromyography was used to confirm localized muscle fatigue using power spectral analysis of four muscles (tibialis anterior, gastrocnemius medialis, rectus femoris, and biceps femoris). The results were as follows: 1) In walking conditions, there was a significantly higher peak pressure in the 10 mm greater than proper shoe size insole sensor 1, 2, 14, and 18 (p<.05). 2) In running conditions, there was a significantly higher peak pressure in the 10 mm greater than proper shoe size insole sensor 5, 14, and 15 (p<.05). 3) In walking conditions, there was a significantly higher first maximal vertical GRF in the 10 mm greater than proper shoe size (p<.05). 4) In running conditions, no GRF components were significantly different between each shoe size condition (p>.05). 5) Muscle fatigue indexes of the tibialis anterior and rectus femoris were significantly increased in the 10 mm greater than proper shoe size condition. These results indicate that wearing shoes that are too large could further exacerbate the problems of increased foot pressure, vertical GRF, and muscle fatigue.

• Korean Journal of Applied Biomechanics
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• v.17 no.1
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• pp.1-8
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• 2007

Ground reaction force (GRF) measures are one of the most commonly used in biomechanical study. GRF system is very useful educational tool to explain and demonstrate the Newton's law of universal gravitation and laws of motion as well. However, accuracy, intra- and inter- force platform measures' consistency, reliability, noise, and the effect of platform mounting to GRF measures were not clearly viewed. The aim of this study was to examine the above. GRFs of a plastic dummy and two subjects' quiet upright standing were collected at four university laboratories eight force platforms. The types of platforms, analysis programs, and platform set-up were various. Three 100s-trials were conducted with sampling frequency of 100 Hz. First two trials' vertical component of GRFs, Fz, and CoP sway ranges of mid-60s-portion of 100s trials were analyzed by the paired t-tests and one-way ANOVA. Six of eight platforms' 1st and 2nd trial dummy Fz were statistically different (p<.05) and all platforms ICC were poor (<.28). Fz of the two platforms in every four laboratories were statistically different (p<.05). There were white noises and/or very distinctive noises at specific frequency ranges in all Fz measures. 5 Hz low-pass filtering made clear the Fz differences. CoP ranges of dummy were less than 0.5 cm and the best was 0.02 cm. This CoP range finding agrees with previous results suggests the importance of force platform mounting and A/D card resolution.

• Journal of the Microelectronics and Packaging Society
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• v.26 no.4
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• pp.69-74
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• 2019

The influence of measuring fixtures on the measurement of dielectric and piezoelectric properties of PMN-PT single crystal specimens was investigated. As the pressure of the jig pin which applied the AC electrical signal to the sample decreased, the lower resonance resistance was measured and the higher mechanical quality factor was calculated. The mechanical quality factor was 418 when the spring tension of the jig pin was 20grf, and the value of the quality factor was reduced by half when the spring tension was 200grf. The 4-probe fixture method is unsuitable for measuring piezoelectric properties, but it was suitable for the measurement of dielectric loss. In order to accurately evaluate the piezoelectric properties of PMN-PT, it is necessary to lower the spring tension of the jig pin as much as possible.