Youngjin Na

A Study on Estimation of Joint Force Through Isometric Index Finger Abduction With the Help of SEMG Peaks for Biomedical Applications

We propose a new method to estimate joint force using a biomechanical muscle model and peaks of surface electromyography (SEMG). The SEMG measurement was carried out from the first dorsal interosseous muscle during isometric index finger abduction. The SEMG peaks were used as the input of the biomechanical muscle model which is a transfer function to generate the force. The force estimation performance (R2) was evaluated using the proposed method with nine healthy subjects, and a former method using a mean absolute value (MAV), which is the full-wave rectified and averaged (or low-pass filtered) signal of SEMG in a time window, was compared with the proposed method; the performance of the proposed method (0.94 ± 0.03) was better than that of MAV (0.90 ± 0.02). The proposed method could be widely applied to quantitative analysis of muscle activities based on SEMG.

Flexible insole ground reaction force measurement shoes for jumping and running

There are only a few insole ground reaction force (GRF) measurement systems for running and jumping because the capacity and durability of the system were not guaranteed to measure the heavy load on the foot. We propose insole-type measurement system that can measure up to 900N to assure the capacity of GRF during running and jumping. The proposed system was implemented using a sensor-embedded flexible shoe including four custom optoelectronic based force sensors with high reliability. In order to evaluate the performance of the proposed sensor, static loading and unloading tests were performed to compare the hysteresis with the force sensitive resistors (FSR) which are widely used in insole-type system. During test, FSR and the proposed force sensor were loaded to the reference force sensor simultaneously; the hysteresis error of the proposed system (4.8%) was better than that of FSR (23.54%). By utilizing these custom force sensors, the proposed system was manufactured and evaluated for walking, running at 8km/h and withstanding high jump experiments with the 6-axis force plate based on NRMSE (Normalized Root Mean Square Error). The results were 14.68± 4.75% for long period walking, 10.9± 6.5% for running, and 14.72± 4.44% for jumping. From these results, the proposed system helps to measure GRFs in real-time for various human movement.

A study on the hydrogenation properties of the MmNi4.5Al0.5Zrx (x = 0−0.2) alloys

Abstract Hydrogenation properties of the MmNi 4.5 Al 0.5 Zr x ( x = 0−0.2) alloys are investigated by means of PC-isotherm measurements, X-ray diffractometry, scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX). As the Zr content increases, the first hydriding reaction is completed in a shorter time, but the plateau pressure is sloped and decreases. At a composition of x = 0.05, a maximum hydrogen storage capacity is obtained with a enhanced activation property. Upon pressure cycling of the MmNi 4.5 Al 0.5 Zr 0.5 alloy, the hydrogen storage capacity is decreased by only 8% after 4300 cycles. From the X-ray diffraction, SEM and EDX tests, new second phases are observed at high Zr content above x = 0.1. The major second phase is confirmed to be the ZrNi 3 phase. It is considered that the improved activation properties of the MmNi 4.5 Al 0.5 Zr x alloys are caused by the strong catalytic effect of ZrNi 3 on the dissociative chemisorption of hydrogen molecule. Among the investigated alloy systems, it is suggested that the most suitable alloy is the MmNi 4.5 Al 0.5 Zr 0.05 as a new hydrogen storage material due to its the large hydrogen storage capacity, the improved activation behavior and the strong resistance to the degradation in the subsequent cyclic operation.

Variation of Dynamic Muscle Model during Fatigue-Inducing Voluntary Contraction

There has been a paucity of studies to identify the variation of muscle properties due to muscle fatigue, although feature changes of surface electromyographic signals due to fatigue have been reported on. In this paper, we investigated the variation of muscle properties in pre-fatigue condition and post-fatigue condition by using the SEMG and the dynamic muscle model. Five subjects performed index finger isometric abduction contraction by using the first dorsal interosseous (FDI) muscle. After fatigue-inducing contraction, the total twitch duration increased by 30.10%, the contraction time and half relaxation time (RT 1/2) were raised to 7.45% and 13.11%, respectively. These results indicate that the response of the twitch force was slowed and prolonged due to the fatigue-inducing contraction. Our results can be used to monitor and identify the muscle properties of patients in rehabilitation programs.

Dynamic Elbow Flexion Force Estimation Through a Muscle Twitch Model and sEMG in a Fatigue Condition

We propose a joint force estimation method to compute elbow flexion force using surface electromyogram (sEMG) considering time-varying effects in a fatigue condition. Muscle fatigue is a major cause inducing sEMG changes with respect to time over long periods and repetitive contractions. The proposed method composed the muscle-twitch model representing the force generated by a single spike and the spikes extracted from sEMG. In this study, isometric contractions at six different joint angles (10 subjects) and dynamic contractions with constant velocity (six subjects) were performed under non-fatigue and fatigue conditions. Performance of the proposed method was evaluated and compared with that of previous methods using mean absolute value (MAV). The proposed method achieved average 6.7 ± 2.8 %RMSE for isometric contraction and 15.6 ± 24.7%RMSE for isokinetic contraction under fatigue condition with more accurate results than the previous methods.

Custom optoelectronic force sensor based ground reaction force (GRF) measurement system for providing absolute force

We propose an optoelectronic force sensor based insole type ground reaction force (GRF) measurement system, which can be used to measure the absolute GRFs produced by the user. Although force sensitivity resistors (FSR) has been widely used to measure the GRFs, they provide only the relative weight of the user. Accurate measurement of GRFs is essential to control robotic devices, such as lower-limb exoskeleton and prosthesis and to make a diagnosis for gait abnormality. To validate the performance of the developed insole type GRF measurement system, we measured GRFs during level ground walking and compared the results with FSRs. For measured GRFs, the GRF data from our system was identical compared with that from a force plate, but the GRF data from FSRs was saturated due to capacity of FSR (~450N). Our insole type measurement system provided the absolute GRFs in walking.

Wearable lower limb biomechanics measurement system for gait analysis during walking and ascending stair

Current existing sensors have inherent limitations such as mobility, wearability, and rigidity for measuring walking information. In order to investigate advantages and disadvantages of each sensor and measure bio-signals during walking on a flat terrain and ascending stairs, we developed a wearable lower limb biomechanics measurement system integrated with force sensors, encoders, and myoelectric sensors. Depending on motions, changes of joint angles and activity patterns for four muscles were recorded for a single healthy subject. As a result, in case of walking on a flat terrain, the tendency of angle variation in each joint are similar whether the subject puts on the wearable monitoring system or not. In ascending stairs, variations of joint angles were larger than that of walking for all joints (hip, knee, and ankle flexion/extension). For muscle activations, vastus lateralis were activated during ascending stairs while it was not activated during walking.

Muscle fatigue estimation with twitch force derived from sEMG peaks

We propose a new method - twitch force - for estimation of the muscle behavior during voluntary contraction for assessing localized muscle fatigue. The proposed method uses the sEMG peaks as input and the measured force as output. The twitch force, which is a transfer function to generate force, was estimated during fatiguing contraction. We verified the estimated twitch force based on the measured results with electrical stimulation. The participants performed isometric little finger flexion until exhaustion. SEMG was recorded on the flexor digiti minimi brevis muscle for the proposed method and the electrical stimulation electrodes on the ulnar nerve induced involuntary contraction for reference. As the muscle fatigue level increased, the twitch peaks decreased in both methods. The proposed method can be widely used in the quantitative analysis of muscle fatigue during voluntary contraction.

Personalized Protocol to Select Usable Movements for Myoelectric Pattern Recognition

Although pattern recognition studies have classified for upper limb movements, there is remaining issues for multi-finger movements. Depending on the user’s characteristic, usable movements for pattern recognition might be different. In addition, different finger movements might show similar surface electromyography (sEMG) from forearm and wrist. Therefore, we present a personalized protocol to select usable movements for each subject. Firstly, all movements were sorted into k classes using a k-means clustering method. Secondly, the movement, which showed different sEMG features for trials, was removed. 20 healthy subjects performed the 64 finger movements and 4 wrist movements. We found that the maximum number of classes (>95%) is different depending on the individual and location of electrodes; 18.5±3.0 on forearm and wrist, 11.7±1.9 on forearm, and 8.9±1.7 on wrist. The large standard deviation supports the personalized protocol for each subject in both locations.

Quantitative Analysis of Ocular Structure after Scleral Buckle Encircling

Scleral buckling is an ophthalmologic surgical procedure that is used to cure a retinal detachment. There are several theories to explain the mechanism of retinal reattachment after this procedure. However, it is not satisfactory to explain the postoperative changes of the eye. We used a geometric model and a computational mechanics model of the nine types of scleral bands to predict the postoperative axial length and the buckled retinal surface area, which can be used to predict the index of refraction. In a computational simulation, the postoperative axial length can be predicted using the linearity of the width-thickness ratio of the band, whereas the buckled retinal surface area is related to the width of the band. Geometric and computational models have been used to reveal retinal redundancy and to further explain the scleral buckling mechanism. Classical vector reversal theories are challenged by insufficient vector traction due to an insufficient increase in the retinal expansion length. The redundancy of the retinal length may be used to correct this problem.