Biomechanical Comparison of the Lower Extremities during Walking on Firm versus Sand Surfaces

Son, Da Kyung; Kim, Su Hyun; Kim, Young Hoon; Da Kyung Son; Su Hyun Kim; Young Hoon Kim

doi:10.15857/ksep.2025.00374

Exerc Sci > Volume 35(1); 2026 > Article

Son, Kim, and Kim: Biomechanical Comparison of the Lower Extremities during Walking on Firm versus Sand Surfaces

Original Article

Exerc Sci. 2026; 35(1): 75-84.

Published online: Feb 28, 2026

DOI: https://doi.org/10.15857/ksep.2025.00374

Biomechanical Comparison of the Lower Extremities during Walking on Firm versus Sand Surfaces

Da Kyung Son MS¹

, Su Hyun Kim MD, MPH, PhD²

, Young Hoon Kim MD, PhD^3,

¹Department of Physical Education, Graduate School, Pukyung National University, Busan, Korea

²Pukyong National University, Industry-University Cooperation Foundation, Busan, Korea

³Department of Marine Sports, College of Information Technology and Convergence, Pukyong National University, Busan, Korea

Corresponding author: Young Hoon Kim Tel +82-10-6651-0010 Fax +82-51-629-5631 E-mail rehabkyh@pknu.ac.kr

Received Jun 8, 2025 Revised Nov 24, 2025 Accepted Jan 13, 2026

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

PURPOSE

Sand walking, a type of marine healing program, has been adopted as a sports training and rehabilitation method because it lowers the risk for injury by reducing muscle and joint loads through shock relief. This study performed a three-dimensional (3D) biomechanical analysis to compare walking on firm versus sandy surfaces in healthy adults in a laboratory environment that closely replicated real beach sand.

METHODS

The 3D joint range of motion (ROM), stance time and muscle activity of the lower extremities were analyzed while 34 healthy adults walked on firm versus sandy surfaces using a constructed sand track (7.2×0.9×0.2 cm).

RESULTS

Sagittal ROMs of the hip and knee joints were significantly greater on the sandy surface than those on the firm surface, whereas the horizontal ROM of the knee joint was significantly lower on the sandy surface than that on the firm surface. Ankle ROMs were significantly greater across all planes. Significantly longer stance times were observed for the sandy surface versus the firm surface. The gastrocnemius and gluteus medius muscles exhibited significantly higher activity on the sandy surface versus the firm surface during the loading and terminal stance phases.

CONCLUSIONS

This study revealed 3D biomechanical changes in the lower extremities during sand walking, increasing joint ROM and muscle activity while extending stance time, suggesting that beach sand is a suitable resource for gait rehabilitation and marine healing programs.

Keywords: Range of motion, Muscle activity, Sand walking, Lower limb biomechanicss

INTRODUCTION

Walking is not only a fundamental aspect of daily life, but also a crucial means of promoting health through physical activity. The continuous adjustment of body movement patterns is required to maintain stability when navigating uneven or demanding surfaces [1]. Particularly, walking on sand offers the advantage of altering exercise patterns owing to its uneven and unpredictable surfaces [1] and decreases the risk of injury by reducing muscle and joint loads through shock relief [2]. Therefore, it is frequently used in real-world settings for sports training or rehabilitation [3]. Marine healing, i.e., marine therapy or ocean healing, refers to the practice of using various marine resources for therapeutic purposes [4]. It aims to prevent disease, promote health, and aid patient rehabilitation [5]. With increasing life expectancy and demand for wellness, it has emerged as a popular form of wellness tourism in several countries [6]. Since 2017, the Korean government has implemented policies to develop extensive marine resources for wellness tourism, with sand resources being one of its key components [7].

Among these, sand walking has drawn attention as a practical and accessible therapeutic activity. An 8-week soft-sand walking program improved muscle strength in older women compared with firm-sand walking [8]. Similarly, after an 8-week sand-running training program for overpronated runners, pronation was reduced, calf muscle activity increased, and pelvic stability was enhanced [9]. Additionally, several studies on the biomechanical analysis of sand walking have demonstrated a reduction in gait speed and ground reaction force, along with an increase in knee and ankle range of motion (ROM), as well as enhanced lower limb muscle activity [1,10].

Laboratory studies on sand walking remain limited owing to the challenges associated with replicating real sand environments. Kinematic variables of the ankle joint have not often been accurately identified, as reflective markers attached to the foot tend to be easily buried in the sand during walking. Furthermore, most studies have focused on the range of joint motion in the sagittal plane, although three-dimensional (3D) analysis is the optimal method for gait assessment. Moreover, research involving healthy individuals with high accessibility to marine healing programs remains insufficient because most studies have primarily been conducted on participants with pathological conditions or altered gait patterns. Therefore, this study aimed to conduct a 3D biomechanical analysis comparing walking on firm and sandy surfaces in healthy adults, using a laboratory environment specifically created to replicate real beach sand.

METHODS

1. Study design and participants

This experimental study aimed to compare biomechanical variables while walking on firm and sandy surfaces in a healthy adult population. This study adhered to the principles of the Declaration of Helsinki and was approved by the Institutional Review Board (IRB) of Pukyong National University (IRB No.: 1041386-202401-HR-02). Written informed consent was obtained from all participants prior to participation.

2. Experimental procedures

1) Establishment of the sand walking experiment environment

The final experimental environment, designed to replicate beach sand and optimize the measurement of 3D biomechanical variables, was established on the basis of previous literature and refined through multiple revisions following the participation of five healthy adults. This process involved determining the optimal placement of body markers and the appropriate type and density of sand.

The track was constructed using iron, with internal dimensions of 7.2 m in length, 0.9 m in width, and 0.2 m in depth. A bottom cut-out was created in the center to expose the embedded force plate (60 cm×50 cm) on the laboratory floor for conventional gait analysis (Fig. 1). To protect the force plate, a 1-mm-thick layer of silicone was applied, and a total of 1.4 tons of dry river sand was evenly filled into the track.

Fig. 1.

Design specifications of the constructed track and a photograph of the actual sand track. (A) Top view. (B) Isometric projection. (C) Front view. (D) Left side view. (E) Actual track, exposed force plate. (F) Actual track, evenly filled with dry river sand.

2) Experimental process

Participants were instructed to complete each of the five walks at a comfortable speed on a ground-by-ground basis. Walks were conducted at 2-week intervals, with one walk on the track's installed sandy surface and the other on a hard surface of equal length to normal ground. The biomechanical parameters of the dominant leg were measured, and data from three of the five walking trials were averaged and analyzed. The three datasets used for the averages were selected on the basis of similar walking speed (range, 1.2-1.4 m/s) and correct foot position on the force plate. To maintain a consistent sand environment, the sandy surface was evenly flattened after each test, room temperature was set at 24°C, humidity was set at 50-60%, and approximately one-third of each participant's calcaneus was checked to be buried in the sand.

3) Data collection

Lower extremity motion was captured using eight infrared real-time motion capture cameras (MIQUS M5; Qualisys AB, Gothenburg, Sweden). To meet the study objectives, a 12-mm reflective marker was placed using the David Hayes marker set and Charnwood Dynamics (CODA) pelvic guide, excluding unnecessary markers. Additionally, four markers were added as follow-up markers, resulting in 15 reflective markers (Fig. 2). To estimate the joint center, static posture data were collected at 100 Hz for 5 seconds, with participants crossing their arms in front of their chest while facing forward.

Fig. 2.

Lower limb reflective marker attachment sites (15).

A portable 3D force plate (9260AA; Kistler, Winterthur, Switzerland), synchronized with the motion capture system, was used to measure stance time. The signals were digitized using a 16-bit Kistler analog-to-digital converter.

Lower extremity muscle activities were recorded at 2,000 Hz using an 8-channel surface electromyography (EMG) system (Ultium EMG; Noraxon, Scottsdale, AZ, USA), which was also synchronized with the motion capture system. Bipolar Ag/AgCl disposable surface electrodes (T246H; SeedTech, Gyeonggi, Korea) were attached to the lower extremity muscles at 2-cm intervals after shaving and skin preparation with alcohol to reduce impedance, following the guidelines of Surface Electromyography for the Non-Invasive Assessment of Muscle [11]. Kinesiology tape was applied to secure the electrodes and prevent electrode displacement during walking. Prior to the walking trials on the firm and sandy surfaces, maximum voluntary isometric contraction (MVIC) tests were conducted three times for each muscle, with each trial lasting 5 seconds. A 3-minute rest was provided to participants between trials to prevent muscle fatigue, and participants were verbally encouraged to exert maximum effort. During the walking trials, EMG data were expressed as a percentage of the MVIC (%MVIC).

All obtained raw data were transmitted to a computer and processed using nonlinear transformation within Qualisys Track Manager Soft-ware (version 2022.2, build 5880). The system automatically detected reflective markers, and a labeling process was applied to recover the positions of lost markers. Missing marker data were interpolated using a third-order spline interpolation.

The trajectory data from the Qualisys Track Manager were converted to a c3d format and exported to Visual 3D (version 5.01; C-Motion Inc., Germantown, MD, USA), and the ROM, stance time of each joint, and walking cycle variables were analyzed. The original ROM data were filtered using a fourth Butterworth low-pass filter with a cutoff frequency of 6 Hz, and a fourth Butterworth low-pass filter with a cutoff frequency of 20 Hz was applied to reduce noise in the GRF data. The 3D spatial coordinates were defined according to the right-hand rule, where the x-axis represents the left-right direction, the y-axis represents the front-back direction, and the z-axis represents the vertical direction.

The EMG data converted to the c3d format were processed using Noraxon software (version MR3 3.20). To remove artifacts from skin movements and electrical noise, a bandpass filter (20-450 Hz) was applied to the primordial EMG signal and smoothed using the root-mean-square method. This process was applied evenly to MVIC and gait EMG data. MVIC data were used to normalize the EMG signals by calculating the mean EMG amplitude for 3 seconds, excluding the initial and final portions of the recording. To ensure data quality and signal reliability, extreme EMG values— operationally defined as those exceeding three standard deviations from the mean or exhibiting clear noise artifacts, inconsistent signal patterns, or poor electrode contact— were excluded from the analysis.

4) 3D ROM of the lower extremity joints

For kinematic variables, the ROMs of the hip, knee, and ankle joints were calculated during walking on sandy and firm surfaces. Joint ROM (°) was defined as the difference between maximum and minimum joint angles:

Joint ROM(∘) = max∠(∘)-min∠(∘)

5) Stance time

The stance phase was defined as the period from the initial contact of the right foot to toe-off of the smae foot, and the initial contact and toe-off were identified using a 10-N force threshold. Stance time was defined as the time when one foot was in contact with the floor (i.e., from initial foot-floor contact until final foot-floor contact) and is expressed as seconds [12].

6) Peak vertical ground reaction force (vGRF) and loading rate

In addition, peak vGRF and vertical loading rate were derived from the synchronized force plate data. Peak vGRF values were obtained from the maximum vertical force and normalized to body weight. Vertical loading rate was calculated as the slope of the vertical force-time curve between initial contact (10-N threshold) and the first vGRF peak. As the accuracy of these kinetic variables may be influenced by the deformable sand surface, detailed results are presented in the Supplementary Material (Supplementary Table 1).

Table 1.

Baseline characteristics of the participants (n=34)

Variables	Mean±standard deviation
Age (year)	23.94±2.94
Height (cm)	168.76±8.18
Weight (kg)	67.12±14.98
Sex (n)	Male (n=16)
	Female (n=18)

7) Muscle activities of the lower extremities

Lower extremity muscle activity during the loading response and terminal stance is expressed as %MVIC for eight muscles: tibialis anterior, peroneus longus, gastrocnemius, vastus medialis, rectus femoris, semitendinosus, biceps femoris, and gluteus medius (Fig. 3). The loading response was defined as the period from initial heel contact to mid-stance, whereas the terminal stance phase was defined as the period from mid-stance to toe-off.

Fig. 3.

Lower limb electrode attachment sites (15) for muscle activity measurement.

8) Statistical analysis

G*Power (version 3.1.9.7; University of Kiel, Kiel, Germany) was used to calculate the required sample size. A total of 34 participants were determined on the basis of a statistical power of 0.8 and a significance level of 0.05. To account for potential data loss due to sand interference, 50 healthy adults without orthopedic or neurological diseases that could affect walking were initially recruited.

Values are expressed as mean±standard deviation. Biomechanical gait variables on firm and sandy surfaces were analyzed using paired t-tests or Wilcoxon signed-rank tests, according to Shapiro-Wilk normality test results. All measured data were analyzed using SPSS (version 29.0; IMB Corp., Armonk, NY, USA), and statistical significance was set at α=0.05.

RESULTS

Data from 34 participants were included in the final analysis after excluding those with marker data loss (n=3), extreme EMG values (n=5), withdrawal after providing consent (n=2), and discontinuation of the experiment (n=6) (Fig. 4). Table 1 shows the baseline characteristics of participants (n=34). Participants’ age ranged from 23.94±2.94 years, 52.9% were women, and 47.1% were men.

Fig. 4.

CONSORT diagram of the study.

1. 3D ROMs of the lower extremity joints

Fig. 5 presents a comparison of the 3D ROMs of the lower extremity joints during firm and sandy surface walking. A statistically significant difference was found in the sagittal ROM of the hip joint, with greater motion observed on the sandy surface (43.33±4.46°) than on the firm surface (40.94±5.30°) (p =.007).

Fig. 5.

Three-dimensional range of motions of the lower extremity joints during firm and sand walking. p-values by paired t-test or Wilcoxon signed rank test. (A) Hip joint. (B) Knee joint. (C) Ankle joint.

For the knee joint, significant differences were observed in the sagittal ROM (firm: 36.48±4.48° vs. sand: 39.77±4.58°, p =.001) and the transverse ROM (firm: 9.30±1.86° vs. sand: 7.59±2.32°, p =.001).

For the ankle joint, ROMs were significantly higher on the sandy surface than on the firm surface across all three planes (p =.001), with greater motion observed in the sagittal (33.25±4.49° vs. 31.05±4.81°), frontal (8.70±1.56° vs. 7.45±1.37°), and transverse planes (18.00±3.37° vs. 17.10±3.46°).

2. Stance time

Stance time was significantly longer on the sandy surface than on the firm surface during walking (0.64±0.03 seconds vs. 0.67±0.05 seconds, p =.001; Fig. 6).

Fig. 6.

Stance time during firm and sand walking. p-value by paired t-test.

3. Peak vGRF and loading rate

Compared with firm surface walking, sand surface walking exhibited a significantly lower peak vGRF, whereas the vertical loading rate did not differ significantly between surfaces (Supplementary Table 1).

4. Muscle activities of the lower extremities

The activities of the gastrocnemius and gluteus medius showed statistically significant differences depending on the surface characteristics in the loading response and terminal stance phases. For the gastrocnemius, muscle activity was significantly higher on the sandy surface than on the firm surface during the loading response phase (11.88±7.72 %MVIC vs. 8.75±6.60 %MVIC, p =.001) and during the terminal stance phase (14.81±12.34 %MVIC vs. 8.96±7.02 %MVIC, p =.001). For the gluteus medius, significantly higher muscle activity was observed on the sandy surface than on the firm surface during the loading response phase (8.43±4.59 %MVIC vs. 5.34±3.42 %MVIC, p =.001) and during the terminal stance phase (6.50±3.40 %MVIC vs. 8.71±5.64 %MVIC, p =.001). However, there were no significant differences in the muscle activities of the tibialis anterior, peroneus longus, vastus medialis, rectus femoris, semitendinosus, and biceps femoris during either the loading response or terminal stance phases (Table 2).

Table 2.

Muscle activities of the lower extremities (%MVIC)

	Loading response			Terminal stance
	Firm	Sand	p-value	Firm	Sand	p-value
Tibialis anterior	13.26±11.33	11.53±10.13	.483^†	14.43±14.89	12.93±14.42	.222^‡
Peroneus longus	27.65±29.39	20.86±20.93	.203^‡	33.73±32.23	27.84±31.80	.309^‡
Gastrocnemius	8.75±6.60	11.88±7.72	.001^‡	8.96±7.02	14.81±12.34	.001^‡
Vastus medialis	14.13±13.29	10.78±12.52	.154^†	13.31±16.01	14.88±19.11	.567^‡
Rectus femoris	7.83±5.08	8.21±8.28	.800^†	7.83±7.95	7.98±6.97	.590^‡
Semitendinosus	12.73±12.40	10.46±9.60	.441^†	11.18±7.50	11.80±11.34	.799^‡
Biceps femoris	7.45±5.83	7.33±5.81	.817^‡	8.22±4.20	9.31±6.73	.416^†
Gluteus medius	5.34 ±3.42	8.43±4.59	.001^‡	6.50±3.40	8.71±5.64	.001^‡

Values are expressed as mean±standard deviation.

^† Paired t-test;

^‡ Wilcoxon signed rank test.

DISCUSSION

This study identified significant differences in the biomechanical variables of the lower extremities during walking on a sandy surface compared with walking on a firm surface in an experimental setting replicating beach sand. Walking on sand resulted in greater joint ROM at the hip, knee, and ankle, increased muscle activity, and longer stance time.

Sand walking affects the joint ROM of the lower extremities to adapt to the unstable nature of the sandy surface. In particular, ankle movement was significantly influenced by walking on unstable surfaces, as demonstrated by increased ankle ROM across all planes in this study. This increase is likely attributable to the complexity and instability of the sandy surface, which undergoes continuous structural changes during movement. These findings align with those of previous studies; however, the existing research has focused on ankle ROM in the sagittal plane. For instance, foot plantar flexion ROM was found to increase significantly in the sagittal plane during sand walking as well as in patients with multiple sclerosis [1,13]. The present study further expands upon previous findings by demonstrating that ankle ROM also increases in the frontal and transverse planes during sand walking. Therefore, the increased ROM observed here may serve as a joint strategy during sand walking [1], potentially enhancing ankle function and improving balance [14]. Additionally, stabilization of the ankle and knee joints may lead to improved motor control [15,16].

For the knee joint, ROM increased in the sagittal plane but decreased in the transverse plane during sand walking. During walking, the foot presses against the ground and transmits force to the knee joint until the lower body stabilizes along with the hip joint. As the knee is a hinge joint, ROM in the sagittal plane increases to facilitate shock absorption when walking on unstable surfaces such as sand [17], which is consistent with findings from a previous study [13]. Conversely, increased motion in the transverse plane is associated with knee instability. The instability of sand surfaces induces greater inward rotation of the tibia relative to the talus during the stance phase, potentially increasing the Q-angle of the knee and exerting greater stress on the knee joint [18]. To compensate for this instability, the body adopts a postural strategy to maintain knee stability, thereby reducing the ROM in the transverse plane.

The current study also demonstrated a significant increase in hip ROM in the sagittal plane, which is consistent with previous findings [13,19,20]. This increased hip ROM is attributed to a postural strategy aimed at maintaining balance, wherein the body's center of gravity shifts forward by tilting the trunk anteriorly on sand [21].

Furthermore, we observed an increase in stance time, which is consistent with the results of previous research [10]. During sand walking, strategies such as reducing gait speed and step length, and increasing stance time and the double support phase are carefully employed to maintain stability [1,10,22].

In addition, kinetic analyses presented in the supplementary table demonstrated significantly lower peak vGRF during sand walking compared with firm walking, which aligns with the previous reports [10,23] that walking or running on sand reduces peak impact forces while modifying temporal gait strategies. However, vertical loading rate did not differ significantly between surfaces in the present study, and prior research has reported inconsistent results across studies regarding this parameter [23–25]. Notably, force plate measurements reflect the net external force transmitted through the sand, without capturing localized foot-sand pressure distribution or energy dissipation within the deformable substrate. Thus, these findings should be interpreted with caution.

Among the lower extremity muscles, significant increases in activity were observed only in the gastrocnemius and gluteus medius during sand walking, particularly during the loading response and terminal stance phases. The gastrocnemius and gluteus medius are key contributors to forward progression during gait, and they play essential roles in propulsion and postural stabilization. The gastrocnemius facilitates push-off, whereas the gluteus medius prevents excessive contralateral pelvic drop, thereby maintaining overall stability. Conversely, other lower extremity muscles exhibited lower activation during gait, likely because of the biomechanical mechanism of walking, in which coordinated muscle action facilitates the natural forward displacement of the body's center of mass during the stance phase, reducing the need for additional muscular effort [26]. Previous studies comparing muscle activity during sand walking and firm surface walking similarly reported increased activation of these muscles. Specifically, long-term gait training on sand enhanced gastrocnemius and gluteus medius activities in individuals with pronated feet. Additionally, significantly higher gastrocnemius activity was observed in the mid-stance phase during sand walking in individuals with flat feet [10]. Furthermore, during sand running, the gastrocnemius and gluteus medius exhibited increased activation, accompanied by greater hip and knee ROM [20].

Sand walking requires greater activation of the gastrocnemius to stabilize the body and prevent excessive postural sway, particularly during the push-off phase [27]. This increased activation may be associated with the greater sagittal plane ankle ROM required to adapt to the unstable nature of the sandy surface. Similarly, the increased activation of the gluteus medius during sand walking and running may be attributed to its role in maintaining lower limb alignment during heel contact, stabilizing the pelvis, and controlling the movements of the femur, knee joint, tibia, and foot [28]. Moreover, overactivation of the gastrocnemius and gluteus medius during sand walking can be attributed to the reduced surface elastic energy of sand, which limits energy return and requires greater muscular effort to maintain stability and propulsion [21]. Therefore, gait training on an unstable sandy surface may contribute to improvements in gait function, lower limb muscle strength and endurance, and neuromuscular activation [29].

From a rehabilitative perspective, the combination of lower peak vGRF with longer stance time, increased joint ROM, and elevated gastrocnemius and gluteus medius activity suggests that sand walking provides a low impact yet neuromuscularly demanding training environment. This unique profile supports its rationale as a therapeutic modality for gait retraining, pelvic stabilization, and proprioceptive training. Unlike walking on soft mats or with treadmill adjustments, the continuous deformation of sand imposes three-dimensional joint excursions and postural control demands that are difficult to reproduce on other surfaces, highlighting the unique therapeutic potential of sand walking [1,8,9,22]. Future studies are warranted to investigate the long-term effects of sand walking and its applicability in real-world clinical rehabilitation settings.

As the biomechanical analysis of sand walking in real-world beach environments presents substantial challenges, we created and optimized an experimental sand track. The dimensions of the experimental sand track were designed to satisfy the recommended specifications for gait analysis [30]. Additionally, sand density is influenced by particle size and water content [31,32]. Thus, dry river sand was selected owing to its commercial availability and uniform particle size (≤2.3 mm), which closely matches that of beach sand (approximately 2 mm). In natural beach environments, sand walking occurs across a spectrum of conditions from dry to fully saturated sand, each producing distinct biomechanical effects. The density of dry sand was higher than that of wet or saturated sand [31,32]. However, in an experimental setting, an excessively low density can result in marker loss due to foot burial, whereas an excessively high density may not sufficiently replicate the instability of natural sand walking. In this study, following multiple refinements in pilot trials to optimize the experiment, the resulting sand density was calculated to be 771.6 kg/m³. Future research should explore the effects of varying sand density in relation to water content on gait mechanics.

This study has some limitations. First, all participants were healthy adults in their 20s and 30s, which limits the generalizability of the findings to other age groups or individuals with pathological conditions. Second, the analysis focused on biomechanical variables during the stance phase without considering other phases of the gait cycle. Third, this study did not fully address the diverse conditions of actual beach sand walking, including variations in sand properties, water content, and density. Finally, we did not include internal kinetic parameters such as joint moments and powers, as the assumptions of inverse dynamics are not fully applicable to deformable sand surfaces, although these could provide additional insights. Future research should address these limitations by expanding the experimental conditions, conducting studies in real beach environments, and exploring the clinical applications of sand walking across diverse populations.

CONCLUSION

Significantly greater ROM in the lower extremity joints, longer stance time, and increased activity of the gastrocnemius and gluteus medius were observed during sand walking compared with firm surface walking. These findings will contribute to establishing a scientific basis for the application of sand-based resources in marine healing programs. Further research, including studies on actual beach walking and a comprehensive analysis of the entire gait cycle, could provide fundamental scientific data for optimizing walking training or exercise interventions in sand-based marine healing programs.

Supplementary Material

Supplementary Table 1.

Comparison of peak vertical ground reaction force and vertical loading rate between firm and sand walking

ksep-2025-00374-Supplementary-Table-1.pdf

Notes

ACKNOWLEDGMENT

We would like to thank Editage (www.editage.co.kr) for English language editing. This manuscript is based on the master's thesis of DK Son, the first author.

CONFLICT OF INTEREST

Authors declare no conflicts of interest.

AUTHOR CONTRIBUTIONS

Conceptualization: DK Son, YH Kim; Methodology: DK Son, YH Kim; Formal Analysis: DK Son, SH Kim; Investigation: DK Son, YH Kim; Writing - Original Draft: DK Son, SH Kim; Writing - Review & Editing: DK Son, YH Kim, SH Kim; Supervision: YH Kim, SH Kim; Visualization: DK Son.

FUNDING

This work was supported by Dongwon Research Foundation in 202310400001.

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