windlass mechanism을 이용한 평발 개선 치료 소도구, 발 아치 회복 2가지 운동법 !!

[문형철]

누구나 쉽게 실천할 수 있는 치료 소도구

문형철 비가역적인 선언!!

문치연은 이러한 치료 소도구를 정확한 적응증을 찾고

              좀더 생체역학적으로 치료 효과가 우수한 장비를 만들겠다!! 

https://pmc.ncbi.nlm.nih.gov/articles/PMC7893268/

The extensibility of the plantar fascia influences the windlass mechanism during human running - PMC

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본 연구는
앉은 자세와 한쪽 다리 서기 자세에서
다양한 발가락 신전 각도(0°, 15°, 30°, 45°) 하에서
족저근막(plantar fascia)과 내재 발 근육(intrinsic foot muscles)이
내측 종아치(medial longitudinal arch, MLA) 지지와
windlass mechanism에 기여하는 정도를 탐구하였다.

51명의 건강한 성인(남성 41명, 여성 10명; 평균 연령 21.3 ± 1.2세)을 대상으로
초음파 탄성촬영(ultrasound elastography)을 사용하여
족저근막과 굴지근(flexor digitorum brevis)의 경도를 측정하고,
등쪽 아치 높이(dorsal arch height)와 아치 높이 지수(arch height index, AHI)를 평가하였다.

앉은 자세에서는
족저근막과 굴지근이 windlass mechanism에 기여하나,
서기 자세에서는 굴지근의 역할이 두드러지며 경도가 앉은 자세의 약 6배에 달한다.

AHI(arch height index)는
앉은 자세에서 더 높았으며,
발가락 신전 각도가 증가할수록 windlass mechanism이 강화되었다.

이는
내재 발 근육 훈련(예: 타월 컬)이
MLA 지지와 windlass mechanism을 개선할 수 있음을 시사하며,
미래 연구는 걸음걸이 같은 동적 활동에서의 역할을 조사해야 한다.

https://www.sciencedirect.com/science/article/pii/S0268003325002669

서론 배경

내측 종아치(MLA)는
보행 중 전방 추진력 지원과 충격 흡수 기능을 하는 발의 핵심 구조로,
인간 보행 패턴 변화에 따라 진화하였다.

MLA 붕괴는
족근관 증후군(tarsal tunnel syndrome),
족저근막염(plantar fasciitis),
무지외반증(hallux valgus) 등의 하하지 병리와 연관된다.

MLA 유지의 중요성은
성과 향상과 부상 예방에 필수적이다.

Windlass mechanism은
발가락 신전에 의한 발 아치 높이 변화를 통해
보행 추진력을 돕고 발 강성을 증가시킨다.

족저근막은
수축 기능이 없는 힘줄로 수동 지지를 제공하며,
810 N의 인장력에서 비가역적 신장을 보이고,
절단 시 아치 유지율이 25% 감소한다.

내재 발 근육
(예: 무지외전근 abductor hallucis, 굴지근 flexor digitorum brevis, 족저방형근 quadratus plantae)은
종아치를 조절하며,
피로 시 하중 하에서 주상골(navicular) 낙하를 유발한다.

족저근막은
수동 지지, 내재 근육은 능동 및 수동 지지를 제공하나,
MLA 지지와 windlass mechanism에 대한 각각의 기여율은 불명확하다.

임상적으로
내재 발 근육 훈련이 MLA 안정성과 windlass 기능을 향상시키나,
기여율 해명이 필요하다.

연구 목적
하중과 비하중 조건에서
다양한 발가락 신전 각도 하 족저근막과 내재 발 근육의 경도를 비교하여
MLA 지지와 windlass mechanism에 대한 기여율을 명확히 하고,
초음파 탄성촬영으로 생체 내 경도를 정량화한다.

방법론 연구 설계
횡단적 연구로,
앉은 자세와 한쪽 다리 서기 자세에서 발가락 신전 각도 변화에 따른
조직 경도와 MLA 특성을 비교 측정하였다.

대상자
최근 3개월 내 하하지 부상이나 통증 이력이 없는 건강한 성인 51명(오른발 대상)의 오른발을 포함.
표본 크기는 G*Power로 ANOVA 효과 크기 0.2, α=0.05, 검정력 0.8로 산출(목표 50명).


재료 및 절차

체성분 분석(InBody470).
발 형태 평가(Foot Posture Index-6, FPI-6: 과회내 -1 이하, 중립 0~+5, 과회내 +6 이상).
아치 특성: 등쪽 아치 높이(DAH, 발 길이 50% 지점)와 AHI(DAH/발 길이)로, 플라스틱 측정기와 전자 캘리퍼스 사용.

실시간 조직 탄성촬영(RTE, Noblus ultrasound, 5-18 MHz 프로브, 음향 커플러 탄성률 22.6 ± 2.2 kPa).
족저근막과 굴지근(FDB)을 주상골 수준 횡단면에서 측정,
발가락 신전 각도 0°~45°(커스텀 플랫폼 사용).
RTE: 리듬 압축(1-2회/초, 변형률 -0.5~0.5), 변형률 비율(대상/참조) 3회 평균.

자세: 앉은 자세(엉덩이/무릎/발목 90° 굴곡), 서기 자세(엉덩이/무릎 신전, 발목 중립).
22°C 실내, 5분 휴식 후 측정. 물리치료사(경력 3년 이상)가 수행.

통계 분석 정규성(Shapiro-Wilk).

정규 데이터:
평균 ± SD, 비정규: 중앙값(사분위 범위). 성별 효과: t-검정 또는 Mann-Whitney U. 상관: Pearson(연령/BMI vs. AHI/RTE). 각도별 경도 차이: Friedman 후 Wilcoxon(Holm 보정). 자세/각도 효과: 이원 반복 측정 ANOVA(정규) 또는 일반화 추정 방정식(GEE, 비정규), 후속 Wilcoxon 또는 t-검정. AHI 후속: Shaffer. 유의 수준 p<0.05, R 4.2.1 사용. RTE 신뢰도: ICC(95% CI), 11명 대상(Landis 기준: 상당 0.61-0.80, 우수 ≥0.81).

결과
주요 발견 및 데이터 대상자: FPI-6 중립 30명, 평발 12명, 고아치 9명. 연령, 성별, BMI가 AHI/RTE에 영향 없음(p>0.05). RTE 경도: 앉은 자세 45°에서 차이 없음, 다른 각도 유의(Fig. 6). GEE: 상호작용 없음; 자세 효과로 족저근막 경도 서기>앉음, FDB 경도 서기>앉음(각도 증가 시 45° vs. 0°/15°에서 증가). AHI: 앉은>서기(모든 각도), 발가락 신전 증가 시 상승(앉은: 0° 61.7±4.7 mm → 45° 65.4±4.2 mm; 서기: 59.0±4.3 → 62.0±4.1 mm). ICC: 0.67-0.93(상당~우수).

주요 그림/표

• Fig. 1: AHI 계산 방법.
• Fig. 2: 발가락 신전 플랫폼.
• Fig. 3: 프로브 위치.
• Fig. 4: 탄성촬영(파랑: 경성, 빨강: 연성).
• Fig. 5: 측정 자세.
• Fig. 6: 경도 변화 그래프.
• Table 1: 대상자 특성.
• Table 2: 상관 분석.
• Table 3: 경도 값.
• Table 4: AHI 값.
• Table 5: RTE 신뢰도.

논의 해석 앉은 자세 windlass mechanism은 족저근막과 FDB가 기여하나, 서기 자세에서는 FDB가 주도적(경도 ~6배 높음)으로, 하중 하 내재 근육 역할 강조. 서기 FDB 경도 증가는 능동 수축에서 기인할 가능성 높음(족저근막은 덜 두드러짐). 발가락 신전은 windlass 강화(AHI 증가 1.3-3.7 mm 앉은, 0.6-3.0 mm 서기), 앉은 AHI 높음(0.247 vs. 0.234 서기), 기존 연구 일치. 젊은 건강인에서 연령/성별/BMI 상관 없음. RTE 신뢰도 상당~우수하나, 서기 불안정과 단축 접근으로 낮음.
함의 족저근막을 주 지지자로 보는 관점에 도전, 하중 하 근육 강화(타월 컬 등) 우선. 인솔 같은 수동 지지보다 근육 훈련 추천.

제한점 FDB만 평가(다른 내재 근육 미포함). RTE 서기 재현성 미최적(불안정, 단축 방법). 서기 자세가 보행 동적 windlass와 다름.

결론 Windlass mechanism 연조직 기여는 하중에 따라 다름: 앉은 자세 족저근막+FDB, 서기 FDB 주도. MLA와 windlass 개선을 위한 내재 발 근육 훈련 지지; 미래 연구는 보행 중 경도와 운동 효과 조사 추천

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Reconsideration of the load-bearing functions of the plantar fascia and intrinsic foot muscles in the windlass mechanism

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• Open access
• Published: 15 April 2025

Reconsideration of the load-bearing functions of the plantar fascia and intrinsic foot muscles in the windlass mechanism

• Hiroshi Shinohara, 
• Yasuyuki Umezaki, 
• Airi Ikeda, 
• Takara Ikeda & 
• Nao Tateyama 

Scientific Reports volume 15, Article number: 12923 (2025) Cite this article

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Abstract

This study aimed to explore the contributions of the plantar fascia and intrinsic foot muscles to medial longitudinal arch (MLA) support and the windlass mechanism under different toe extension angles in seated and single-leg standing postures. The study included 51 healthy adults (41 males, 10 females; mean age: 21.3 ± 1.2 years) with no history of lower extremity injuries or pain in the past 3 months. Ultrasound elastography was used to measure the stiffness of the plantar fascia and flexor digitorum brevis under toe extension angles of 0°, 15°, 30°, and 45°. Dorsal arch height and arch height index (AHI) were eval‎uated to assess MLA changes. The plantar fascia and flexor digitorum brevis contributed to the windlass mechanism in the seated posture, while in the standing posture, the flexor digitorum brevis played a more pronounced role, with stiffness approximately six times greater than in the seated posture. The AHI was significantly higher in the seated posture than in the standing posture, and greater toe extension angles enhanced the windlass mechanism. These findings highlight the importance of intrinsic foot muscle training, such as towel curls, to improve MLA support and the windlass mechanism. Future studies should investigate their role in dynamic activities like walking.

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Introduction

The medial longitudinal arch (MLA) is a key structural component of the foot that supports forward propulsion during walking and serves as a shock absorber. It evolves in response to changes in human gait patterns1,2. Collapse of the MLA has been reported to lead to various lower limb pathologies such as tarsal tunnel syndrome3, plantar fasciitis4,5, and hallux valgus6,7. Maintaining the MLA is essential not only for enhancing performance but also for preventing injuries.

A phenomenon associated with the MLA is the windlass mechanism. The windlass mechanism refers to the phenomenon in which the height of the foot arch changes when the toes are extended. This is also known as the “winding-up mechanism” and has been reported to assist forward propulsion during walking8. This mechanism occurs because of the tensioning of soft tissues in the plantar region and is often associated with foot rigidity. Previous studies have indicated that the plantar fascia undergoes irreversible elongation only when subjected to a tensile force of 810 N and is considered a key structure that provides passive support to the MLA9,10. Furthermore, cutting the plantar fascia reportedly results in a 25% reduction in the ability to maintain the arch11. The intrinsic foot muscles also play a role in maintaining the MLA under load, and their fatigue has been shown to cause a navicular drop during loading12. Among the intrinsic foot muscles, the abductor hallucis, flexor digitorum brevis, and quadratus plantae contribute to the regulation of longitudinal arch13,14. Thus, the plantar fascia and intrinsic foot muscles are essential soft-tissue structures that support the windlass mechanism.

Contributions of the plantar fascia and intrinsic foot muscles to MLA support and the windlass mechanism have been suggested; however, the extent of their respective contributions remains unclear. The plantar fascia, which is a tendon structure without a contractile function, provides passive support. In contrast, intrinsic foot muscles possess contractile functions and offer both active and passive support. Intrinsic foot muscle training is commonly employed in clinical practice to enhance MLA stability and windlass function. Elucidating their contribution rates would be valuable in eval‎uating the effectiveness and necessity of intrinsic foot muscle training for improving MLA support and the windlass mechanism.

In recent years, the development of ultrasound elastography has made it possible to measure the stiffness of soft tissues, allowing for the quantification of the stiffness of soft tissues that support the arch in vivo. Previous studies investigated the relationship between foot soft tissue stiffness and performance using ultrasound elastography15. Referring to prior research16, we hypothesize that measuring the stiffness of the plantar fascia and intrinsic foot muscles under loading conditions could help elucidate their contribution to load bearing and the windlass mechanism.

This study aimed to clarify the contribution rates of the plantar fascia and intrinsic foot muscles to MLA support and windlass mechanism by comparing the stiffness of the plantar fascia and intrinsic foot muscles under both loaded and unloaded conditions with varying toe extension angles.

Methods

Participants

The study included the right feet of 51 healthy adults (41 males and 10 females) with no history of lower-extremity orthopedic injuries or pain within the past three months. Prior to participation, all participants were provided with a detailed explanation of the study, both verbally and in writing, and informed consent was obtained. The sample size for this study was determined using G*Power (version 3.1.9.6, University of Düsseldorf, Germany). A priori power analysis was conducted for ANOVA (or regression, depending on the study design) with an effect size (Cohen’s f ) of 0.2, an α error probability of 0.05, and a statistical power (1-β) of 0.8. Based on these parameters, the required sample size was calculated to be 50 participants. This sample size was used as a reference for recruitment in the present study. This study was approved by the Ethics Review Committee of the Takarazuka University of Medical and Health Care (approval number: 2207201). The study was conducted according to the Declaration of Helsinki.

Body composition

Body composition was assessed using a body composition analyzer (InBody470, manufactured by InBody Japan, Japan).

Measurement of foot morphology and arch characteristics

Foot morphology was assessed using the Foot Posture Index-6 (FPI-6)17. The FPI-6 is widely used to eval‎uate static foot morphology, with a total score ranging from − 12 to + 12. Scores were categorized as follows: -1 or lower, supinated foot; 0 to + 5, neutral foot; and + 6 or higher, pronated foot17.

Foot arch characteristics were eval‎uated based on the method described by McPoil et al.18,19 using dorsal arch height (DAH) at 50% foot length as an indicator. The arch height index (AHI) was calculated as the ratio (%) of dorsal arch height to foot length (Fig. 1). The measurement process involved first determining foot length using a plastic foot measurement device, followed by marking the 50% point of the foot length with tape. The dorsal arch height at this point was measured using electronic calipers (DHD-01, FregocS). All foot morphology and arch characteristic assessments were performed by the same examiner to ensure consistency.

Fig. 1

Method for calculating AHI. The AHI was calculated by dividing the dorsal arch height by the foot length.

Full size image

Real-time tissue elastography (RTE)

Real-time tissue elastography (RTE) was performed using an ultrasound diagnostic device (Noblus, Hitachi Aloka, Tokyo, Japan) equipped with a 5–18 MHz linear probe (L64, Hitachi Aloka). An acoustic coupler (EZU-TECPL1, Hitachi Aloka Medical, Tokyo, Japan) was attached to the probe using a plastic attachment (EZU-TEATC1, Hitachi Aloka Medical) as the hardness reference material. The elasticity modulus of the reference material was 22.6 ± 2.2 kPa, based on relative values provided by the manufacturer. An echo gel was applied to the probe before attaching an acoustic coupler to prevent air bubble formation. In addition, an echo gel was applied to the tip of the coupler to ensure proper acoustic coupling. In this study, the frequency was set to 15 MHz, the Brightness Gain (BG) to 20 dB, and the Dynamic Range (DR) to 65 dB.

To measure the stiffness of the plantar soft tissues, the foot was placed on a custom apparatus and the probe was applied to the plantar region in the RTE mode (Figs. 2 and 3). This study depicted only the flexor digitorum brevis (FDB) as an intrinsic foot muscle. The RTE images were obtained at the navicular level, depicting the plantar fascia and flexor digitorum brevis in the transverse plane (Fig. 4). RTE images were captured by positioning the probe at the scanning site and applying rhythmic manual compression and relaxation. Compression cycles were performed at a rate of 1–2 times per second, with a strain graph used to maintain consistent compression force (strain ratio range: 0.5 to 0.5).

Fig. 2

Custom platform for adjusting toe extension angles. The custom platform was designed to allow adjustment of the metatarsophalangeal (MP) joint to 0°, 15°, 30°, and 45° by inserting wedges. A rectangular opening on the base of the platform (67 × 73 mm) enabled the insertion of the ultrasound probe to obtain images of the plantar region.

Full size image

Fig. 3

Placement of the ultrasound probe on the plantar surface. The heel section of the apparatus (gray) was adjustable in the anterior-posterior direction, allowing individual adjustments to ensure that the probe was positioned on the plantar surface at the navicular level. The heel section served as a guide, helping to maintain the probe in a vertical orientation.

Full size image

Fig. 4

Explanation of ultrasound elastography. In the elastography images, stiff tissues are displayed in blue, whereas softer tissues are shown in red. The strain ratio of the target tissue relative to that of the acoustic coupler (strain value of the target tissue/strain value of the acoustic coupler) was calculated by applying an acoustic coupler with a known elastic modulus and simultaneously deforming it.

Full size image

The RTE images were displayed as real-time, semi-transparent, and color-coded overlays on the B-mode images. Measurements were performed thrice for each posture and toe-extension condition. The strain within the region of interest (ROI) of the plantar fascia, flexor digitorum brevis, and acoustic coupler was calculated from the obtained images (Fig. 4). The tissue stiffness was expressed as the strain ratio (target tissue/reference ratio), which was calculated by dividing the strain measured in the target tissue ROI by that measured in the acoustic coupler ROI. The average strain ratio from three trials was used for statistical analysis. All imaging and ROI positioning procedures were performed by the same examiner to ensure consistency.

The room temperature was maintained at 22 °C, and all subjects were instructed to rest for at least 5 min before the measurement to ensure consistency. All RTE measurements were conducted by a single licensed physical therapist with over three years of experience. The tester regularly uses ultrasound in clinical and research settings, ensuring familiarity with the measurement procedures. This consistency in the examiner helped to minimize inter-rater variability and enhance the reliability of the measurements.

Measurement postures and tasks

The measurement postures for AHI and plantar RTE included a seated position and a single-leg standing position. In the seated position, the hip, knee, and ankle joints were set at 90° flexion. In the single-leg standing position, the supporting-side hip and knee joints were extended, the ankle joint was in a neutral position, and the swing-side knee joint was flexed to 90°, with the hip and ankle joints in neutral positions (Fig. 5). To stabilize the single-leg standing posture, participants were instructed to lightly place one hand on a platform at the level of the ASIS, with the elbow joint flexed at 30°. Additionally, participants were required to keep their gaze forward and maintain a stable posture without noticeable body sway before measurements were taken.

Fig. 5

Measurement postures. The left figure shows the measurement posture in the seated position, and the right figure shows the posture in the standing position.

Full size image

In each posture, measurements were conducted under four conditions in which the metatarsophalangeal (MP) joints were passively extended to 0°, 15°, 30°, and 45°.

Statistical analysis

The normality of the data was eval‎uated using the Shapiro–Wilk test. Variables with normal and non-normal distributions were presented as mean ± SD and median (interquartile range), respectively. To examine the influence of gender on AHI and RTE, an independent t-test was used when normality was confirmed, while a Mann-Whitney U test was applied when normality was not met. Additionally, Pearson’s correlation analysis was performed to assess the relationships between age and AHI, age and RTE, BMI and AHI, and BMI and RTE. Significant variables were further analyzed to determine their role as predictors. These analyses were conducted only under the toe extension 0° condition in both seated and standing postures.

To compare the stiffness of the plantar fascia and flexor digitorum brevis at different toe extension angles, Friedman tests were conducted for each angle, followed by post hoc Wilcoxon tests using Holm’s method for multiple comparison adjustments.

To examine the effects of posture (seated, single-leg standing) and toe extension angle (0°, 15°, 30°, 45°) on the stiffness of the plantar fascia, flexor digitorum brevis, and arch height index, two-way repeated measures ANOVA was performed for normally distributed data, and generalized estimating equations (GEE) were used for non-normally distributed data. Interactions and main effects were assessed. If interactions or main effects were identified in the GEE, Wilcoxon tests with Holm’s method for multiple comparisons were conducted to clarify the differences between levels. For the two-way repeated-measures ANOVA, if interactions were detected, paired t-tests or Wilcoxon tests were performed to identify differences between the levels.

Post hoc analyses of the arch height index were conducted using Shaffer’s method for multiple comparisons. Statistical analyses were performed using R version 4.2.1 for MacOS (CRAN, freeware), with a significance level set at 5%.

To assess the reliability of RTE, we conducted three repeated measurements in each posture on 11 subjects (mean age: 21.1 ± 1.3 years, mean height: 175.2 ± 5.4 cm, mean weight: 71.3 ± 6.8 kg). The intraclass correlation coefficient (ICC) was used to quantify the reliability of the measurements. As proposed by Landis et al., a reliability coefficient ≤ 0.2 was considered slight, 0.21–0.40 was considered fair, 0.41–0.60 was considered moderate, 0.61–0.80 was considered substantial, and ≥ 0.81 was considered excellent20, with the confidence interval (CI) set at 95%.

Results

The participants’ basic characteristics are listed in Table 1. Based on the FPI-6 results, there were 30 neutral feet, 12 flat feet, and nine high-arched feet.

Table 1 Characteristics of the participants (mean ± SD).

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Effect of gender, age, and BMI on AHI and RTE

The independent t-test showed no significant differences in AHI (seated and standing conditions at 0° toe extension) between male and female participants (seated: p = 0.184, standing: p = 0.769). For RTE, the Mann-Whitney U test indicated no significant differences between genders across all conditions at 0° toe extension (seated-PF: p = 0.175, seated-FDB: p = 0.275, standing-PF: p = 0.767, standing-FDB: p = 0.328). Therefore, gender was not included as a predictor in the regression analysis. Additionally, Pearson’s correlation analysis showed no significant correlations between age, BMI, and AHI or RTE under all conditions (all p > 0.05). These results are summarized in Table 2. Based on these findings, gender, age, and BMI were not considered as predictors in the regression analysis.

Table 2 Correlation analysis between physical characteristics, plantar soft tissue hardness and AHI.

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Results of ultrasound eval‎uation

The median and interquartile ranges of the ultrasound eval‎uations are shown in Table 3. The results of the one-way repeated measures analysis of variance (Friedman test) revealed no significant differences in the stiffness of the flexor digitorum brevis (FDB) and plantar fascia (PF) at 45° toe extension in the seated posture. However, significant differences were observed in the stiffness of the intrinsic foot muscles (IFM) and PF across other toe extension angles in both the seated and standing postures (Fig. 6).

Table 3 Stiffness values of plantar soft tissues.

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Fig. 6

Changes in FDB and PF stiffness with variations in toe extension angles (left: sitting, right: standing). PF plantar fascia, FDB Flexor digitorum brevis.

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Comparative analysis of stiffness in the plantar fascia and intrinsic foot muscles

Furthermore, the results of the GEE analysis indicated no interaction effects for the stiffness of the plantar fascia or flexor digitorum brevis. A main effect of posture was observed on the stiffness of the plantar fascia, with significantly higher values in the standing posture than in the seated posture. Regarding the stiffness of the flexor digitorum brevis, main effects were found for both posture and toe extension angle. Stiffness was significantly higher in the standing posture than in the seated posture. Regarding the toe extension angle, significantly higher stiffness was observed at 45° toe extension than at 0° and 15°.

Results of MLA

The mean ± standard deviation of dorsal arch height for each toe extension angle (0°, 15°, 30°, 45°) was as follows: in the seated posture, 61.7 ± 4.7 mm, 63.0 ± 4.2 mm, 64.3 ± 4.1 mm, and 65.4 ± 4.2 mm, respectively; in the standing posture, 59.0 ± 4.3 mm, 59.6 ± 4.1 mm, 60.6 ± 4.0 mm, and 62.0 ± 4.1 mm, respectively. For the arch height index, interaction and main effects of the two factors were observed. Under all conditions (0°, 15°, 30°, and 45° toe extension), the arch height index was significantly higher in the seated posture than in the standing posture. Additionally, the arch height index increased significantly as the toe extension angle increased (Table 4).

Table 4 Arch height index (AHI) across different toe extension angles.

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Results of RTE reliability

The intraclass correlation coefficient (ICC) results are summarized in Table 5. The reliability of RTE measurements ranged from 0.67 to 0.93. Based on the classification by Landis et al., the ICC values indicated substantial to excellent reliability20.

Table 5 The reliability of real-time elastography (RTE) based on ICC (1,3).

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Discussion

The results of this study suggest that the windlass mechanism in the seated posture is primarily influenced by the plantar fascia and the flexor digitorum brevis, whereas in the single-leg standing posture, it is mainly influenced by the flexor digitorum brevis. This finding may contribute to the ongoing discussion regarding the extent to which intrinsic foot muscle activity is necessary to maintain the shape of the MLA in the resting state12,14,21,22,23.

In the seated posture, a difference in stiffness was observed between the plantar fascia and the flexor digitorum brevis; however, the difference was not as pronounced as that in the standing posture. The stiffness of the flexor digitorum brevis in the standing posture was approximately six times higher than that in the seated posture. This suggests that the role of the intrinsic foot muscles becomes significantly more pronounced under a load. While previous studies have measured the stiffness of the plantar fascia in standing postures, no study has compared it with that of the intrinsic foot muscles, making this study a novel finding. According to previous studies, the support provided by muscle activity is considered secondary, whereas primary support is provided by the plantar fascia. It has been suggested that the intrinsic foot muscles play a supporting role when excessive strain is applied to the plantar fascia, such as when bearing additional weight24. Based on the results of this study, it can be interpreted that, in conditions with minimal load, such as the seated posture, the windlass mechanism is primarily facilitated by both the plantar fascia and the flexor digitorum brevis. By contrast, under a full load, such as in a standing posture, the windlass mechanism is predominantly driven by the flexor digitorum brevis.

It is highly intriguing that primary tissues are responsible for inducing changes in the windlass mechanism under a load. Previous studies have reported that the intrinsic foot muscles exhibit increased activity when supporting the MLA24. Another study observed the EMG changes in the tibialis anterior and intrinsic foot muscles under loads exceeding 400 pounds in individuals with normal arches22. In our study, EMG measurements were not performed; thus, whether the increased stiffness of the flexor digitorum brevis was due to the increased muscle activity or elongation-induced stiffness remains unclear. However, we hypothesized that the flexor digitorum brevis would exhibit increased muscle activity. If the stiffness of the flexor digitorum brevis is solely due to elongation, one would expect the plantar fascia, which runs parallel to the same region, to exhibit a similar increase in stiffness. Although both tissues exhibited increased stiffness under loading, the flexor digitorum brevis exhibited a significantly greater increase. This indicates that the heightened stiffness was not solely due to elongation but also due to the active contraction of the flexor digitorum brevis.

In this study, the windlass mechanism was eval‎uated by extending the toes to 0°, 15°, 30°, and 45°. The results revealed that the increase in dorsal arch height was 1.3 mm, 2.6 mm, and 3.7 mm in the seated posture and 0.6 mm, 1.6 mm, and 3.0 mm in the standing posture. Previous studies using X-rays reported increases in the MLA height of 4.1 mm, 5.0 mm, and 6.1 mm, while studies using foot models reported increases of 2.0 mm, 2.9 mm, and 3.9 mm25. In our study, the AHI was 0.247 in the seated posture and 0.234 in the standing posture, and the MLA was lower in the standing posture than in the seated posture. This finding is consistent with previous studies26. Furthermore, prior research involving participants with an average age of 26.7 years reported an AHI of 0.253 for standing posture18. Compared with these previous studies, the results of our study are considered to be within a reasonable range. However, during the late stance phase, the tension in the plantar fascia is estimated to be nearly equivalent to body weight, and the arch height increases by an average of 6 mm during this phase27,28. In our study, the increase in arch height was not pronounced. This difference may be attributed to differences in posture and toe extension angles compared with those in the late stance phase. In this study, the changes in arch height observed demonstrate the interaction effects between the seated and standing postures. This was interpreted as a result of greater stiffness in the flexor digitorum brevis and plantar fascia in the standing posture, leading to a more pronounced windlass mechanism induced by toe extension.

Next, we discuss the relationship between foot morphology and plantar soft tissue stiffness. Previous studies have reported that plantar soft tissue stiffness increases with age in individuals aged 41 and older29. Additionally, it has been suggested that foot morphology alone may not fully explain variations in plantar soft tissue stiffness30. However, since the participants in this study were young adults, most of whom had normal foot morphology within the range defined by FPI-6, it was not possible to examine whether foot morphology is associated with plantar soft tissue stiffness.

In this study, no significant correlation was found between age and AHI or RTE, indicating that age was not a predictor. This lack of significant correlation may be due to the limited age range of the participants, as all were young adults with normal foot morphology. Similarly, gender was not significantly associated with AHI or RTE, consistent with previous studies showing no significant gender differences in plantar soft tissue stiffness31. BMI also did not show a significant correlation with AHI or RTE, suggesting that it was not a predictor. One possible reason is that participants had BMI values within a standard range, whereas the impact of BMI may be more pronounced in populations with extremely low or high BMI. These results suggest that gender, age, and BMI do not significantly influence AHI or RTE in young, healthy adults. Future research including older adults and individuals with a broader BMI range is needed to further explore these factors and assess the potential influence of plantar soft tissue properties in more diverse populations.

The reliability of RTE in this study was confirmed to range from substantial to excellent (0.67 to 0.93). While reliability tended to be lower in the standing position compared to the seated position, this may be attributed to instability in weight distribution that occurred during the single-leg stance. Additionally, previous studies have reported that reproducibility decreases when measurements are taken in the short axis due to the influence of vascular structures and fat pads32,33. These factors may have also contributed to the results observed in this study. Previous studies using strain RTE have reported excellent ICC values, such as 0.88 for the resting biceps brachii muscle in patients with Parkinson’s disease and 0.95 for the gastrocnemius muscle34. However, other studies have documented ICC values in the moderate range, such as 0.51 for the supraspinatus muscle (ICC 1,1)35 and low reliability (ICC 0.35) for the proximal quadriceps tendon36. Therefore, the results obtained in this study cannot be considered inadequate.

The study results provide valuable insights for therapeutic exercises. In clinical practice, exercises such as towel curls are commonly performed to train the intrinsic foot muscles. The study findings suggest that training intrinsic foot muscles may lead to improvements in MLA support and the windlass mechanism. Insoles and taping can be considered as methods to support the plantar fascia, and prioritizing the improvement of performance through one’s own strength is essential. Furthermore, based on the results of this study, it can be interpreted that performing therapeutic exercises, such as towel curls, with at least some degree of loading may be more effective.

This study has several limitations. First, the intrinsic foot muscle assessed in this study was limited to the flexor digitorum brevis. Other intrinsic foot muscles, including the abductor hallucis, quadratus plantae, and lumbricals, were not eval‎uated. Therefore, the extent to which these muscles influence the findings remains unclear. In particular, as the abductor hallucis is both anatomically and functionally closely related to the MLA37,38,39, future studies should consider its potential impact. Second, the reproducibility of the RTE measurement method in single-leg standing was not established with supporting data. Since single-leg standing is inherently unstable, it may have affected the reliability of the measurements. Additionally, the reliability of RTE may have been further reduced due to the use of the short-axis approach during measurement. In fact, the reliability in the standing position was not particularly high. Therefore, it is necessary to explore more stable measurement techniques or methodological refinements to enhance the reliability of RTE in standing postures. Third, the stiffness of the intrinsic foot muscles during reproduction of the windlass mechanism was measured in a posture in which the plantar surface of the foot was in full contact with the ground. Although this posture involves loading, it differs from a posture in which a windlass mechanism typically occurs during functional movements.

The results of this study suggest that soft tissues involved in the windlass mechanism may differ depending on the load conditions. This finding challenges the established notion that the plantar fascia predominantly functions in both seated and standing postures. Concurrently, it supports the necessity of intrinsic foot muscle training, such as towel curls, commonly used in rehabilitation settings. Future research should investigate whether exercises targeting the intrinsic foot muscles affect muscle stiffness and examine the stiffness of these muscles during walking. We hope that this study will contribute to advancements in rehabilitation practice.

Data availability

The datasets generated and/or analyzed during the current study are not publicly available due to privacy and ethical considerations but are available from the corresponding author upon reasonable request.

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