Relationship between plantar fascia thickness in static standing and calf raise positions and the medial longitudinal arch height ratio in healthy adults

Abstract

[Purpose] To evaluate plantar fascia thickness during static standing and calf raise positions using ultrasound imaging and to examine its relationship with medial longitudinal arch height ratio and physical factors. [Participants and Methods] We measured plantar fascia thickness in 30 healthy adults (18 males, 12 females) using ultrasound at a site 1.5 cm distal to the calcaneal origin in static standing and calf raise positions. Additionally, we assessed their medial longitudinal arch height ratio (arch height using navicular height divided by foot length) and other physical characteristics to examine possible correlations. [Results] Ultrasound measurements of plantar fascia thickness showed high reliability in both static standing and calf raise positions, with thickness reduced significantly from standing to the calf raise position. In the overall analysis, plantar fascia thickness was positively correlated with body weight in both positions, and a positive correlation between plantar fascia thickness and height was observed in the standing position. In contrast, no significant correlations were found between plantar fascia thickness and the medial longitudinal arch height ratio or body mass index in the overall analysis, nor between plantar fascia thickness and physical parameters in sex-specific analyses. In males, the change in plantar fascia thickness between positions was negatively correlated with age; however, no significant correlations were observed in the overall sample or in females. [Conclusion] Plantar fascia thickness decreases during calf raises compared with standing and is influenced by body weight, supporting that the calf raise position places stretching stress on the plantar fascia and may be used for evaluating load response and physical function.

INTRODUCTION

The medial longitudinal arch of the foot is a supporting mechanism responsible for shock absorption and propulsion during walking. The plantar fascia is a key structure supporting the medial longitudinal arch; it originates from the medial calcaneal tubercle and inserts into the metatarsals¹⁾. Excessive stress on the plantar fascia can cause inflammation and thickening, leading to plantar fasciitis²⁾.

Computed tomography (CT) and magnetic resonance imaging (MRI) are commonly used for imaging in plantar fasciitis; however, recently, ultrasonography is gaining popularity owing to its minimal invasiveness, ease of measurement, and capacity for real-time visualization³⁾. Sabir et al.⁴⁾ reported comparable accuracy between ultrasound and MRI for measuring plantar fascia thickness. Likewise, Morares do Carmo et al.⁵⁾ compared the reliability of ultrasound and MRI measurements in cadaver specimens and found no significant differences in plantar fascia thickness between the two modalities. Nakajima⁶⁾ also reported no difference in plantar fascia thickness measurements between ultrasound and MRI.

Plantar fascia thickness is commonly assessed using ultrasound in patients with plantar fasciitis; however, some studies have measured its thickness in healthy adults as well. The evaluation is usually performed at rest in the supine or prone position, or in varying limb positions, including toe dorsiflexion and knee flexion; however, only a few studies have measured plantar fascia thickness during a weight-bearing (standing) position. When supporting the medial longitudinal arch, the plantar fascia undergoes repeated elongation and contraction through the “windlass” and “truss” mechanisms⁷⁾. In weight bearing, the medial longitudinal arch is moderately elevated to provide shock absorption; in this position, the plantar fascia contributes to shock absorption through its flexibility. Excessive stress on the plantar fascia during weight bearing leads to inflammation, resulting in increased plantar fascia thickness⁸⁾. Additionally, some studies have suggested a correlation between plantar fascia thickness and physical factors, such as body weight and body mass index. Therefore, the flexibility of the plantar fascia and its stress tolerance vary among individuals.

Calf raises are a common rehabilitation exercise that apply more stretching stress to the plantar fascia through toe dorsiflexion and contraction of the triceps surae. Despite being a common weight-bearing exercise used in rehabilitation protocols, very few studies have evaluated the thickness and dynamics of the plantar fascia during calf raises. Accordingly, the present study evaluated plantar fascia thickness in healthy adults using ultrasonography during static standing and calf raise positions and compared the measurements between these conditions to determine the relationships between plantar fascia thickness and medial longitudinal arch height ratio, height, weight, and other physical factors.

PARTICIPANTS AND METHODS

The study was performed in accordance with the ethical standards of the Declaration of Helsinki and the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) guidelines. Ethical approval was obtained from the Ethical Review Committee of JCHO Gunma Central Hospital (Approval number: 2022-003).

In this study, we included 30 healthy adults. All participants were briefed on the purpose and methods of the study, and measurements were conducted after obtaining informed consent from all participants.

Plantar fascia thickness was measured in two conditions: standing and calf raises (maximum lift) using a Viamo ultrasound system (SSA-640A; Toshiba Medical Systems, Tochigi, Japan) with a 7.5 MHz linear probe. All measurements were taken along the longitudinal axis of the plantar fascia of the dominant foot (defined as the foot used to kick a ball), and thickness was determined using the measurement software of the system. To ensure consistency, the measurement site was selected as 1.5 cm distal to the calcaneal origin (Figs. 1, 2). A previous study reported good reliability and no significant differences in plantar fascia thickness at the areas located 1 cm and 2 cm distal to the calcaneal origin⁹⁾.

Fig. 1.

Ultrasound image of plantar fascia thickness measurement in standing position.

Superior: Plantar side, Inferior: Dorsal side, Left: Distal side, Right: proximal side.

Point A: Calcaneal origin of the plantar fascia, Point B: Measurement site of plantar fascia thickness.

Dist A: Distance from the calcaneal, Dist B: Plantar fascia thickness.

Fig. 2.

Ultrasound image of plantar fascia thickness measurement during calf raise.

Superior: Plantar side, Inferior: Dorsal side, Left: Distal side, Right: proximal side.

Point A: Calcaneal origin of the plantar fascia, Point B: Measurement site of plantar fascia thickness.

Dist A: Distance from the calcaneal, Dist B: Plantar fascia thickness.

For the static standing position, a 40-cm-high platform was constructed, consisting of an Erector^® framework (Yazaki Kako Corporation, Shizuoka, Japan), with the top surface made of plywood sufficiently thick to withstand weight bearing. A 10 × 10 cm measurement window was created on the top surface to allow probe manipulation and visualization of the heel region. During standing measurements, a 2-mm transparent plastic sheet was placed over the window (Fig. 3).

Fig. 3.

Measurement of plantar fascia thickness during static standing.

For the calf raise position, the participant stood on the platform without the plastic sheet, and measurements were obtained with the probe placed directly on the calcaneal origin. Calf raises were performed, with hands placed on a wall directly in front to ensure stability; participants were verbally instructed to “lift the heel as high as possible”, and measurements were performed while confirming visually that the foot did not deviate into excessive varus (Fig. 4). For each condition, measurements were taken three times during both the standing and calf-raise positions, and the mean value was used as the representative value. All measurements for all participants were conducted by the same examiner.

Fig. 4.

Measurement of plantar fascia thickness during calf raise.

The arch height ratio was calculated as the navicular height (cm) divided by foot length (cm). The participant sat on the same platform with the foot placed flat on the floor. Using a ruler, navicular height was measured as the distance from the floor to the lower end of the navicular bone, and foot length was measured from the heel to the tip of the toes with the ruler placed flat on the floor.

Data for other physical factors were obtained through a questionnaire; BMI was calculated by dividing weight (kg) by squared height (m²).

Statistical analyses were performed using SPSS (IBM SPSS Statistics for Windows, version 28; IBM Corp., Armonk, NY, USA). After confirming normality using the Shapiro–Wilk test, statistical analyses were conducted. Intra-rater reliability was assessed by calculating intraclass correlation coefficients (ICC) for normally distributed data and Cohen’s kappa (κ) coefficients for non-normally distributed data. Differences in plantar fascia thickness between positions were examined using paired t-tests. Correlations between plantar fascia thickness and physical factors in each position, as well as correlations between changes in plantar fascia thickness between positions and physical factors, were evaluated using Pearson’s correlation coefficient (r) for normally distributed data and Kendall’s rank correlation coefficient for non-normally distributed data.

RESULTS

The participants’ characteristics and physical factors are summarized in Table 1; data for arch height ratio are shown in Table 2. Based on the measurements, we obtained an ICC (95% confidence interval) of 0.911 (0.844–0.953) for calf raise position. For standing plantar fascia thickness, Cohen’s κ values were 0.881 for the first and second measurements, 0.870 for the second and third measurements, and 0.846 for the first and third measurements, all showing high reliability.

Table 1. Basic demographic characteristics of study participants.

Sex	n	Age (years)	Height (cm)	Weight (kg)	BMI (kg/m2)
Male	18	25.3 ± 3.1	171.5 ± 5.3	66.7 ± 11.0	22.7 ± 3.8
Female	12	24.4 ± 2.6	158.8 ± 5.2	50.3 ± 6.5	20.0 ± 1.8
Total	30	25.0 ± 3.0	166.4 ± 8.2	60.1 ± 12.4	21.6 ± 3.4

Data are presented as the mean ± standard deviation. BMI: body mass index.

Table 2. Medial longitudinal arch height ratio values.

Sex	Arch height ratio
Male	0.174 ± 0.021
Female	0.170 ± 0.024
Total	0.172 ± 0.024

Data are presented as the mean ± standard deviation.

The plantar fascia was significantly thinner during calf raises than during standing position in the overall cohort, as well as in males and females separately (p<0.05 each; Table 3). Additionally, the mean thickness across three measurements was consistently lower in the calf raise position than in standing for all participants.

Table 3. Comparison of plantar fascia thickness during static standing and calf raise positions.

Sex	Standing (mm)	Calf raise (mm)	t-test
Male	2.44 ± 0.17	2.17 ± 0.22	*
Female	2.33 ± 0.21	2.05 ± 0.22	*
Total	2.39 ± 0.193	2.12 ± 0.22	*

Data are presented as the mean ± standard deviation. *p<0.05.

Correlation coefficients between physical factors, including arch height ratio, and plantar fascia thickness were shown in Table 4. No significant correlation between physical factors and plantar fascia thickness during standing or calf raise was observed in men or female. In the overall analysis, significant correlations were found between standing plantar fascia thickness and height and weight, as well as between calf raise plantar fascia thickness and weight.

Table 4. Correlation coefficient values for the correlation between plantar fascia thickness in standing and calf raise positions and physical factors.

		Arch height ratio	Height	Weight	Age	BMI
Male	Standing	0.272	0.304	0.297	0.150	0.172
	Calf raise	0.036	0.125	0.357	0.338	0.294
Female	Standing	0.192	0.382	0.470	−0.238	0.461
	Calf raise	0.222	0.370	0.375	−0.383	0.340
Total	Standing	0.246	0.428*	0.430*	−0.03	0.226
	Calf raise	0.141	0.342	0.432*	0.033	0.247

*p<0.05. BMI: body mass index. Kendall’s rank correlation coefficient was used only for age and total BMI. Pearson’s correlation coefficient was used for the other variables.

The mean change in thickness between standing and calf raise is summarized in Table 5, and the correlation results are presented in Table 6. The mean change in plantar fascia thickness was 0.263 ± 0.105 mm for male, 0.271 ± 0.104 mm for female, and 0.266 ± 0.103 mm overall. There was no significant difference in the change in plantar fascia thickness between males and females. Significant correlations between the change in plantar fascia thickness and physical factors were not observed overall or in females. In men, however, a significant negative correlation was identified with age.

Table 5. Mean change in plantar fascia thickness between standing and calf raise positions.

	Change in thickness (mm)
Male	0.263 ± 0.105
Female	0.271 ± 0.104
Total	0.266 ± 0.103

Data are presented as the mean ± standard deviation.

Table 6. Correlation between the change in plantar fascia thickness from static standing to calf raise positions and physical factors.

Sex	Arch height ratio	Height	Weight	Age	BMI
Male	0.343	0.269	–0.222	–0.556*	–0.307
Female	–0.079	–0.003	0.170	0.236	0.230
Total	0.144	0.074	–0.109	0.142	0.052

*p<0.05, BMI: body mass index. Kendall’s rank correlation coefficient was used only for age and total BMI. Pearson’s correlation coefficient was used for the other variables.

DISCUSSION

The plantar fascia thickness measurements in this study demonstrated high ICCs for both static standing and calf raise positions; the use of Cohen’s κ further confirmed reliability. The high reliability of measurements in this study is likely attributable to the measurement site (1.5 cm distal to the calcaneal origin). Huerta JP et al.⁹⁾ measured plantar fascia thickness at 1 cm and 2 cm distal to the calcaneal origin and reported no differences between these sites. Therefore, the measurements in the present study were conducted at a site with minimal variation in plantar fascia thickness, which may have contributed to the high reliability observed. Furthermore, the tendency for measurements during calf raises to show higher reliability than those during standing may be attributable to methodological factors. The plastic sheet used during standing measurements is thought to have increased acoustic shadowing and reduced image quality¹⁰⁾. Nevertheless, the reliability characteristics demonstrated in both positions provide a robust foundation for subsequent analyses.

The plantar fascia thickness observed in this study is likely influenced by both the truss and windlass mechanisms. The truss mechanism refers to the force that stretches the plantar fascia when the talus is stressed under load, causing anterior displacement of the supporting calcaneus and elongation of the entire foot, thereby contributing to shock absorption during gait. During standing, this truss mechanism appears to be active under body weight, resulting in elongation of the plantar fascia and involvement in plantar fascia thickness. The windlass mechanism, on the other hand, is a phenomenon in which extension of the toes tensions the plantar fascia, elevates the medial longitudinal arch, increases foot rigidity, and generates propulsive force during push-off in walking. In the present study, calf raises performed under weight-bearing conditions involve toe extension, which likely activated the windlass mechanism, thereby imposing elongation stress on the plantar fascia and affecting its thickness. Furthermore, contraction of the triceps surae during calf raises exerts traction on the calcaneus, which may further increase the stretching stress on the plantar fascia. Therefore, calf raises reflect the combined effects of hallux dorsiflexion, body weight loading, and calcaneal traction, which may explain why plantar fascia thickness decreased compared to the standing condition.

Among the overall group, body weight was the only factor that showed a significant correlation with plantar fascia thickness during standing and calf raises, and no differences in correlation coefficients were observed between the two conditions. This finding is consistent with the assumption that greater body weight increases stretching stress during loading, and similar stress resisting body weight and gravity also occurs during calf raises. This correlation with body weight suggests that plantar fascia thickness in the calcaneal region may increase due to long-term and repetitive stretching stress experienced throughout daily life. Pascual Huerta and Alarcón García⁹⁾ defined this as the “overloading effect”. In contrast, no significant differences were observed when analyzed by sex, which may be attributable to the small sample size in each sex-specific group.

We expected plantar fascia thickness to be correlated with the medial longitudinal arch height ratio because of its role in absorbing shock during loading; however, the results did not suggest such a relationship. This discrepancy may have occurred due to the age of the participants, who were in their 20s and 30s, limiting our ability to assess the potential influence of aging. And, because the plantar fascia extends from the calcaneus to the proximal phalanx of the foot, stretching stress during standing and calf raises may vary at other sites.

The correlations between changes in plantar fascia thickness and physical factors are shown in Table 6. In this study, a negative correlation between the change in plantar fascia thickness and age was observed only in males. A generally higher body weight of males compared with females may have contributed to the loss of plantar fascia flexibility. Although these results did not establish definitive correlations between changes in plantar fascia thickness and physical factors during standing and calf raises, further analyses could consider the influence of methodological factors such as sample size, plantar fascia measurement sites, and the degree of calf raise, as well as physical factors including ankle dorsiflexion range of motion and toe-grip strength.

Future research plans include examining the relationships with other physical factors and the effects of different measurement sites on plantar fascia thickness. The plantar fascia originates from the calcaneus and inserts into the proximal phalanges of the first to fifth toes. Therefore, it is likely associated with foot and toe function, and, as mentioned above, the relationships with ankle dorsiflexion range of motion and toe-grip strength should also be investigated. In this study, plantar fascia thickness was measured 1.5 cm distal to the calcaneal origin; however, the plantar fascia spans the navicular and metatarsal bones. Accordingly, future research should clarify how variations in measurement site and changes in plantar fascia thickness relate to physical factors.

Funding

The authors declare that this research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Conflict of interest

The authors have no conflicts of interest to declare that are relevant to the content of this article.