Morphological characteristics of the heel fat pad in the dominant and non-dominant feet under varying loads

Abstract

[Purpose] Morphological changes in the dual-layered structure of the heel fat pad under varying loads, differences between the dominant and non-dominant feet, and changes over one month are not understood. This study aimed to examine these factors, as understanding normal conditions provides insights into identifying abnormal conditions. [Participants and Methods] Forty healthy Japanese adults (80 feet) participated in this study. The heel fat pad was divided into a macrochamber layer (extending from the calcaneal tuberosity to the fibrous septum, including the macrochambers) and a microchamber layer (extending from the microchambers to the skin). The thickness of each layer in the dominant and non-dominant feet was measured under four conditions: non-load, sitting, 50% load, and 80% load. The compressibility indices were calculated and compared. The same investigations were performed one month later. [Results] Changes in thickness from non-load to 80% load mainly occurred in the macrochamber layer (compressibility index=0.40), with minimal changes observed in the microchamber layer (compressibility index=0.76). No significant differences were observed, although a difference of a few millimeters was observed. Similar results were obtained in the second examination, which was conducted after one month. [Conclusion] These results likely represent the morphological changes in the normal heel fat pad under varying loads.

INTRODUCTION

The heel fat pad (HFP) is divided into the macrochambers in the deep part and the microchambers in the superficial part^1,2,3). The fibrous septum (FS) connects the upper and lower parts²⁾ (Fig. 1). HFP is a soft tissue that is reportedly associated with plantar heel pain (PHP) and plantar fasciitis. These diagnostic terms have often been used synonymously for a long period^{4, 5)}. Some recent reports have defined PHP as a comprehensive term encompassing various diagnoses, including plantar fasciopathy or fasciitis, HFP syndrome, nerve irritation or entrapment, calcaneal stress fracture, and lumbar radiculopathy^6,7,8,9). HFP syndrome is an independent pathology and the second most prevalent cause of PHP, and its details remain poorly understood⁷⁾. Therefore, HFP related to HFP syndrome is worth investigating. However, despite the fact that the foot is a region where non-load and full-load conditions are repeated, the morphological changes in the normal two-layer structure of HEP under unloading and loading conditions have not been clearly visualized, and its kinetics have not been investigated. Furthermore, fundamental information, such as the difference between dominant and non-dominant feet and diurnal and monthly variations, is not known. Understanding these points may further provide clues for identifying abnormal conditions during physical therapy for HEP.

Fig. 1.

Heel fat pad ultrasonograms under non-load condition.

Ultrasonograms of the heel fat pad in a 34-year-old woman are shown. The macrochambers at the deep part, microchambers at the superficial part, and a fibrous septum that connects the upper and lower parts are observed.

Therefore, we developed an apparatus to evaluate the morphology of the HFP from non-load to full-load conditions using an ultrasound (US) device in a previous study¹⁰⁾. We also reported the reliability of the measurements using this evaluation apparatus¹¹⁾ (Fig. 2).

Fig. 2.

Evaluation apparatus, measurement positions, and scanning methods.

a: Evaluation apparatus. The evaluation apparatus consisted of an evaluation platform (left) and a weighing platform (right) made of iron. The tabletop, made of 20 mm polycarbonate resin, lies on the evaluation platform. It has cross-shaped openings (dotted circles) with a 5-mm polymethylpentene resin plate (PMP) placed on the tabletop. b: Measurement positions under the non-load condition. A plastic lamina (10 mm) was placed on the PMP, with the forefoot placed onto the top and the ankle joint set at an intermediate position. The space between the hindfoot and the PMP was filled with gel. c: Measurement position under the sitting condition. The load on the opposite foot to the measurement side by US was measured. d: Measurement position under the 50% load condition. Fifty percent of the weight was loaded onto the weighing platform; this was defined as the measurement position for the 50% load condition. e: Measurement position under the 80% load condition. Twenty percent of the weight was loaded onto the weighing platform; this was defined as the measurement position for the 80% load condition. f: Ultrasonographic scanning method. The probe was manually held in position. Scanning was performed from the sole through the PMP. The ultrasonogram of the heel fat pad was obtained at the most convex part of the calcaneal tuberosity.

Herein, we investigated the morphological changes in the HFP (a dual-layered structure) of the dominant and non-dominant feet by obtaining the mean thickness, amount of change (AC), and compressibility index (C-index) under non-load, sitting, 50% load, and 80% load conditions in healthy participants using our evaluation apparatus. The dominant and non-dominant sides were compared. Furthermore, to investigate the monthly variation of the dominant and non-dominant feet, the same examinations were repeated 1 month after the initial examination, and the thicknesses of each layer under each load condition were compared. This study aimed to examine the morphological characteristics of the two-layer structure of HEP in the dominant and non-dominant foot under varying loads and to understand the normal conditions.

PARTICIPANTS AND METHODS

This research included 40 healthy Japanese adults, accounting for a total of 80 feet. We enlisted the participants at Kuwana City Medical Center (Table 1). The participants were self-sufficient in their daily lives. Patients with a history of orthopedic disease of the lower limb, dermatological disorders of the foot, or internal diseases, such as rheumatoid arthritis, gout, diabetes, and connective tissue disorders, were not recruited.

Table 1. Participants’ characteristics.

	Overall (n=40)	Male (n=20)	Female (n=20)
Age (years)	45.5 ± 15.4	44.8 ± 15.3	46.3 ± 15.5
Age range (years)	24–80	24–80	24–72
Number of participants* (feet)
24–29 years	8 (16)	4 (8)	4 (8)
30–44 years	12 (24)	6 (12)	6 (12)
45–64 years	12 (24)	6 (12)	6 (12)
65–80 years	8 (16)	4 (8)	4 (8)
Height (cm)	164.9 ± 8.6	171.8 ± 5.7	158.1 ± 4.8
Body mass (kg)	60.5 ± 11.5	69.0 ± 9.4	52.1 ± 5.9
Body mass index (kg/m2)	22.1 ± 2.9	23.3 ± 2.5	20.9 ± 2.7
Foot length (cm)	24.4 ± 1.5 (n=80)	25.6 ± 1.1 (n=40)	23.3 ± 0.7 (n=40)

*Classifications of the participants by age were conducted in accordance with methods from a previous study¹⁸⁾.

This study was conducted in accordance with the Declaration of Helsinki and the Ethical Guidelines for Medical and Health Research Involving Human Subjects in Japan. The experiments were performed after obtaining approval from the Institutional Review Board of Kuwana City Medical Center (Ethics Approval Number 2019/120). We explained the purpose, methods, and ethical considerations of this study to the participants and obtained written informed consent from them. The experiments were conducted between January 2019 and January 2020.

The foot used to kick a ball was considered the dominant foot, whereas the other foot was considered the non-dominant foot^{12, 13)}. Before the experiment, we asked the participants to identify their dominant foot, and the foot length was measured at the 50% load condition.

The evaluation apparatus was positioned at 7° from the sagittal plane. The apparatus uses a 5-mm polymethylpentene resin plate (PMP). Using the PMP, we obtained a clear ultrasonogram of the HEP under load conditions, as the acoustic impedance of the PMP was similar to that of human tissue. The US device used was LOGIQ e Premium (GE Healthcare, Tokyo, Japan) equipped with a 12 L high-frequency (8–13 MHz) linear probe. HFP scanning was performed by the first author, who has more than 10 years of experience in ultrasonography in clinical practice. The HFP was imaged under non-load, sitting, 50%, and 80% load conditions using a long-axis scan with the highest intra- and inter-examiner reliability¹¹⁾ (Fig. 2). HFP imaging under non-load conditions was performed with the participants seated. A 10-mm plastic lamina was placed on the PMP, with the forefoot positioned on top and the ankle joint set at an intermediate angle. The hindfoot was in a non-load state, and the space between the hindfoot and the PMP was filled with gel. This configuration constituted the measurement position under the non-load condition (Fig. 2b). Scanning was performed from the sole through the PMP, with the probe held manually (Fig. 2f). The most convex part of the calcaneal tuberosity was identified, and the ultrasonogram of the HFP was captured at this location. The probe was then separated from the PMP, and a second image was obtained. The measurement position under the sitting condition was defined as the ankle in an intermediate position, the knee joint at 90°, the hip joint at 90°, both hands on the anterior superior iliac spine, and the back straight (Fig. 2c). At that time, the most convex part of the calcaneal tuberosity was positioned at the center of the cross-shaped openings, and the line passing through this point and the second metatarsal head was positioned at a 7° angle to the sagittal plane. Furthermore, the opposite foot was placed on the weighing platform with both feet open at 7° angle, matching the measurement side, and both feet were positioned shoulder-width apart. In this sitting position, the HFP on the measurement side was imaged, and the load on the opposite side was measured. For the measurement position under the 50% load condition, participants were asked to stand without moving their foot position from the seated condition, and 50% of their body weight was placed on the weighing platform to obtain images of the HFP. At that time, both hands were placed beside the greater trochanter (Fig. 2d). For the 80% load condition, 20% of the body weight was placed on the weighing platform to obtain HFP images at 80% load (Fig. 2e). The foot selected for the first measurement was randomized, and after measuring one side, the opposite side was subsequently measured. The same imaging was conducted after 1 month (28–33 days). We did not impose any restrictions on activity during this period. Procedures were performed as in our previous study¹¹⁾. In this experiment, we chose an 80% load instead of a full load because maintaining a steady single-leg stance is difficult. This decision was also made to avoid falls, especially because eight participants were older adults. Additionally, heel impact while walking is equivalent to 80% of the body weight¹⁴⁾.

All ultrasonograms obtained were measured the following day. Consistent with previous studies^{3, 15,16,17,18)}, all components of the HFP were defined as the entire layer (from the calcaneal tuberosity to the skin). It was then divided into the MAC layer (from the calcaneal tuberosity to the FS, including the macrochambers) and the MIC layer (from the microchambers to the skin) for measurement. Both the initial and follow-up measured values after 1 month were the means of two consecutive measurements. The intra-examiner reliability of repeated measurements ranged from 0.92 to 0.99.

Sample size calculations were performed using G*Power 3.1.9.7 (Heinrich-Heine-Universität, Düsseldorf, North Rhine-Westphalia, Germany). SPSS (version 29; IBM Japan Ltd., Tokyo, Japan) was used for other statistical analyses. The normality of the data was confirmed using the Shapiro–Wilk test, and parametric tests were subsequently employed for statistical analysis.

The mean thicknesses and standard deviations (SD) of the dominant and non-dominant foot lengths under the 50% load condition were calculated. Moreover, differences in foot length between the dominant and non-dominant feet were calculated. Subsequently, the minimal detectable changes based on 95% confidence intervals (MDC₉₅) were obtained. The MDC₉₅ value was calculated by multiplying the SD by 1.96. We calculated the mean thickness in each layer, AC, and C-index to investigate the morphological characteristics of each layer under non-load, sitting, 50%, and 80% load conditions for the dominant and non-dominant feet. Here, the AC was defined as the non-load thickness minus each load thickness, and the C-index was the load thickness divided by the non-load thickness. Furthermore, for each of the dominant and non-dominant feet, we performed a one-way repeated-measures analysis of variance to evaluate the significant differences across all layers under non-load, seated, 50%, and 80% load conditions. The same procedures were also carried out for MAC and MIC layer. Subsequently, a Tukey’s multiple comparison test was performed. The alpha level was set at 0.05. The priori power analysis determined that a sample size of 24 was required, given an effect size (f) of 0.25, an alpha error probability of 0.05, and a statistical power (1-β probability) of 0.8.

We calculated Pearson’s product-moment correlation coefficient for each layer under each load condition to compare the dominant and non-dominant feet. These results were classified according to the criteria outlined in a previous study¹⁸⁾: poor correlation=0.00–0.20, fair correlation=0.21–0.40, moderate correlation=0.41–0.60, good correlation=0.61–0.80, and excellent correlation=0.81–1.00. Paired t-tests were conducted to compare the sides. The alpha level was set at 0.05. For Pearson’s product-moment correlation coefficient, the priori power analysis determined that a sample size of 29 was required, given an effect size (r) of 0.5, an alpha error probability of 0.05, and a statistical power (1-β probability) of 0.8. In addition, under each load condition, the differences between the sides of each layer were calculated as the mean, SD, standard error of the mean (SEM), 95% confidence interval (CI), and MDC₉₅.

We conducted paired t-tests between the initial measured values and those taken after 1 month to investigate the changes in the HFP over a 1-month period for each dominant and non-dominant foot. The alpha level was set at 0.05. Moreover, we computed the Pearson’s product-moment correlation coefficient between the values measured initially and those measured after 1 month. The differences between the two sets of measured values were calculated as the mean, SD, SEM, 95% CIs, and MDC₉₅.

RESULTS

The dominant foot was on the right side in 34 participants and on the left side in 6 participants. The dominant foot length under 50% load was 24.4 ± 1.48 cm, and the non-dominant foot length was 24.5 ± 1.55 cm. The MDC₉₅ was 0.57 cm.

Table 2 presents the mean thickness, SD, AC, and C-index for every layer under each load condition for the dominant and non-dominant feet. It also includes Pearson’s product-moment correlation coefficient between the two sides and their mean differences, SD, SEM, 95% CI, and MDC₉₅. An example of the measurement results is shown in Fig. 3. The mean load value applied to the foot under the sitting condition was 5.1 ± 1.0 kg, corresponding to an 8.4% load condition. First, although not listed in Table 2, significant differences between each layer under each load condition were evaluated for the dominant and non-dominant feet. No significant differences were observed between the 50% and 80% load conditions in the MIC layer of the non-dominant foot; however, differences were observed under the other conditions. The AC and C-index of the entire layer from the non-load to the 80% load conditions were 8.7 and 0.49 (dominant feet) and 9.0 and 0.47 (non-dominant feet), respectively. The morphological change in the HFP mainly occurred in the MAC layer, with its AC and C-index being 7.8 and 0.40 (dominant feet) and 8.1 and 0.39 (non-dominant feet), respectively, whereas those of the MIC layer were 0.9 mm and 0.76, respectively. Moreover, the AC and C-index of the entire layer from the non-load to the 50% load conditions were 7.8 and 0.54 (dominant feet) and 7.9 and 0.54 (non-dominant feet), respectively.

Table 2. Morphological characteristics in each layer with load changes and comparison between the dominant and non-dominant feet.

	Dominant			Non-dominant			r	Mean difference
	n=40			n=40				n=40
	Mean ± SD	AC	C-index	Mean ± SD	AC	C-index		Mean ± SD	SEM	95% CI	MDC95
	(mm)	(mm)		(mm)	(mm)			(mm)	(mm)	(mm)	(mm)
Non-load
Entire	16.9 ± 2.3			17.0 ± 2.4			0.95***	−0.11 ± 0.77	0.12	−1.62 to 1.40	1.51
MAC	13.1 ± 2.0			13.2 ± 1.9			0.91***	−0.11 ± 0.87	0.14	−1.83 to 1.60	1.71
MIC	3.8 ± 0.6			3.8 ± 0.7			0.63***	0.02 ± 0.57	0.09	−1.11 to 1.14	1.12
Sitting (8.4% load)
Entire	11.5 ± 3.1	5.4	0.68	11.4 ± 3.0	5.6	0.67	0.89***	0.05 ± 1.46	0.23	−2.81 to 2.90	2.85
MAC	8.1 ± 2.8	5.0	0.62	8.0 ± 2.6	5.2	0.61	0.88***	0.06 ± 1.34	0.21	−2.55 to 2.68	2.62
MIC	3.4 ± 0.6	0.4	0.89	3.4 ± 0.7	0.4	0.89	0.81***	−0.02 ± 0.41	0.06	−0.82 to 0.78	0.80
50% load
Entire	9.1 ± 2.4	7.8	0.54	9.1 ± 2.2	7.9	0.54	0.92***	0.06 ± 0.94	0.15	−1.78 to 1.89	1.83
MAC	6.0 ± 2.0	7.1	0.46	6.0 ± 1.8	7.2	0.45	0.90***	0.02 ± 0.90	0.14	−1.74 to 1.77	1.76
MIC	3.1 ± 0.6	0.7	0.82	3.1 ± 0.7	0.7	0.82	0.87***	0.04 ± 0.32	0.05	−0.59 to 0.68	0.64
80% load
Entire	8.2 ± 1.7	8.7	0.49	8.0 ± 1.7	9.0	0.47	0.94***	0.17 ± 0.62	0.11	−1.04 to 1.38	1.21
MAC	5.3 ± 1.3	7.8	0.40	5.1 ± 1.3	8.1	0.39	0.93***	0.19 ± 0.53	0.09	−0.84 to 1.22	1.03
MIC	2.9 ± 0.5	0.9	0.76	2.9 ± 0.5	0.9	0.76	0.87***	−0.02 ± 0.28	0.05	−0.56 to 0.52	0.54

Entire: entire layer; MAC: macrochamber layer; MIC: microchamber layer; SD: standard deviation; AC: amount of change; C-index: compressibility index; r: Pearson’s product-moment correlation coefficient; ***p<0.001; SEM: standard error of the mean; CI: confidence interval; MDC₉₅: minimal detectable change based on a 95% confidence interval.

Fig. 3.

Conditions for each layer under various loads and measurement sections.

a: Non-load condition b: Sitting condition c: 50% load condition d: 80% load condition. The fibrous septum is indicated by white triangles. The MAC layer includes the plantar fascia, macrochambers, and fibrous septum. The MIC layer includes the microchambers and the skin. Entire: entire layer; PMP: 5-mm polymethylpentene resin plate; MIC: microchamber layer; MAC: macrochamber layer; FS: fibrous septum.

The Pearson’s product-moment correlation coefficient between the dominant and non-dominant feet indicated that the MIC layer under the non-load condition exhibited a “good” correlation, whereas that under the other conditions demonstrated an “excellent” correlation. There were no significant differences between the sides. The MDC₉₅ values between the two sides under the non-load condition ranged from 1.12 mm to 1.71 mm. Under load conditions, from sitting to 80% load, the MDC₉₅ values ranged from 0.54 mm to 2.85 mm. In each layer, MDC₉₅ decreased with increased load.

Table 3 summarizes the Pearson’s product-moment correlation coefficient between the initial measurement and that after 1 month and the mean differences, SD, SEM, 95% CI, and MDC₉₅ for the dominant and non-dominant feet. One month (28–33 days) later, the participants comprised 32 adults (16 males and 16 females, mean age 39.6 ± 10.8 years, mean height 165.7 ± 9.2 cm, mean body mass 61.3 ± 11.8 kg, mean body mass index 22.2 ± 2.7) with 64 feet (mean length 24.6 ± 1.6 cm). Although not listed in Table 3, no significant differences were observed between the initial measured values and those obtained after 1 month, regardless of the load condition, layer, or whether the foot was dominant or non-dominant. Pearson’s correlation coefficients showed good to excellent correlations for all conditions. The MDC₉₅ values under the non-load condition ranged from 0.68 mm to 1.25 mm, regardless of the layer or whether the foot was dominant or non-dominant. Under load conditions, the MDC₉₅ values ranged from 0.40 mm to 3.12 mm. In each layer, the MDC₉₅ tended to decrease with increased load.

Table 3. Comparison between the initial measurements and measurements after 1 month.

	Dominant					Non-dominant
	n=32					n=32
	r	Mean ± SD	SEM	95% CI	MDC95	r	Mean ± SD	SEM	95% CI	MDC95
		(mm)	(mm)	(mm)	(mm)		(mm)	(mm)	(mm)	(mm)
Non-load
Entire	0.95***	−0.06 ± 0.64	0.11	−1.30 to 1.19	1.25	0.96***	−0.01 ± 0.55	0.10	−1.09 to 1.06	1.08
MAC	0.93***	−0.07 ± 0.62	0.11	−1.27 to 1.14	1.21	0.96***	−0.11 ± 0.50	0.09	−1.10 to 0.87	0.98
MIC	0.81***	0.01 ± 0.35	0.06	−0.67 to 0.70	0.68	0.85***	0.10 ± 0.34	0.06	−0.58 to 0.77	0.68
Sitting (8.4% load)
Entire	0.83***	−0.12 ± 1.59	0.28	−3.00 to 3.24	3.12	0.88***	−0.11 ± 1.31	0.23	−2.69 to 2.46	2.58
MAC	0.83***	0.14 ± 1.38	0.24	−2.57 to 2.84	2.70	0.88***	−0.12 ± 1.18	0.21	−2.44 to 2.20	2.32
MIC	0.82***	−0.02 ± 0.37	0.06	−0.73 to 0.70	0.72	0.74***	0.01 ± 0.45	0.08	−0.87 to 0.89	0.88
50% load
Entire	0.90***	−0.20 ± 0.84	0.15	−1.84 to 1.45	1.64	0.90***	0.13 ± 0.76	0.13	−1.36 to 1.63	1.50
MAC	0.87***	−0.23 ± 0.78	0.14	−1.77 to 1.31	1.54	0.88***	0.13 ± 0.67	0.12	−1.18 to 1.44	1.31
MIC	0.93***	0.03 ± 0.21	0.04	−0.37 to 0.44	0.40	0.89***	0.00 ± 0.28	0.05	−0.54 to 0.54	0.54
80% load
Entire	0.96***	0.01 ± 0.47	0.08	−0.91 to 0.93	0.92	0.91***	0.03 ± 0.70	0.12	−1.34 to 1.41	1.37
MAC	0.96***	0.05 ± 0.43	0.08	−0.80 to 0.89	0.84	0.92***	0.03 ± 0.56	0.10	−1.06 to 1.13	1.10
MIC	0.78***	−0.03 ± 0.34	0.06	−0.69 to 0.63	0.66	0.85***	0.00 ± 0.30	0.05	−0.58 to 0.58	0.58

Entire: entire layer; MAC: macrochamber layer; MIC: microchamber layer; r: Pearson’s product-moment correlation coefficient; ***p<0.001; SD: standard deviation; SEM: standard error of the mean; CI: confidence interval; MDC₉₅: minimal detectable change based on 95% confidence interval.

DISCUSSION

This study investigated the morphological characteristics of the HFP, dividing it into the MAC and MIC layers under varying load conditions. The experiments were designed to (1) investigate the morphological changes in the HFP by obtaining the mean thickness, AC, and C-index under non-load, sitting, 50% load, and 80% load conditions for the dominant and non-dominant feet of healthy participants; (2) compare the two sides; and (3) investigate the change over a 1-month period. Changes in the HFP mainly occurred in the MAC layer, with slight changes observed in the MIC layer. Moreover, for every layer under each load condition, no significant differences were observed between the sides or between the initial measurements and those after 1 month.

First, to verify the validity of the mean thickness of each layer and the C-index obtained from this experiment, we compared the thickness and C-index of the entire layer under non-load and 50% load conditions with those reported in previous studies^{1, 19,20,21)}. The studies targeted healthy participants and reported the mean thickness of the entire layer to be 18 mm (range: 12–22 mm) and 14.6–19.5 mm under the non-load condition, with results similar to our findings. Prichasuk et al.^{20, 21)} reported a C-index of 0.52 and 0.53 from non-load to 50% load condition, respectively. Their study participants included patients’ relatives, hospital staff, and medical students, and their participant characteristics were similar to ours. Moreover, the C-index (0.54) from non-load to the 50% load condition of the entire layer of both sides (Table 2) was similar to those reported in these studies. Therefore, our findings, which include the mean thicknesses, AC, and C-index for every layer under each load condition, may represent the normal condition of the HFP in Japanese adults.

The primary function of the HFP is to absorb shock during walking and running. Hsu et al.³⁾ reported that macrochambers mitigate the impact via deformation, whereas microchambers limit the immoderate deformation of the macrochambers, functioning as an inherent heel cup. Their analysis was conducted on the basis of experiments under partial weight-bearing conditions of 196 N applied to the heel. In this study, because the change in each layer of the HFP from the non-load to 80% load conditions mainly occurred in the MAC layer, with only slight changes observed in the MIC layer, the results supported their reports, along with actual verification under high load conditions.

The dominant foot is used to perform skilled tasks that require specific neuromuscular control, such as kicking a ball, whereas the non-dominant foot provides the stability required for the dominant foot to perform these actions^{13, 22,23,24)}. If the usage of the dominant and non-dominant feet differs, morphological differences may occur. Additionally, left-right differences may occur in foot morphology, such as foot length and width²⁵⁾. Some participants had a difference in foot length between their dominant and non-dominant feet, as the MDC₉₅ between the two sides was 0.57 cm. Hence, we investigated the differences in the HFP between dominant and non-dominant feet. No significant differences were observed in the thicknesses of each layer between the two sides under any loading condition. The validity of these results needs to be verified. Uzel et al.²⁶⁾ compared the left and right entire layer thicknesses of healthy participants under non-load and 50% load conditions and reported no significant differences between them. Approximately 90% of people are reported to be right-handed²⁷⁾; for many individuals, their dominant hand and foot coincide^28,29,30,31). Therefore, the results of the left-right investigation may approximate those of the dominant and non-dominant feet. This study revealed no significant difference between the dominant and non-dominant foot in the thickness of every layer under each load condition; however, a few millimeter differences existed (Table 2). These conditions can be considered the normal conditions between the dominant and non-dominant foot of the HFP.

Monthly variations in the HFP are not well understood. Moreover, it is unclear which conditions are considered normal. Therefore, we attempted to clarify these changes during a 1-month period. This study revealed no significant difference between the initial and 1-month measurements in the thickness of every layer under each load condition; however, the mean difference and MDC₉₅ results indicated that differences of a few millimeters may occur between them (Table 3). These conditions may be considered the normal conditions during 1 month of the HFP.

Our findings can potentially be used in clinical settings. The mean thickness, AC, and C-index in each layer of the dominant and non-dominant sides under varying load conditions may serve as indices in healthy Japanese adults because these were comprehensive values obtained from participants of wide age ranges. Moreover, these conditions may be used to compare with patients with PHP, such as those with plantar fasciitis or calcaneal fractures. However, we reported that the mean thickness in each layer under the non-load condition was different by age and sex¹⁸⁾. The changes in each layer under varying load conditions should be investigated by age and sex to understand the HFP further.

The MDC₉₅ values under non-load conditions between the dominant and non-dominant feet and between the initial measurements and those after 1 month may be particularly useful. The non-load condition is a state without any intervention and can be measured without the evaluation instrument. If the difference in HFP thickness between the dominant and non-dominant foot or over a 1-month period exceeds the MDC₉₅ values for each layer, it may serve as an index to suspect a potential abnormality. In the case that evaluation under load conditions is possible, the MDC₉₅ value could be a useful index alongside the C-index. Furthermore, values obtained under non-load and load conditions might serve as a reference for periodic and longitudinal examinations. Additionally, the morphological changes over 1 month in patients with PHP are not well understood, which could serve as a potential research question. The results of this study may serve as a basis for comparison in future studies.

This study has some limitations. The results of different races and minors at the developmental stage are unknown and need to be investigated in the future. Moreover, the load value on the heel could not be measured under the load conditions. Improvements to the evaluation apparatus are necessary, such as developing a device to fix the probe and using a load cell to quantify the load on the heel in future studies. Furthermore, the effects of the other US devices remain unknown. If a different US device is used, we recommend assessing its reproducibility against that used in previous studies^{10, 11)}.

In conclusion, the change in the HFP mainly occurred in the MAC layer, with slight changes observed in the MIC layer. The thickness of the HFP for every layer under each load condition, the AC, and the C-index may represent normal conditions in healthy Japanese adults. Moreover, there was no significant difference between the dominant and non-dominant feet or between the initial measurements and those after 1 month. However, a difference of only a few millimeters may have existed. This minimal difference may also represent a normal HFP condition in healthy Japanese adults.

Funding and Conflicts of interest

None.