Quantifying mechanical and morphological properties of plantar foot soft tissues: a systematic review of techniques, methods and their clinimetric properties

Abstract

Background

Plantar foot soft tissues play a crucial role in absorbing and distributing mechanical stresses during weight-bearing activities. Accurate quantification of their mechanical and morphological properties is essential for understanding load distribution, tissue integrity, and managing conditions like diabetic foot disease. However, the best technique to investigate these properties remains unknown. This systematic review evaluates the clinimetric attributes (i.e. reliability, validity, and repeatability) of existing techniques to identify those most suitable for research and clinical practice.

Methods

Following PRISMA guidelines, we systematically searched MEDLINE, EMBASE and Web of Science for studies evaluating the mechanical or morphological properties of plantar foot soft tissues. Studies reporting reliability, validity, and/or repeatability of used techniques were evaluated and clinimetric outcomes were interpreted using published cut-off values. Finally, the methodological quality of studies was assessed using the COSMIN Risk of Bias tool.

Results

Of 4115 screened studies, 37 were included. Nine techniques to assess mechanical properties (e.g. elastography- and indentation-based techniques, durometer, optical coherence tomography) and six to assess morphological properties (e.g. CT, MRI, ultrasound) were identified. Reliability (29/37 studies) and repeatability (23/37) were frequently reported, while validity assessments were limited (4/37) likely due to the methodological complexity of in-vivo validity assessment, including the absence of an established reference standard for plantar soft tissue mechanics. Inconsistent statistical approaches (26/37) and doubtful blinding (26/37) were the most reported methodological limitations across studies. Only Shear Wave Elastography (SWE) and ultrasound had reliability and validity comprehensively investigated. Ultrasound demonstrated good-to-excellent reliability (ICC = 0.70–0.99) and moderate-to-strong validity (r = 0.5–1.0) using MRI and radiography as reference for measurement of morphological properties. Despite excellent intra-rater reliability (ICC > 0.90) and strong validity (R² = 0.91) to assess mechanical properties, SWE exhibited systematic measurement bias, likely related to tissue anisotropy, probe pressure, and boundary condition effects. For both techniques, operator-dependent factors (i.e. probe positioning, pressure, inclination, and quantity of coupling gel applied) might influence measurement performance.

Conclusions

SWE and ultrasound are currently supported by the most consistent and comprehensive clinimetric evidence among available techniques and emerged as most suitable techniques to quantify the mechanical and morphological properties of plantar foot soft tissues. Nevertheless, heterogeneity in study design, test conditions, and analysis methods limits inter-study and inter-technique comparisons, and highlight the need for more research into clinimetric properties, and into development of more robust and/or valid techniques. Standardized measurement protocols (e.g. probe positioning and pressure, coupling medium, foot positioning) and statistical reporting of clinimetric properties would substantially improve comparability across future studies with potential for future integration into quantitative monitoring approaches for plantar tissue health.

Supplementary information

The online version contains supplementary material available at 10.1186/s42490-026-00106-x.

Introduction

The plantar soft tissues in the foot consist of the skin and the subcutaneous fat tissue [1]. The fat tissue is composed of adipocytes surrounded by fibrous tissue septa, that provide shock absorption, insulation, and protection [2], while septal walls anchor the fat pads to the skin and bones, ensuring structural stability [3]. The skin forms the outer protective barrier against mechanical damage and infection [4–6].

The plantar soft tissues exhibit complex viscoelastic mechanical properties, often summarized by stiffness or elasticity, which contributes to the absorption and distribution of mechanical loads during weight-bearing activities [7, 8]. Tissue thickness reflects a morphological state, where changes in thickness indicate reflect alterations in tissue integrity and function [9–11]. Investigating the plantar soft tissues mechanical and morphological properties is essential for understanding injury mechanisms and foot conditions such as diabetic foot disease [11, 12].

Diabetic foot disease comprises a series of complications [13], among which foot ulcers (i.e. chronic wounds through at least the skin layers) significantly contribute to morbidity and mortality [14] and precede up to 85% of non-traumatic lower limb amputations [15]. Ulcer development is complex and multifactorial, arising from the interaction of extrinsic biomechanical forces (i.e., load) and the intrinsic structural and functional properties of the plantar foot soft tissues (i.e., load-bearing capacity). Additionally, persistent hyperglycemia leads to a series of histochemical alterations of soft tissues, affecting the tissues’ mechanics [16, 17]. For instance, increased stiffness and reduced thickness impair tensile stress distribution, and compromise the foot’s ability to dissipate load effectively [18], increasing ulcer risk [14, 19]. Accurate quantification of the plantar foot soft tissues’ mechanical and morphological properties is therefore essential to understand the effects of diabetic foot disease on soft tissue integrity, identifying individuals at risk of ulceration and support the development of targeted prevention strategies [20].

Related works

A recent systematic review [21] on plantar soft tissue thickness and stiffness in individuals with diabetes reported significant heterogeneity of measurement techniques, which complicated inter-study comparisons and may have contributed to inconsistent findings. For example, more than half of the studies (55%, 11/20) reported non-significant differences in tissue thickness between participants with or without diabetes, 10% (2/20) found significantly reduced thickness and 5% (1/20) found significantly increased thickness. The remaining 30% (6/20) showed mixed results, found with plantar tissues being significantly thicker, thinner, or not significantly different depending on the anatomical site or subgroup examined. Similarly, 47% of the studies (9/19) reported increased tissue stiffness in people with diabetes compared to healthy controls, with 26% (5/19) reporting significant increases across all examined sites, while 21% (4/19) reporting increases in most sites. The remaining 26% (6/19) showed region-dependent variations. The causes for these inconsistencies may stem from study design, true disease effect, or, most importantly, the diversity of assessment techniques used, each with its inherently different clinimetric properties (i.e., reliability, validity and repeatability). Variations in device configuration, measurement conditions and operator dependence further limited comparability, potentially masking true physiological effects.

Methodological heterogeneity was also highlighted by a narrative review comparing ex-vivo and in-vivo methods to assess biomechanical properties of plantar soft tissues in people with diabetic foot disease [20]. Ex-vivo tests (e.g., uniaxial compression), provide controlled mechanical data to replicate the physiological loading and boundary conditions that influence tissue behavior in-vivo [22–30]. Accordingly, marked differences in both stiffness (150 kN/m in-vivo, 900–1100 kN/m in-vitro) and energy dissipation (95% in-vivo vs 31% in-vitro) between in-vivo and in-vitro conditions have been reported [29], and have been attributed to different factors influencing tissues’ viscoelastic behaviour, for example: tissue vascularization, change in boundary conditions, fluid interactions, temperature, and oxygenation [31, 32].

In-vivo measurements have been increasingly applied to investigate the mechanical properties of the foot under physiological conditions. Most of these methods rely on quasi-static testing, where external loads are applied to the foot using loading devices to observe tissue deformation and derive its stiffness. This approach typically involves indentation, elastography, or similar setups that measure the tissue’s response to gradually applied forces. Spherical indentation systems quantify viscoelastic properties through stress-relaxation testing, and revealed the time-dependent response and sensitivity to strain rate of the tissues [33–35]. Elastography-based techniques, including Shear Wave Elastography (SWE) [36–44] and Magnetic Resonance Elastography (MRE), have further advanced in-vivo assessment by generating spatial maps of tissue stiffness and viscosity. These methods detect subtle mechanical differences across tissue layers and between health and disease states, allowing visualization of the shear modulus distribution and its variation under load [37, 42, 45]. Finally, imaging-assisted (ultrasound or MRI) indentation systems further integrate structural and mechanical data to estimate of stiffness, strain, and energy dissipation in vivo [39, 46–51].

Dynamic in-vivo methods aim to better reproduce the loading patterns occurring during gait. Methods combining X-Ray Fluoroscopy with plantar pressure measurements enable estimation of tissue deformation and ground reaction forces during walking, under physiological loading [52–54]. More recent studies using Dual-Plane Fluoroscopy have reconstructed three-dimensional heel pad deformation and strain during walking, providing a more accurate representation of tissue mechanics [55, 56]. Nevertheless, these methods remain technically demanding, require complex synchronization of imaging and pressure data, and involve radiation exposure, which has limited their widespread application.

Aim

While existing reviews provide valuable overviews of existing methods and their advantages and disadvantages, they do not systematically evaluate their clinimetric attributes. Thus, uncertainty remains regarding which techniques are most robust and suitable for research and clinical use. A thorough evaluation of the clinimetric attributes of the measurement techniques is essential to support research focused on understanding the role of soft tissue properties in the development, healing, and prevention of foot ulcers in diabetes. We therefore aimed to systematically review the clinimetric attributes (i.e., reliability, validity and repeatability) of techniques used to measure the mechanical and morphological properties of plantar foot soft tissues.

Methods

The conduct and reporting of this systematic review was in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA)-statement [57] and registered on Prospero (ID: CRD42024602247).

Eligibility criteria

Eligibility criteria were defined a priori using population, measurement techniques, comparators (when applicable), and clinimetric outcomes (PICO). In this context:

• Population (P): Studies involving adults aged 18 and older, with no specific restrictions concerning their medical or foot condition (e.g., healthy individuals, those with diabetes, arthropathies, or other foot-related conditions).

• Intervention (I): Measurement techniques or instruments used to assess the mechanical or morphological properties of plantar foot soft tissues.

• Comparator (C): When applicable, another measurement technique, protocol, or operator used for comparison, or different populations assessed with the same technique.

• Outcome (O): Clinimetric properties of the measurement techniques, including reliability, validity, measurement error, and repeatability.

We included papers assessing mechanical and morphological properties of soft tissues of the plantar foot tissues - structures comprising the layer of adipose tissue beneath the bony prominences of the foot together with the plantar skin [58] - that used a specific measurement technique, compared different measurement techniques, or compared different populations. No restriction was set for study design. During eligibility screening, studies that investigated the clinimetric properties of the measurement instrument(s) used were further selected for full-text screening. We excluded studies conducted in body parts other than plantar foot soft tissues, technical studies solely evaluating the physics or engineering aspects of measurement techniques, and in-vitro studies, since these do not accurately reflect the in-vivo characteristics of the tissue, as previously discussed [31, 32].

Information sources and search strategy

A comprehensive search was performed in the databases: MEDLINE (Ovid), Embase (Ovid) and Web of Science from inception up to April 12^th 2025, in collaboration with a medical information specialist (AM). The search included controlled terms and free text terms for synonyms of ‘plantar soft tissue’ combined with synonyms for mechanical and morphological parameters, and types of measurement techniques. The search was performed without restrictions for date or languages. The full search strategies can be found in Appendix A. Duplicate articles were excluded by an in-house made deduplication tool (DedupEndNote (version 1.0.1 20,240,114); http://dedupendnote.nl/).

The study selection process was documented using a PRISMA 2020 flow diagram [57], reported in Fig. 1.

Fig. 1.

PRISMA flowchart of the study selection

The Rayyan web application was utilized for assessing eligibility of the papers [59]. Two reviewers (A.V. and M.A.M.) blindly screened the title and abstract of 4115 papers for eligibility based on predefined criteria. Cohen’s kappa was calculated for agreement between reviewers (i.e., <0.40: poor, 0.40–0.75: fair to good; >0.75 excellent agreement) and disagreements during screening were resolved through discussion. A kappa score of 0.87 (98.97% agreement) for the first 385 papers indicated excellent agreement, allowing the remaining papers to be screened by one reviewer (A.V.). In screening full-text papers for eligibility, two reviewers (A.V. and M.A.M.) both assessed 20 papers. A kappa score of 0.755 (92.3% agreement) was achieved, and the remaining papers were screened by one reviewer (A.V.).

Data extraction

Data extraction focused on participant characteristics, study design, anatomical sites examined, and outcomes specifically related to tissue (i.e., mechanical and morphological) and clinimetric properties (reliability, validity, and repeatability). Data extraction was conducted independently by two reviewers (A.V. and M.A.M.), with disagreements resolved through consensus. Additional data were sought from study investigators when information was unclear or missing.

The primary focus was on evaluating the clinimetric properties of measurement techniques used to assess the plantar foot soft tissues mechanical (e.g., stiffness, elasticity) and morphological (i.e., thickness) properties. Reviewers examined whether reliability, validity, and/or repeatability were investigated for each technique. Eligibility screening and outcome classification were guided by COSMIN terminology. Studies were included if they explicitly investigated measurement properties and reported outcomes were mapped to reliability, validity, or repeatability, defined as follows:

• Reliability: The consistency of measurements between raters (inter-rater) or within the same rater over time (intra-rater) [60].

• Validity: This review focused on criterion validity, defined as agreement with a reference standard [61].

• Repeatability (or reproducibility): The stability of results over repeated measurements under identical conditions [62]. Often assessed using test-retest reliability.

No single clinimetric construct was prioritized over another in this review. All reported clinimetric properties were therefore evaluated and interpreted using published cut-off values as follows:

• ICC (Intra Class Correlation): Excellent (ICC > 0.90); Good (ICC 0.75-0.90); Moderate (ICC 0.50 – 0.75); Poor (ICC < 0.50) [60].

• r (Correlation coefficient): Perfect Negative Association (r = −1), Strong Negative Correlation (−1 < r≤-0.8), Moderate Negative Correlation (−0.8 < r≤-0.4), Weak Negative Correlation (−0.4 < r < 0), No Association (r = 0), Weak Positive Correlation (0 < r < 0.4), Moderate Positive Correlation (0.4≤r < 0.8), Strong Positive Correlation (0.8≤r < 1) and Perfect Positive Correlation (r = 1) [63].

• R² (Coefficient of determination): No Correlation (R² = 0) and Perfect Correlation (R² = 1) [64].

• Cut-off values for reliability reported with CV (Coefficient of Variation) and Std. Dev. (Standard Deviation) were reported in none of the studies.

Risk of bias assessment

Methodological quality was assessed independently by two reviewers (A.V. and M.A.M.) using the COSMIN (consensus-based standards for the selection of health status measurement instruments) Risk of Bias tool [65]. The tool comprised two parts: 1) seven elements that make up a comprehensive research question of the study, and 2) standards on design requirements and preferred statistical methods of studies on reliability and measurement error. Each standard has a legend that defines the ratings, but in general a standard on a design requirement is rated as:

• “Very Good” if evidence or convincing arguments were provided that the standard is met;

• “Adequate” when it is assumable, although not explicitly described, that the standard is met;

• “Doubtful” when it is unclear that the standard is met;

• “Inadequate” when there is evidence that the standard is not met.

To rate the quality of the studies, the COSMIN Risk of Bias relies on the worst-score-counts method, meaning the lowest rating in a box determined the overall study quality score. Agreement in Risk of Bias between the two reviewers was quantified using the Cohen’s kappa coefficient, while disagreements were discussed and resolved when a consensus was reached. In the case that one or both of the reviewers were involved in the study being assessed, another independent assessor (S.A.B.) scored the Risk of Bias.

Results

Study selection

The flowchart for study selection is shown in Fig. 1. Out of 4115 screened papers, 134 evaluated the mechanical and/or morphological properties of plantar soft tissues. Among these, 37 investigated the clinimetric properties of the measurement techniques, with 15 investigating the mechanical properties, 16 the morphological properties, and 6 examining both, evaluating various techniques and their performance characteristics (Tables 1 and 2). Due to substantial heterogeneity in study designs, populations, and measurement outcomes, a quantitative synthesis (meta-analysis) was not feasible. Semi-quantitative approaches such as vote counting by direction or hierarchical scoring models were also deemed inappropriate, as many included studies were not primarily designed as clinimetric investigations and reported outcomes were not directly comparable. Therefore, a narrative synthesis was performed. For techniques investigating the mechanical properties, reliability was assessed in 16/21 studies, validity in 2/21 and repeatability in 15/21; for morphological properties, reliability was assessed in 18/22 studies, validity in 2/22 and repeatability in 11/22. The anatomical sites assessed most frequently were the heel (29/37 studies), metatarsal head (MTH) 1 (14/37), MTH-2 (7/37), MTH-3 (8/37), and MTH-5 (10/37).

Table 1.

Study overview

No	Technique	Author (Year)	Aim of the study	Population	Parameters evaluated	Anatomical sites examined	COSMIN Risk of Bias rating
							Reliability			Measurement Error
Mechanical Properties
1	Durometer	Cuaderes et al. (2009) [66]	Assess reliability and limitations of Durometer and PressureStat in diabetic feet.	80 participants with diabetes.	Stiffness as SH (0–100 scale).	Hallux, Third and fifth toes, MTH-1, MTH-3, MTH-5, Medial and lateral midfoot, Dorsal aspect between the hallux and second toe, Heel.
2	Durometer (Shore Meter)	Periyasamy et al. (2012) [67]	Investigate age-related changes in hardness.	13 young participants (26–45 years); 13 older participants (46–65 years).	Stiffness as SH.	Hallux, MTH-1, MTH-3, MTH-5, Hindfoot, Midfoot.
3	Durometer (Shore-00)	Tonna et al. (2024) [68]	Assess reliability and validity of SH.	20 healthy participants.	Stiffness as SH values, SW speed, Skin and bulk tissue thickness.	Hallux, MTH-1, MTH-3, MTH-5, Rearfoot, Midfoot.
4	Indentometer	Klaesner et al. (2001) [69]	Develop and test the reliability and accuracy of a portable soft tissue indentor.	5 healthy participants.	Tissue stiffness, effective Young’s modulus.	Heel pad.
5	Indentometer	Xiong et al. (2010) [70]	Assess foot tissue discomfort, pain thresholds, and mechanical properties with a custom-built device.	5 healthy participants.	Stiffness, F-D curve, PDT, PPT.	Heel pad, Foot center.
6	MRE	Cheung et al. (2006) [45]	Evaluate the heel fat pad stiffness and thickness using MRE.	16 participants: 12 healthy participants; 4 with diabetes and neuropathy.	Stiffness (Shear modulus).	Heel pad.
7	MRE	Weaver et al. (2005) [71]	Assess MRE feasibility for in vivo shear modulus quantification.	1 healthy participant.	Stiffness (Shear modulus).	Heel pad (Medial, Lateral).
8	RSWE	Naemi et al. (2022) [37]	Investigate plantar soft tissue stiffness using RSWE in diabetes and prediabetes.	11 participants with prediabetes; 40 adults with diabetes.	SW speed.	MTH-1, MTH-3, Heel.
9	RSWE	Romero et al. (2022) [36]	Assess RSWE feasibility for plantar soft tissue elasticity.	5 healthy participants.	SW speed.	MTH-1, MTH-3, Heel.
10	SWE	Chatzistergos et al. (2018) [40]	Assess feasibility and reliability of SWE for heel pad biomechanics.	5 healthy participants.	Stiffness (Shear modulus).	Heel pad.
11	SWE	Jiao et al. (2024) [72]	Examine the relationship between the elastic modulus of foot soft tissues and gait characteristics in individuals with flatfoot.	42 participants: 20 with flatfoot; 22 healthy controls.	Stiffness.	Heel pad.
12	SWE	Lin et al. (2015) [42]	To evaluate the reliability of SWE and the relationship between heel pad stiffness and plantar heel pain.	36 participants: 20 healthy (reliability tested in 12/20); 16 with plantar heel pain.	Stiffness.	Heel pad.
13	Strain elastography	Naemi et al. (2016) [38]	Assess differences in plantar soft tissue stiffness and thickness between ulcerated and non-ulcerated feet.	39 diabetic neuropathy patients: 10 ulcerated; 29 non-ulcerated.	Relative stiffness, Thickness.	Heel, MTH-1.
14	Tissue interrogation device	Wendland et al. (2018) [73]	Assess plantar skin stiffness and thickness under varying environmental conditions and over time.	8 healthy participants; 8 with diabetes.	Tangential stiffness, Epidermal and dermal thickness.	Hallux, MTH-1.
15	Ultrasound + indentometer	Chatzistergos et al. (2014) [39]	Investigate the heel pad mechanical properties and their clinical correlations.	52 participants: 35 with diabetes; 17 without diabetes.	Thickness, Stiffness, Energy Absorbed.	Heel pad.
16	Ultrasound + indentometer	Hsu et al. (2005) [46]	Investigate effects of aging on plantar soft tissue properties under different impact velocities.	19 participants: 9 young participants (19–35 years); 10 older participants (42–72 years).	EDR, Thickness.	MTH-1, MTH-2, MTH-3, MTH-4, MTH-5.
17	Ultrasound + load cell	Hsu et al. (2007) [3]	Assess mechanical and morphological differences between people with and without diabetes.	21 participants: 13 patients with type 2 diabetes mellitus; 8 age-matched healthy controls.	EDR, Thickness.	MTH-1, MTH-2, MTH-3, MTH-4, MTH-5.
18	Ultrasound + load cell	Ng et al. (2015) [74]	Assess reliability of ultrasound foot scanner and compare posture effects.	15 healthy participants.	Thickness, Stiffness.	Hallux, MTH-1, MTH-2, MTH-5, Heel.
19	Ultrasound + load cell	Tsai et al. (1999) [75]	Assess mechanical properties of painful vs painless heels.	20 patients with unilateral plantar heel pain.	Stiffness, EDR, Thickness.	Heel.
20	Ultrasound + load cell	Wang et al. (1999) [76]	Assess mechanical properties of reconstructed vs. uninjured heel pads.	7 patients with heel flap reconstruction: 6 from direct trauma; 1 diabetic ulceration.	Stiffness, Thickness, Energy Dissipation Ratio.	Heel.
21	CT	Weijers et al. (2005) [77]	Analyze relationships between plantar pressure, sole thickness, and energy absorption.	10 healthy participants.	Stiffness, Thickness.	MTH-1, MTH-5, Sesamoids.
22	DECT	Mens et al. (2023) [78]	Assess DECT reliability for fat pad changes and correlation with plantar pressure.	56 participants: 45 with diabetes; 11 age-matched controls.	Thickness.	Heel.
23	MRI	Bus et al. (2006) [79]	Evaluate reproducibility of foot structure measurements using MRI.	28 participants: 23 with diabetes; 5 healthy controls.	Thickness.	MTH-1, MTH-2, MTH-5, Proximal phalanx of hallux and fifth toe.
24	MRI	Oh et al. (2018) [10]	Assess diabetes-induced pedal soft tissue atrophy.	52 patients with diabetes and without peripheral arterial disease; 47 healthy participants without diabetes.	Thickness.	MTH-1, MTH-5.
25	Orthotic embedded ultrasound	Telfer et al. (2014) [80]	Assess the feasibility and repeatability of orthotic heel inserts with embedded ultrasound for measuring dynamic heel pad behavior in-shoe during gait. Evaluate measurement reproducibility and compare the effects of two inserts: a flat heel raise and a contoured heel cup.	16 healthy participants.	Stiffness, EDR and Thickness.	Heel.
26	SXCT	Commean et al. (2002) [81]	Assess reliability and validity of SXCT and F-Scan in diabetic feet.	16 participants: 8 with diabetes; 8 controls.	Thickness.	MTH-1, MTH-2, MTH-3, MTH-4, MTH-5, Midfoot, Heel.
27	Ultrasound	Johannsen et al. (2016) [82]	Evaluate ultrasound reliability in measuring the heel fat pad thickness.	17 patients with chronic heel pain.	Thickness.	Heel pad.
28	Ultrasound	Ko et al. (2009) [83]	Explore plantar pressure and soft tissue strain under varying heel heights.	21 healthy participants.	Thickness, Strain.	MTH-1, MTH-2, MTH-3, MTH-4, MTH-5.
29	Ultrasound	López-López et al. (2019) [9]	Assess sub calcaneal fat pad thickness in patients with and without heel pain.	375 participants: 185 heel pain; 190 controls.	Thickness.	Heel pad.
30	Ultrasound	Maemichi (2024) [84]	Investigate age-related changes in mechanical morphology of heel fat pad.	55 healthy participants: 21 of age ≤ 29 years; 16 of age 30–44 years; 18 of age ≥ 45 years.	Thickness.	Heel pad.
31	Ultrasound	Matsumoto et al. (2022) [85]	Evaluate ultrasound reliability to assess the heel fat pad under weight-bearing and non weight-bearing conditions.	40 healthy participants.	Thickness (whole, MAC, MIC layers).	Heel pad.
32	Ultrasound	Matsumoto et al. (2023) [86]	Evaluate ultrasound reliability of heel fat pad thickness measurements.	40 healthy participants.	Thickness (whole, MAC, MIC layers).	Heel pad (long/short axis).
33	Ultrasound	Morrison et al. (2021) [1]	Assess ultrasound reliability to assess plantar thickness and tissue characteristics.	22 participants: 13 without diabetes; 9 with diabetes.	Thickness, Tissue characteristics.	MTH-2, MTH-3, Heel, Lateral Sesamoid.
34	Ultrasound	Rome et al. (1998) [87]	Evaluate intra- and inter-operator reliability of heel pad thickness measurements using ultrasound.	10 healthy participants.	Thickness.	Heel.
35	Ultrasound	Sabir et al. (2005) [88]	Assess validity of sonography for diagnosing plantar fasciitis compared with MRI.	154 participants: 77 symptomatic patients with heel pain; 77 age- and sex-matched controls.	Thickness	Plantar fascia, Heel pad.
36	Ultrasound	Uzel et al. (2006) [89]	Assess ultrasound for heel pad compressibility compared to radiography.	42 patients with plantar heel pain syndrome.	Thickness, Compressibility Index.	Heel pad.
37	Ultrasound	Wang et al. (2003) [90]	Assess plantar soft tissue thickness and metatarsal alignment.	25 healthy participants.	Thickness.	MTH-1, MTH-5.
15	Ultrasound + indentometer	Chatzistergos et al. (2014) [39]	Investigate the heel pad mechanical properties and their clinical correlations.	52 participants: 35 with diabetes; 17 without diabetes.	Thickness, Stiffness, Energy Absorbed.	Heel pad.
17	Ultrasound + load cell	Hsu et al. (2007) [3]	Assess mechanical and morphological differences between people with and without diabetes.	21 participants: 13 patients with type 2 diabetes mellitus; 8 age-matched healthy controls.	EDR, Thickness.	MTH-1, MTH-2, MTH-3, MTH-4, MTH-5.
18	Ultrasound + load cell	Ng et al. (2015) [74]	Assess reliability of ultrasound foot scanner and compare posture effects.	15 healthy participants.	Thickness, Stiffness.	Hallux, MTH-1, MTH-2, MTH-5, Heel.
19	Ultrasound + load cell	Tsai et al. (1999) [75]	Assess mechanical properties of painful vs painless heels.	20 patients with unilateral plantar heel pain.	Stiffness, EDR, Thickness.	Heel.
14	Ultrasound (Tissue interrogation device)	Wendland et al. (2018) [73]	Assess plantar skin stiffness and thickness under varying environmental conditions and over time.	8 healthy participants; 8 with diabetes.	Tangential stiffness, Epidermal and dermal thickness.	Hallux, MTH-1.

Legend: DECT (Dual-Energy Computed Tomography), EDR (Energy Dissipation Ratio), F-D (Force-Deformation), MAC (Macrochambers), MIC (Microchambers), MRE (Magnetic Resonance Elastography), MRI (Magnetic Resonance Imaging), MTH (Metatarsal Head), PDT (Pressure Discomfort Threshold), PPT (Pressure Pain Threshold), RSWE (Reverberant Shear Wave Elastography), SH (Shore Hardness), SW (Shear Wave), SWE (Shear Wave Elastography), SXCT (Spiral X-Ray Computed Tomography)

Table 2.

Clinimetric properties report for the mechanical and morphological properties assessments

Method	Study	Outcome metrics	Reliability	Validity	Repeatability	Reported limitations of the instrument
Mechanical Properties
Durometer	Cuaderes [66]	Hardness (0–100).	Intra-Rater r: 0.62–0.94; Inter-Rater r: 0.25–0.90	Not assessed.	Not assessed.	Difficult at thin sites (e.g., toes, midfoot).
Durometer (Shore Meter)	Periyasamy [67]	Shore Hardness (dimensionless: 0–100).	Variance p > 0.05	Not assessed.	Variance p > 0.05	Lack of tissue thickness measurements.
Durometer (Shore-00)	Tonna [68]	Shore Hardness (dimensionless: 0–100).	Intra-rater ICC: 0.60–0.90 (varied by site); Inter-rater ICC: 0.29–0.80	Correlated with Shear Wave [91] speed (skin and microchamber layers) for rearfoot, 1st MTH, and 3rd MTH.	Bias: −2.1 to 4.2 LoA: Narrow for rearfoot (−2.1 to 5.5), wide for MTH-5 (−4.0 to 10.1).	Limited reliability and validity in hallux, MTH-5, and midfoot due to morphology.
Indentometer	Klaesner [69]	Stiffness (kPa).	Intra-Rater ICC: 0.99; Inter-Rater ICC: 0.99	Not assessed.	Not assessed.
Indentometer	Xiong [70]	F-D curves, stiffness (N/mm).	Inter-Session ICC ≥ 0.88 (within-day), ICC ≥ 0.91 (between-day).	Not assessed.	CV: <0.2% (displacement), <0.6% (force).	Limited to healthy participants.
MRE	Cheung [45]	Shear modulus (kPa).	Not Assessed.	Not Assessed.	CV: 5%–8%	Requires precise vibration control.
MRE	Weaver [71]	Shear modulus (kPa).	Not assessed.	Not assessed.	Std. Dev.: 3.6%	Inconsistent applied pressure during acquisition.
Orthotic embedded ultrasound	Telfer [80]	Stiffness (kPa), Energy Dissipation Ratio (%).	Inter-session ICC (0.94–0.95); Inter-rater ICC (0.89)	Not assessed.	Not assessed.	Gel displacement during walking reduces accuracy.
RSWE	Naemi [37]	Shear wave speed (m/s).	Not assessed.	Not assessed.	CV: 6.74%
RSWE	Romero [36]	Shear Wave Speed (m/s).	CV: 5.19–9.88%	Not assessed.	CV: 5.19–9.88%	Artifacts without gel pad.
SWE	Chatzistergos [40]	Shear modulus (kPa).	Not assessed.	Significant correlation with Finite Element results; systematic underestimation by SWE (64% ± 16%).	Not assessed	SWE results sensitive to compressive state; limited to superficial layer.
SWE	Jiao [72]	Elastic Modulus (kPa).	Intra-Rater ICC: 0.973	Not assessed.	Intra-Rater ICC: 0.973
SWE	Lin [42]	Stiffness (kPa).	Intra-Rater ICC: 0.93–0.96	Not assessed.	CV: 6.1–8.8%.	Presence of artifacts during measurements.
Strain elastography	Naemi [33]	Relative stiffness (dimensionless).	Not assessed.	Not assessed.	Percentage Error: Heel: 6.6%; MTH-1: 10.6%	Qualitative elastography.
Tissue interrogation device	Wendland [73]	Tangential stiffness (N/mm).	Inter-Session ICC: Hallux (0.31–0.83), MTH-1 (0.74–0.31–0.74) depending on timing intervals.	Not assessed.	Not assessed.	Decreased reliability over longer time intervals; dependent on participant positioning and repeated measures setup.
Ultrasound + indentometer	Chatzistergos [39]	Stiffness (N/mm), Energy Absorbed.	CV = 4% (stiffness), 8% (energy).	Not Assessed.	CV = 4% (stiffness), 8% (energy).	Manual loading introduces variability.
Ultrasound + load cell	Hsu [46]	Energy, Energy Dissipation Ratio (%).	Intra-Rater CoV: 1%–3% (E), 2.56%–8.45% [9].	Not assessed.	CoV: 1%–3% (E), 2.56%–8.45% [9]	Requires precise manual handling.
Ultrasound + load cell	Hsu [3]	Energy Dissipation Ratio (%).	Intra-Rater CV: 2.56%-8.45%	Not assessed.	CV: 2.56%–8.45%
Ultrasound + load cell	Ng [74]	Stiffness (kPa).	ICC > 0.9	Not assessed.	ICC > 0.9
Ultrasound + load cell	Tsai [75]	Energy Dissipation Ratio (%).	CV: 8%	Not assessed.	Not assessed.	Limited load range to prevent ultrasound damage.
Ultrasound + load cell	Wang [76]	Compressibility Index (CI) %, Elastic Modulus (kg/cm2), Energy Dissipation Ratio (%).	CV: 1%-3% for UHPT/Ep; 8% for EDR.	Not assessed.	CV: 1%-3% for UHPT/Ep, 8% EDR.	Limited to compression testing only.
Morphological Properties
CT	Weijers [77]	Sole thickness (mm).	Not assessed.	Not assessed.	Median difference −0.01 m
DECT	Mens [78]	Thickness.	Inter-Rater ICC: 0.938	Not assessed.	Not assessed.
MRI	Bus [79]	Thickness (mm).	Intra-Rater ICC: 0.978–0.991; Inter-Rater ICC: 0.976–0.979	Not assessed.	Bland-Altman LoA: ±0.5–0.8 mm
MRI	Oh [10]	Thickness (mm).	ICC > 0.8	Not assessed.	ICC > 0.8
Orthotic embedded ultrasound	Telfer [80]	Thickness (mm).	Inter-session ICC (0.94–0.95); Inter-rater ICC (0.89)	Not assessed.	Not assessed.	Gel displacement during walking reduces accuracy; Spatial resolution.
SXCT	Commean [45]	Thickness (mm).	Intra-Rater Bias: < 0.5 mm	Not assessed.	Not assessed.
Ultrasound	Johannsen [82]	Thickness (mm).	Without Pressure Intra-Rater ICC: 0.89–0.92: Inter-Rater ICC: 0.81–0.89; LoA%: 16.1–21.0%	Not assessed.	TE: 0.61–0.70 mm, LoA%: 12.1–13.7%	Results may vary due to systematic observer differences.
			With Pressure Intra-Rater ICC: 0.88–0.97; Inter-Rater ICC: 0.82–0.91		TE: 0.33–0.87 mm, LoA%: 9.7–23.4%; Time: 1 month.
Ultrasound	Ko [83]	Thickness (mm).	Intra-Rater ICC: Supine: 0.74–0.96; Standing: 0.49–0.75	Not assessed.	Not assessed	Decreased reliability in weight-bearing conditions.
Ultrasound	Lopez-Lopez [9]	Thickness (mm).	Intra-Rater ICC: 0.93–0.95	Not assessed.	Not assessed
Ultrasound	Matsumoto [85]	Thickness (mm).	Inter-Rater ICC: 0.840–0.999	Not assessed.	MDC95: 0.13–0.78 mm	Darker ultrasonograms may reduce microchambers visibility.
Ultrasound	Matsumoto [86]	Thickness (mm).	Inter-Rater ICC: 0.75–0.99; Intra-Rater ICC: 0.75–0.99	Not assessed.	MDC95: 0.25–1.31 mm.	Examiner proficiency variability.
Ultrasound	Maemichi [84]	Thickness (mm)	Intra-observer ICC > 0.90	Not assessed	Not assessed	Heel pad
Ultrasound	Morrison [1]	Thickness (mm).	Inter-Rater: ICC 0.75–0.93; Intra-Rater: ICC 0.92–0.98	Not assessed.	SEM < 10%.	Operator variability under compressed state.
		Echogenicity.	Inter-Rater ICC: Kappa 0.70–0.78; Intra-rater ICC: Kappa 0.80–0.90		Not assessed.	Poor inter-rater agreement in compressed state.
Ultrasound	Rome [87]	Thickness (mm).	CV: 3.2%, LoA: ±0.39 mm	Not assessed.	Measurement Error: 0.21 mm
Ultrasound	Sabir [88]	Thickness (mm).	Not assessed.	Sensitivity: 80.9%, Specificity: 85.7% (vs MRI in detecting fasciitis).	Not assessed.
Ultrasound	Uzel [89]	Thickness (mm).	Not assessed.	Correlation with radiography: r = 0.847 (unloaded); r = 0.880 (loaded).	Not assessed.	Operator dependence of ultrasonography.
Ultrasound	Wang [90]	Thickness (mm).	Inter-Rater ICC: MTH-1: 0.94–0.75, MTH-5: 0.81–0.30	Not assessed.	Not assessed.	Poor reproducibility for MT5 under load.
Ultrasound + indentometer	Chatzistergos [39]	Thickness (mm).	CV: 5%	Not assessed.	CV: 5%
Ultrasound + load cell	Hsu [3]	Thickness.	Not assessed.	Not assessed.	Intra-Rater CV: 1%–3%
Ultrasound + load cell	Ng [74]	Thickness (mm).	ICC > 0.9	Not assessed.	ICC > 0.9
Ultrasound + load cell	Tsai [75]	Fat pad thickness (mm).	CV: 1–3%	Not assessed.	Not assessed.	Limited load range to prevent ultrasound damage.
Ultrasound (Tissue interrogation device)	Wendland [73]	Epidermal and dermal thickness (mm).	Inter-Rater ICC: Hallux (Epidermal: 0.94–0.79; Dermal: 0.87–0.48); MTH-1 (Epidermal: 0.93–0.79; Dermal: 0.63–0.18).	Not assessed.	Not assessed.	Variation in reliability across time periods, particularly for dermal thickness.

Legend: ICC (Intra Class Correlation), CV (Coefficient of Variation), LoA (Limits of Agreement), MDC95 (Minimal detectable change with 95% confidence), r (Correlation coefficient), R² (Coefficient of determination), SEM (Standard Error of Measurement), Std. Dev (Standard Deviation)

Risk of bias

The methodological quality of the included studies is reported in Table 3. For studies assessing reliability, 2 papers were evaluated as “Very Good”, 1 was “Adequate”, 16 were “Doubtful”, 15 were “Inadequate” and 2 were not rated. For measurement error, 2 studies were evaluated as “Very Good”, 2 were “Adequate”, 14 were “Doubtful”, 6 were “Inadequate” and 12 were not rated. The standards of the COSMIN checklist “1. Stability of the patient”, “2. Time interval“, and “3. Similar measurement conditions“were rated as “Very Good” in the majority of the studies (23/37, 26/37 and 32/37, respectively). The most reported methodological limitations were the standards “4. Administration of measurements“, and “5. Assignment of the score or determination of the biological value” that were rated as “Doubtful” in 27/37 and 26/37 studies, respectively). For reliability, the standard “7. Preferred statistical methods for continuous scores“was rated as “Inadequate” in 12/37 studies, as these studies did not calculate ICC values.

Table 3.

Risk of Bias assessment using COSMIN risk of Bias tool to assess the quality of a study on reliability or measurement error

Mechanical properties

Nine techniques were used to evaluate the mechanical properties of plantar foot soft tissues (Table 2).

Durometer

Intra-rater ICC values ranged from 0.60 to 0.90, while inter-rater ICC values ranged from 0.29 to 0.80, depending on the anatomical site [66, 68]. Validity assessment reported weak to moderate positive correlations (r = 0.24–0.64) with shear wave speed [68].

Indentometer

Indentometer demonstrated inter- and intra-rater ICC values of ≥0.99 [69, 70]. Repeatability on the heel was strong, with CV of <0.2% for displacement and <0.6% for force [70].

MRE

Repeatability for MRE ranged from 5% to 8% CV, with a reported standard deviation of 3.6% [45, 71].

Reverberant shear wave elastography (RSWE)

RSWE demonstrated repeatability ranging from 5.19% to 9.88% CV [36, 37], with inter-rater ICC values of 0.89 [36].

SWE

SWE achieved intra-rater ICC values between 0.93 and 0.97 [42, 72] while CV values ranged from 6.1% to 8.8% [42] for repeatability. Criterion validity was assessed in one study, showing a strong positive correlation with finite element modeling (R² = 0.91) [40]. However, SWE systematically underestimated the shear modulus by 64%±16%, indicating potential measurement bias [40].

Strain elastography

Reliability and validity data for strain elastography were not assessed. Repeatability for stiffness measurements was reported as 6.6%±4.4% for the heel, and 10.6%±7.3% for MTH-1 [38].

Tissue interrogation device

The device showed inter-session ICC values that varied depending on anatomical site and time interval (ICC: 0.31–0.74) [73].

Ultrasound coupled with indentometer

This combined technique measured stiffness and energy absorption, achieving a CV of 4% for stiffness and 8% for energy absorbed [39].

Ultrasound coupled with load cell

Intra-rater ICC values exceeded 0.90 in one study on the hallux, MTH-1, MTH-2, MTH-5 and the heel [74]. Repeatability ranged from 1% to 8% CV for MTH-2 in two studies [3, 75].

Overall, compression-based methods (durometer, indentometer, load cell, and tissue interrogation device) demonstrated high intra-rater reliability but inconsistent in inter-rater performance across anatomical sites, particularly in the forefoot. Elastography-based techniques (MRE, RSWE and SWE), provided quantitative stiffness estimates in standardized units (kPa), were constrained by systematic bias or limited validation. Together, the differences in measurement units and reporting formats further complicated direct comparisons between techniques.

Morphological properties

Six techniques were used to quantify the morphological properties.

Computed Tomography (CT)

Repeatability of CT in measuring the sole thickness on the MTH-1, MTH-5 and sesamoids, reported a median difference of −0.01 mm [77] within measurements.

Dual-Energy Computed Tomography (DECT)

DECT demonstrated inter-rater ICC values of 0.938 on the heel [78].

MRI

MRI achieved inter- and intra-rater ICC values ranging from 0.976 to 0.991 [79]. Repeatability was supported by Bland-Altman Limits of Agreement (LoA) ±0.5 mm and ±0.8 mm [79]. MRI was used as a reference to assess validity of ultrasound in measuring the heel pad and plantar fascia thickness in people with plantar fasciitis [88].

Orthotic embedded ultrasound

Intra-rater ICC values ranged from 0.94 to 0.95, while inter-rater ICC values were moderate at 0.89 during heel assessments [80].

Spiral X-Ray Computed Tomography (SXCT)

SXCT demonstrated intra-rater reliability with a bias of <0.5 mm when measuring MTH-1, MTH-2, MTH-3, MTH-4, MTH-5, midfoot and the heel [81].

Ultrasound

Ultrasound was the most used technique [84] for measuring soft tissue thickness, with intra- and inter-rater reliability ranging from moderate to excellent (ICC: 0.75–0.99) across studies on the heel [82, 84–86]. Repeatability was generally high, with CV values between 1 and 3% [75] and SEM values below 10% [1]. However, operator dependency affected measurement consistency, particularly under weight-bearing conditions, where ICC values decreased across the five metatarsal heads [83]. In supine position, intra-rater ICC values ranged from 0.74 to 0.96, whereas in a standing position, they dropped to 0.49–0.75. Similarly, under load, inter-rater ICCs were lower than intra-rater ICCs for both thickness (inter-rater: 0.75–0.93; intra-rater: 0.92–0.98) and echogenicity (inter-rater Kappa: 0.70–0.78; intra-rater Kappa: 0.80–0.90) Variable repeatability under loading (ICC: 0.30–0.81) was also found for MTH-5 under load in another study [90]. Validity assessments supported moderate to strong correlations (r = 0.5–1.0) between ultrasound and MRI [88] and Radiography [89].

Overall morphological imaging methods consistently achieved excellent reliability (ICC > 0.90), with Ultrasound was the most frequently evaluated but also the most operator-dependent technique, particularly under loading conditions [90]. MRI, by contrast, showed high intra- and inter-rater reliability with small limits of agreement across studies and served as a reference method to assess criterion validity of ultrasound.

Discussion

Summary of the findings

This study aimed to evaluate the clinimetric attributes (i.e., reliability, validity, and repeatability) of techniques used to quantify the mechanical and morphological properties of plantar foot soft tissues, and to identify the most accurate and reliable methods for these assessments. Across the techniques used to assess the mechanical properties (9 techniques) and morphological properties (6 techniques) of plantar foot soft tissues, reliability was the most frequently investigated clinimetric property, whereas evidence for validity was limited. Only SWE (for mechanical properties) and ultrasound (for morphological properties) had both reliability and validity comprehensively investigated, displaying good-to-excellent clinimetric properties. SWE demonstrated excellent intra-rater reliability (ICC > 0.90) [42, 72] and strong validity (R² = 0.91) when compared with finite-element modeling, although it underestimated the shear modulus (≈64% ± 16%), indicating potential measurement bias [40]. Ultrasound showed good-to-excellent reliability (ICC: 0.70–0.99) and moderate-to-strong criterion validity (r = 0.5–1.0) relative to MRI (r = 0.5–1.0) [88] and radiography (r = 0.847–0.880) [89].

Criterion validity was evaluated in only four of the 37 included studies, limiting confidence in conclusions and precluding strong evidence statements beyond a small subset of methods. This scarcity likely reflects the methodological complexity of in-vivo validity assessment, including the lack of an established reference standard for plantar soft tissue mechanics and the challenges of reproducing controlled boundary and loading conditions. Consequently, the primary aim of identifying the most accurate and reliable method for the quantification of plantar soft tissues’ mechanical and morphological properties was only partially achieved, and conclusions should be interpreted as conditional and technique-specific.

These findings complement prior narrative reviews that described available methods and their theoretical advantages and limitations, including the review by Yang and colleagues focusing on diabetic foot disease and comparisons between in-vivo and ex-vivo approaches [20]. The present review extends prior work by applying a COSMIN-based evaluation of clinimetric outcomes and methodological quality, shifting the focus from describing techniques to assessing the robustness of the evidence supporting their use.

Methodological quality and risk of Bias

Across the 37 included studies, only two studies achieved a “Very Good” rating for reliability, while the majority were rated as Doubtful (17/37) or Inadequate (15/37). Measurement error assessment was particularly limited, as just two studies were rated “Very Good” and two “Adequate”. The most prevalent methodological shortcomings were lack of blinding (27/37 studies) and unclear score assignment procedures (26/37 studies), both of which introduce potential biases. The high proportion of studies rated “Doubtful” or “Inadequate” indicate gaps in methodological rigor rather than excessive stringency of the COSMIN framework. Furthermore, the heterogeneity in study designs, measurement protocols, and statistical models precluded quantitative pooling and meta-analysis, limiting direct comparisons across techniques. Future studies should adopt more methodological transparency, consistent use of appropriate reliability statistical models (i.e., ICC), and implementation of blinding procedures where feasible.

Mechanical properties

The included techniques assessed the mechanical properties either by compression (i.e., Durometer, Load Cell, Indentometer and Tissue Interrogation Device) or by shear waves propagation (i.e., SWE, MRE), with Strain Elastography, providing qualitative stiffness contrasts.

Among compression-based techniques, indentometer [69, 70] and Load Cell [3, 75] demonstrated high intra-rater reliability (ICC ≥ 0.90) and repeatability (ICC ≥ 0.99, CV 1–8%), while Durometer [66, 68] and Tissue Interrogation Device [73] showed variable reliability across anatomical sites [66, 68] and measurement intervals [73]. Durometer performed poorly in thinner regions like the hallux and midfoot (ICC: 0.60–0.90 intra-rater, 0.29–0.80 inter-rater) and lacked strong correlation with SWE in some locations [68]. These compression-based techniques measure tissue gross deformation of the composite fat pad/skin structure without differentiating region-specific mechanical properties of the different layers, crucial for conditions like diabetic foot disease, where soft tissue atrophy and distal migration are common in the forefoot [95, 96]. Because measurements are typically acquired at a single time point, these methods also fail to account for the time-dependent viscoelastic behaviour of the soft tissues, such as stress-relaxation and creep [97–99]. These properties reflect how the tissue gradually redistributes internal stresses and fluid under sustained load [100], and may explain the mechanical alterations of the tissues characteristics in presence of diseases like fibrosis or diabetes [101].

Techniques based on shear wave propagation provide a more advanced assessment of soft tissues, as they can quantify and map tissues viscoelasticity by coupling the propagation of shear waves with the anatomical reference provided by medical imaging (either ultrasound or MRI). Elastography quantifies tissue stiffness by observing how mechanical waves or external compression cause deformation within soft tissues [102]. Because different tissues have distinct elastic moduli [103], their deformation under compression varies [104]. In ultrasound elastography, real-time changes in echo-signal displacement before and after compression are converted into color maps: tissues with lower elastic moduli deform more and appear red, whereas stiffer tissues deform less and appear blue [105]. In shear-wave elastography, the propagation speed and damping of shear waves in soft tissue increase with harmonic driving frequency, reflecting the dispersion of elastic waves in viscous media [68]. RSWE uses an external vibration source to generate a reverberant field of shear waves that propagate in multiple directions and interact with tissue boundaries. By analyzing this diffuse interference field, RSWE can estimate tissue stiffness without requiring prior knowledge of the main propagation direction [106, 107]. Compared to SWE, this approach may better handle complex boundary conditions and heterogeneous tissues, offering improved resolution in softer materials, although traditional SWE tends to provide higher signal-to-noise ratios in stiffer regions. RSWE demonstrated strong inter-rater reliability [36] and repeatability [36, 37], while SWE exhibited strong intra-rater reliability [42, 72] and correlated well with finite element modeling (R² = 0.91) [40], but systematically underestimated shear modulus values by approximately 64%±16%, highlighting the need for cautious interpretation. These techniques rely on ultrasound for anatomical reference and share the same operator-dependent limitations, including probe pressure, transducer angle and user experience. were identified as sources of variability [43, 108–110]. MRE emerged as a promising alternative: pilot studies reported good repeatability (CV: 5%-8%) [45, 71] without probe-load effects and operator dependency, offering quantitative mapping of the mechanical properties [111]. MRE applies the same physical principle as SWE but uses magnetic resonance imaging to visualize low-amplitude harmonic waves (10–1000 Hz, typically 30–80 Hz in vivo) generated on the body surface and imaged in three dimensions [112, 113]. The resulting wave motion is related to local tissue properties through linear viscoelastic wave equations [113]. MRE has been successfully applied to various organs and body parts [45, 71, 93, 112, 114, 115], demonstrating its ability to differentiate healthy and diseased tissues [45, 114, 115] and to characterize tissue mechanical anisotropy in phantoms and bovine muscles [116]. However, current evidence is limited to two studies and require further validation across plantar regions and loading conditions [45, 93, 117, 118]. The repeatability of Strain Elastography varied across anatomical sites, as the Percentage Error was 6.6% for the Heel but 10.6% for MTH-1 [38]. Reliability and validity were not assessed.

The heterogeneity in study methodologies, measurement techniques, test conditions, and anatomical locations complicates direct comparisons between the different techniques. Furthermore, the use of different metrics and units, hindered direct comparison: Shore Hardness (0–100), force–displacement–derived stiffness (N/mm), shear modulus (kPa), and energy metrics (e.g., Energy Dissipation Ratio; energy absorbed) were all used, often under different preload, posture, and boundary conditions [3, 39, 40, 42, 66, 69–75]. Taken together, Elastography-based methods (RSWE, SWE, MRE, Strain Elastography) offer stronger repeatability and anatomical mapping than compression-only approaches, but current evidence is tempered by systematic bias (SWE), operator dependence (Ultrasound-based methods), and limited validation (MRE). Standardized acquisition (i.e., preload, probe tilt and positioning, ROI depth, and loading state), reporting (i.e., metrics and measurement units), and statistical analysis of the clinimetric properties (i.e., ICC, SEM, LoA) are needed to improve the comparability of techniques. Despite its systematic measurement bias [40] likely related to tissue anisotropy, probe pressure, and boundary condition effects, SWE emerged as the most reliable technique, though further validation studies are required to refine its accuracy.

Morphological properties

Ultrasound was the most frequently assessed technique, demonstrating good-to-excellent intra-rater [1, 9, 82, 83, 85–87] and inter-rater reliability [80, 82, 85, 86, 119] and repeatability [39, 46, 74, 80, 82, 83, 90, 119]. However, operator-dependency, particularly related to probe pressure and positioning, affected measurement consistency. Heel pad studies reported intra-rater ICCs of 0.75–0.99 and inter-rater ICCs of 0.70–0.93 [82, 85, 86], likely due to operator-dependency reported also in other studies [1, 82, 86, 89]. Reliability decreased under loading conditions, highlighting the importance of probe stabilization and standardized acquisitions [1, 83, 90]. Despite these limitations, ultrasound showed moderate to strong correlations with MRI (r = 0.5–1.0) [88] and Radiography (r = 0.847–0.880) [89], supporting its criterion validity. Orthotic Embedded ultrasound demonstrated excellent intra-rater (ICC: 0.94–0.95) and inter-rater reliability (ICC: 0.89) when assessing tissue thickness during gait [23].

MRI is often considered as the reference standard, and exhibited excellent reliability (ICC: 0.976–0.991) [10, 79], with repeatability supported by small Bland-Altman limits of agreement [88]. While MRI provides high-resolution imaging, its use for validating ultrasound is likely based on its established role as a gold standard in musculoskeletal imaging, rather than on specific validation for plantar foot soft tissues.

CT-based techniques (SXCT, CT, and DECT) were each evaluated in a single study, demonstrating excellent reliability [78]. CT reported a median difference of −0.01 mm for sole thickness repeatability [77], but radiation exposure limit feasibility for routine clinical use.

Overall, ultrasound was the most frequently and comprehensively investigated method showing the high reliability and validity when properly standardized. However, sensitivity to factors like anatomical region, examiner expertise, and loading conditions continues to limit the comparability across studies. Further validation of imaging techniques in multiple studies and cohorts is essential for broader clinical applicability.

Technique strengths, limitations, and implications for clinical and research practice

Based on the findings from the previous sections, SWE for mechanical properties and ultrasound for morphological are currently supported by the most comprehensive clinimetric evidence among available techniques, demonstrating good-to-excellent reliability and validity. Alongside durometer, they were the only methods with comprehensive clinimetric investigations; however, durometer exhibited inconsistent reliability and validity, particularly in forefoot measurements [68].

SWE provides a quantitative and spatially resolved measure of tissue stiffness, making it well suited for detecting local mechanical variations within the plantar fat pad. Its integration into conventional ultrasound systems makes it a practical and accessible solution for both research and clinical use. Nevertheless, SWE remains sensitive to probe pressure, transducer angle, and user expertise [42, 108–110]. In foot imaging, errors stem from transducer stability, inclination, and gel homogeneity [42] and these factors may influence shear-wave propagation and measurement repeatability, particularly on curved or narrow surfaces such as the metatarsal heads. SWE struggles to differentiate deeper heel pad structures under compression. One study reported difficulty in distinguishing microchambers, macrochambers, and fibrous septa, while another study [42] achieved high intra-rater reliability. From a clinical perspective, the use of SWE should therefore be considered conditional on strict standardization of acquisition protocols, including control of probe pressure, transducer orientation, foot positioning, and coupling conditions.

Ultrasound, used for morphological measurements, demonstrated good-to-excellent reliability and validity across multiple studies but is limited by its operator dependency and sensitivity to acquisition conditions [42, 83, 90, 108, 110]. Accurate localization of the region of interest requires technical skill, and small changes in probe positioning, pressure, inclination, and gel use impact performance [108, 110]. Limited sound wave penetration reduces spatial resolution, making deep tissue visualization less precise than MRI [108, 110]. Because of these technical limitations, MRI is often used as the reference standard for validating ultrasound measurements [120, 121]. As shown in different applications (i.e., musculoskeletal and neurological imaging), MRI provides a more comprehensive evaluation of deep soft tissues, articular cartilage, and bone marrow than ultrasound [120] due to its contrast sensitivity and multiparametric capability to detect subtle microstructural changes [122]. In clinical radiology, MRI has repeatedly been considered the “gold standard” imaging modality for the assessment of indeterminate or complex soft-tissue lesions when ultrasound findings are inconclusive [121, 123]. However, although MRI is generally considered the gold standard for soft-tissue imaging, no superior in-vivo reference standard exists specifically for plantar foot soft tissues. Consequently, although ultrasound shows good validity when compared with MRI, these findings should be understood in light of MRI’s assumed, but unverified, accuracy as a reference technique. Accordingly, ultrasound-based morphological assessment is most appropriate when acquisition conditions are carefully standardized, particularly with respect to probe positioning, applied pressure, and loading state.

MRI provides detailed morphological imaging and high reproducibility for assessing plantar soft-tissue thickness [10, 79]. Dixon imaging enables consistent visualization of plantar soft tissues with minimal artifacts, improving diagnostic quality in neuropathic or osteomyelitic feet [124]. Beyond morphology, Dixon imaging quantifies tissue composition through the fat–water ratio, a marker of lipid content that reflects compositional rather than viscoelastic properties. In diabetic neuropathy, a reduced fat fraction of the heel pad has been reported, correlating with higher plantar pressures and suggesting compositional degradation associated with mechanical dysfunction [95].

MRE demonstrated excellent repeatability in assessing heel pad viscoelasticity [45, 71]. In one study on healthy volunteers, the heel fat pad shear modulus increased from 8 kPa to 12 kPa under higher loading, confirming the strain-dependence of tissue stiffness [71]. A follow-up pilot study in diabetic patients reported mean elastic moduli of 4.85 kPa in controls and 5.26 kPa in diabetes, indicating a trend toward increased stiffness [45]. Unlike SWE, MRE is unaffected by probe pressure or operator dependency and provides high spatial resolution with 3D stiffness mapping and eliminates probe-related artifacts common to ultrasound-based elastography [117]. Despite its potential, MRE’s validation remains limited because these early studies used prototype hardware with non-standardized vibration sources, small sample sizes, and incomplete reporting of acquisition parameters such as wave frequency, amplitude, and boundary conditions. The complex geometry of the foot further complicates homogeneous wave propagation and makes consistent loading difficult to achieve.

Future research should prioritize multicenter validity studies and direct head-to-head comparisons between elastography-based techniques under realistic loading, particularly in narrow, high-risk, areas like the forefoot [125, 126], alongside improved standardization of acquisition and reporting as suggested by the Guidelines for reporting reliability and agreement studies (GRRAS) [127].

Strengths and limitations of this study

This study systematically assessed the clinimetric attributes of measurement techniques for plantar soft tissues. Compared to the narrative review by Yang and colleagues [20], which qualitatively summarized methods used in patients with diabetic foot disease, the present review included studies on both healthy and pathological populations, allowing a broader methodological perspective. In contrast to the paper from Yang and colleagues, who provided a descriptive overview of the development, advantages, and disadvantages of existing techniques, this review applied a structured COSMIN-based risk of bias evaluation to systematically evaluate their clinimetric properties (reliability, validity, and repeatability). Another strength is the comprehensive analysis of techniques for both mechanical and morphological assessments across multiple imaging modalities.

Although plantar soft tissue properties vary between anatomical regions (e.g., heel and forefoot), substantial heterogeneity in regional definitions, measurement units, test conditions, and anatomical locations, complicated inter-study and inter-technique comparisons and the feasibility of region-specific synthesis. As a result, quantitative pooling and semi-quantitative synthesis approaches (e.g., vote counting or hierarchical scoring) were considered inappropriate for the reasons already mentioned in 4.2. The COSMIN Risk of Bias tool provided a structured approach to quality assessment but often led to high-risk ratings reflecting lacking blinding procedures or statistical transparency. The COSMIN’s “worst score counts” principle further contributed to these high-risk ratings by emphasizing the weakest methodological aspect of each study. As many studies (20/37) were not primarily designed as reliability and measurement error studies, some COSMIN criteria were only partially applicable, contributing to the observed high risk-of-bias. Despite these limitations, the COSMIN tool remains useful to systematically appraise the methodological quality of studies investigating measurement techniques and showing their methodological shortcomings. Formal assessment of publication bias was not performed, as measurement properties were typically reported as secondary outcomes. Missing data were handled as reported in the original studies, and no deviations from the registered review protocol were identified.

Conclusion

This systematic review highlights that, despite the wide range of techniques used for assessing mechanical and morphological properties of plantar foot soft tissues, robust clinimetric evidence remains limited. Ultrasound (for morphological assessment) and SWE (for mechanical assessment) are currently supported by the most consistent and comprehensive clinimetric evidence for both reliability and validity, although confidence in these findings remains conditional due to methodological limitations and sparse validity testing. Standardization of acquisition protocols, such as probe pressure, coupling medium, foot positioning, and reporting of reliability metrics, would substantially improve comparability across future studies. Addressing these gaps through validation and standardized methodology is essential to support reliable clinical application, particularly in high risk population such as individuals with diabetic foot disease.

Electronic supplementary material

Below is the link to the electronic supplementary material.

Abbreviations

COSMIN

Consensus-based standards for the Selection of health status Measurement Instruments

CT

Computed Tomography

CV

Coefficient of Variation

DECT

Dual-Energy Computed Tomography

EDR

Energy Dissipation Ratio

F-D

Force-Deformation

GRRAS

Guidelines for reporting reliability and agreement studies

ICC

Intra Class Correlation

LoA

Limits of Agreement

MAC

Macrochambers

MDC95

Minimal detectable change with 95% confidence

MIC

Microchambers

MRE

Magnetic Resonance Elastography

MRI

Magnetic Resonance Imaging

MTH

Metatarsal Head

PDT

Pressure Discomfort Threshold

PECO

Exposure, Comparator, Outcome

PICO

Population, Intervention, Comparator, Outcome

PPT

Pressure Pain Threshold

PRISMA

Preferred Reporting Items for Systematic Reviews and Meta-Analyses

RSWE

Reverberant Shear Wave Elastography

SEM

Standard Error of Measurement

SH

Shore Hardness

Std De

Dev: Standard Deviation

SW

Shear Wave

SWE

Shear Wave Elastography

SXCT

Spiral X-Ray Computed Tomography

Author contributions

Alessandro Vicentini 1,2,*: Writing – review & editing, Writing – original draft, Methodology, Investigation, Conceptualization, Data curation, Formal analysis. Marieke A. Mens 3: Methodology, Investigation, Data curation, Formal analysis. Arjan Malekzadeh 4: Methodology, Investigation. Jaap J. van Netten 1,2: Writing – review & editing, Methodology, Investigation, Conceptualization, Supervision. Mario Maas 5: Writing – review & editing, Methodology, Investigation, Conceptualization, Supervision. Sicco A. Bus 1,2: Writing – review & editing, Methodology, Investigation, Conceptualization, Data curation, Formal analysis, Supervision.

Funding

This research received funding the European Union’s Horizon Europe research and innovation programme under the Marie Skłodowska-Curie Doctoral Network grant agreement No 101073533. The grant agreement No 101073533 was signed on 13-07-2022.

Data availability

All data extracted and analyzed during this review are included in this article (Tables 1–3) and/or its supplementary materials (Appendix A), where applicable. Additional datasets are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

Not applicable. This study is a systematic review of published literature and did not involve the collection of new human or animal data.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.