Shear wave elastography of the plantar fascia: A cross-sectional observational study in asymptomatic runners and sedentary controls

Abstract

The plantar fascia (PF) is exposed to repetitive loading in runners; it remains unclear whether chronic training in young, asymptomatic athletes leads to changes in PF thickness or stiffness. To compare PF thickness and elasticity between asymptomatic elite runners and sedentary controls using B-mode ultrasound and shear-wave elastography (SWE), and to test the hypothesis that prolonged mechanical loading induces adaptive changes in PF stiffness. In this cross-sectional study, 56 participants were included: 38 elite runners (12 short-distance, 11 middle-distance, 15 long-distance) and 18 sedentary controls. Bilateral B-mode ultrasound and longitudinal 2-dimensional SWE virtual touch tissue imaging quantification were performed at proximal (≈1 cm from the calcaneal insertion) and distal (≈6 cm) PF sites using an Acuson S2000 system (Siemens, Munich, Germany) with a 4 to 9 MHz linear array transducer. Primary outcomes were PF thickness (mm) and shear-wave velocity (m/s). Gender distribution did not differ between groups (χ², P = .748). Age differed across subgroups (analysis of variance, P = .011), and body mass index was lower in middle- and long-distance runners than in sedentary controls (P = .042). Thickness: No differences were observed between runners and sedentary controls (all P > .10); among runner subgroups, a difference was detected only at the left-proximal site (P = .043), with the highest values in short-distance runners. Proximal thickness exceeded distal thickness (P < .001). SWE: No differences were found between-group at any site (all analysis of variance, P > .45); effect sizes were small (Hedges, g  −0.07 to −0.29), and post hoc power to detect such small effects was low (~6%–16%). Proximal SWE was higher than distal SWE (P < .001); at the distal site, left-sided values were higher than right-sided values (Wilcoxon, P = .032). Among sedentary participants, females had higher proximal SWE than males (P ≈ 0.014–0.053), whereas no gender effect was observed in runners. Reliability: Intraclass correlation coefficient was 0.965 to 0.979 for distal SWE (excellent) and 0.755 for right-proximal SWE (good); thickness intraclass correlation coefficients ranged from 0.765 to 0.814. In young, asymptomatic elite runners, chronic training is not associated with a measurable change in PF thickness or stiffness compared with sedentary peers. Differences are predominantly site-dependent (proximal > distal), and a gender effect was observed only among sedentary individuals. Adaptation of the PF to mechanical loading may occur without macroscopic hypertrophy or increased stiffness.

1. Introduction

The plantar fascia (PF) is a robust dense connective-tissue structure.^[1] It consists predominantly of longitudinal type I collagen fibers extending from proximal to distal, with elastic fibrils interspersed among areas of loose connective tissue.^[2] It is the most important of the bands that form the longitudinal arch of the foot. The principal stress on the PF occurs during running.^[1] Hicks likened the human foot to the arch of a bridge; in this analogy, the bones and joints are the stones of the arch, whereas the PF functions as the tie linking the calcaneus and the toes.^[3] During heel strike the PF slackens, whereas maximal tension develops during push-off/toe-off.^[1] Owing to its hyperelastic behavior, it stores energy during loading and releases it upon recoil to its original state during relaxation.^[4,5] The PF protects the plantar aspect of the foot from trauma and supports and stabilizes the arches.^[6] Because athletes feet are exposed to repetitive loading, PF pathologies are more prevalent in this population.^[7,8] Human data indicate that tendon tissue is metabolically sensitive to tensile load and that the adaptive response of connective tissue to loading occurs through increased synthesis and turnover of matrix proteins – particularly collagen.^[9,10]

Sonoelastography is a technique that enables assessment of the mechanical properties of soft tissues.^[11] Soft-tissue elasticity is defined as the capacity for deformation determined by the tissue structure and composition.^[12] Shear-wave elastography (SWE) quantifies tissue elasticity and stiffness in an operator-independent manner, reporting values in kilopascals (kPa) or meters per second (m/s).^[13] Malignant tissues are generally stiffer and therefore yield higher elasticity (elastic modulus/velocity) values. Alterations in soft-tissue elasticity are also observed in nonneoplastic disease processes such as tendinopathy, neuromuscular disorders, and stages of wound healing.^[11] SWE is widely used in elastic tissues such as the PF.^[11,14–17]

In this study, we hypothesized that prolonged mechanical loading in elite runners would initiate adaptation and remodeling of the PF at a metabolic level – particularly by increasing connective-tissue collagen synthesis – thereby leading to an increase in tissue elasticity. The aim of this study was to identify differences in PF elasticity among short-, middle-, and long-distance elite runners and healthy volunteers.

2. Materials and methods

This single-center, cross-sectional observational study included 56 participants: 38 elite runners and 18 sedentary controls. The sample size for this study was planned based on prior evidence on PF stiffness^[30]. Assuming a large effect (Cohen d ≈ 0.80), with a significance level of α = 0.05 and statistical power (1 − β) = 0.80, a priori calculations using G*Power (v3.1.9.7) indicated a minimum of 21 participants per group (total n = 42) for a 2-sample comparison of means. Ultimately, 56 participants were included: elite runners (n = 38) and sedentary controls (n = 18). Although this unequal allocation entails a modest loss of efficiency relative to a 1:1 design, the overall sample size remains sufficient to maintain approximately 80% power for detecting a large effect.

A total of 56 participants were enrolled: 38 elite runners and 18 healthy sedentary controls. Elite runners were licensed short-, middle-, or long-distance track athletes who had been participating in structured running training programs for an average of 4 to 8 years and typically at least 6 days/wk under the supervision of a coach. Both right and left feet were evaluated. Among the runners, 12 specialized in short-distance (200–400 m), 11 in middle-distance (800–1500 m), and 15 in long-distance (≥5000 m). The American Orthopaedic Foot and Ankle Society Foot and Ankle Scale was administered to all runners and controls by an experienced sports physician. This structured assessment includes items on pain, function, and alignment; only individuals scoring 100 points (indicating absence of foot and ankle pain or functional limitation) were included. During the same clinical encounter, participants were systematically questioned about heel and plantar pain, pain with the 1st step in the morning, exercise-related symptoms, any prior diagnosis of plantar fasciitis, and any history of lower-extremity surgery or injections. Age, gender, weight, height, and body mass index (BMI) were recorded. The control group was selected to approximate the distribution of age, gender, weight, and BMI observed in the runner cohort. Exclusion criteria included current heel or plantar pain, a known history of plantar fasciitis, rheumatologic or metabolic disease, prior corticosteroid injection around the heel, and prior foot surgery or major trauma. On B-mode assessment, inclusion required a homogeneous PF echotexture, a thickness < 4 mm, and absence of edema or fluid accumulation in surrounding tissue planes. The same inclusion and exclusion criteria were applied to controls. Participants were instructed to abstain from sports activities for the 3 days preceding imaging to minimize acute training effects and isolate findings consistent with chronic adaptation. Participants were referred to the radiology unit at scheduled intervals.

All sonoelastography examinations were performed by a radiologist with 5 years of experience, who was blinded to group allocation. Imaging was conducted on an Acuson S2000 ultrasound system (Siemens, Munich, Germany) using a 4 to 9 MHz linear transducer. Longitudinal and short-axis B-mode imaging was followed by longitudinal 2-dimensional (2D) SWE using virtual touch tissue imaging quantification (VTTIQ), a 2D quantitative SWE technique. Within the color-coded shear-wave map, VTTIQ synthesizes information from up to 256 sequential beam lines to derive localized shear-wave velocity (SWV) values across a broad measurement range (0.5–10 m/s) using small shear-wave regions of interest (region of interest [ROI], 1.5 × 1.5 mm), enabling homogeneous sampling with small ROIs at targeted sites.

All participants were examined in the same room maintained at 25°C under a standardized protocol. Assessments were performed with the subject prone, the knee in full extension (0°), and the ankle in a neutral position. The transducer was positioned parallel to the PF in the sagittal (long-axis) plane with light contact. All examinations consistently began on the right side.

On B-mode imaging, PF thickness was measured at 2 sites: ~1 cm proximal to the calcaneal insertion and ~6 cm distal to the insertion at the point of maximal thickness in the anterior–posterior plane. These measurement sites, rather than the exact calcaneal enthesis, were intentionally selected to minimize artifacts related to the bone–soft tissue interface and to allow clearer delineation of the fascial margins and more reproducible PF thickness and SWE measurements. Subsequently, elastographic assessment was performed on PF sections selected from the same proximal and distal regions using dedicated software (Virtual Touch Imaging and Quantification; Siemens Medical Solutions). Owing to pronounced elastographic anisotropy of tendon fibers, all measurements were acquired in the longitudinal orientation, parallel to fiber direction. The target measurement area was 1st delineated with a freehand electronic caliper; then, using the smallest available ROI (1.5 × 1.5 mm), sequential multipoint SWV measurements were obtained to cover the entire area. Each site was measured twice, and the mean value was recorded. To evaluate repeatability, all measurements in a randomly selected subset of 18 participants were repeated 24 hours later, and intraclass correlation coefficients were calculated.

The mean duration of imaging for each foot was approximately 15 minutes. Institutional review board approval was obtained, and written informed consent was secured from all participants prior to imaging. As shown in Figure 1, the study flow chart outlines participant recruitment, inclusion/exclusion, group allocation, and the imaging/analysis pipeline.

Figure 1.

Flow chart of the study. 2D = 2-dimensional, AOFAS = The American Orthopaedic Foot & Ankle Society, SWE = shear-wave elastography, VTTIQ = virtual touch tissue imaging quantification.

To minimize bias, participants were selected under the supervision of 2 faculty members – a sports physician (ŞK) and a faculty member from the department of sports management (ZÖ) – in accordance with The American Orthopaedic Foot and Ankle Society Foot and Ankle Scale Calculator guidelines. As planned, elite short-, middle-, and long-distance runners and sedentary controls were matched for age, sex, and BMI. B-mode and 2D-SWE examinations were performed using a 4 to 9 MHz linear array transducer with standard preset settings. SWE measurements were acquired with VTTIQ, a 2D quantitative technique, using small regions of interest (ROI, 1.5 × 1.5 mm) across a broad measurement range (0.5–10 m/s) at targeted sites. All examinations were conducted by an experienced radiologist blinded to the study. Repeat measurements were obtained to assess measurement repeatability. To ensure cohort homogeneity, strict exclusion criteria were applied. Ethical approval was obtained, and written informed consent was secured in accordance with the declaration of Helsinki.

2.1. Statistical analysis

Statistical analyses were performed using SPSS Statistics 24.0 (IBM Corp., Armonk) and Python 3.11 (SciPy and statsmodels libraries; Python Software Foundation, Beaverton). Depending on the distributional properties of the data, both parametric and nonparametric methods were applied. Descriptive statistics are presented as mean ± standard deviation. For between-group comparisons, 1-way analysis of variance (ANOVA) with least significant difference post hoc testing was used when normality assumptions were met; otherwise, the Kruskal–Wallis test was applied. Pairwise comparisons between pooled elite runners and sedentary controls employed Welch t test and the Mann–Whitney U test. Categorical variables were analyzed with chi-square tests (4 × 2 and 2 × 2), with Fisher exact test used for sparse cells. For side-to-side and site-level (proximal–distal) comparisons, the paired t test was used for parametric data and the Wilcoxon signed-rank test for nonparametric data. Intraobserver reliability was assessed using the intraclass correlation coefficient (ICC) with 95% confidence intervals. Effect sizes were calculated as Hedges g and Cohen dz. Post hoc power analyses were conducted to evaluate the ability to detect differences across groups and gender strata. Correlations were analyzed with Pearson method for normally distributed variables and Spearman method when distributional assumptions were not met. All statistical tests were 2-sided, with P < .05 considered significant.

3. Results

A total of 56 participants were included: 38 elite runners and 18 healthy sedentary controls. The athletic cohort was comprised of 12 short-distance runners (mean age, 19.8 ± 2.3 years), 11 middle-distance runners (18.1 ± 1.8 years), and 15 long-distance runners (20.4 ± 2.0 years). The control group consisted of 18 sedentary individuals (20.6 ± 1.9 years). Thirty-six participants (64.3%) were male and 20 (35.7%) were female; gender distribution did not differ between groups (4 × 2 χ², P = .748; 2 × 2 χ², P = .798; Fisher exact test, P = 1.000). Age differed significantly across groups (ANOVA, F = 4.06; P = .011). The youngest subgroup was the middle-distance runners (18.1 ± 1.8 years), whose mean age was significantly lower than that of long-distance runners (20.4 ± 2.0 years; P < .05) and sedentary controls (20.6 ± 1.9 years; P < .05). Short-distance runners showed an intermediate mean age (19.8 ± 2.3 years) with no significant differences from the other groups (both P > .05). No age difference was detected between sedentary individuals and long-distance runners (P > .05). Baseline demographics, PF thickness, and SWE metrics are summarized as mean ±  standard deviation; P values derive from 1-way ANOVA (continuous) and χ² tests (categorical) (Table 1).

Table 1.

Demographic characteristics, PF thickness, and SWE values across study groups.

Group	N	Gender (M/F)	Age (yr)	BMI (kg/m2)	Thickness proximal (mm)	Thickness distal (mm)	SWE proximal (m/s)	SWE distal (m/s)
Short-distance	12	9/3	19.8 ± 2.3	20.06 ± 2.09	3.02 ± 0.38	1.80 ± 0.23	6.69 ± 1.01	5.97 ± 0.97
Middle-distance	11	6/5	18.1 ± 1.8	19.42 ± 2.54	2.68 ± 0.35	1.74 ± 0.19	7.13 ± 1.13	6.06 ± 0.91
Long-distance	15	9/6	20.4 ± 2	19.65 ± 1.69	2.77 ± 0.34	1.72 ± 0.23	6.67 ± 1.56	6.01 ± 0.77
Sedentary	18	12/6	20.6 ± 1.9	21.79 ± 3.19	2.96 ± 0.36	1.77 ± 0.21	7.03 ± 1.04	6.30 ± 0.98
P value (ANOVA/χ2)		0.748	0.011	0.042	0.059	0.740	0.685	0.737

Data is presented as mean ± standard deviation. P values were calculated using 1-way ANOVA for continuous variables and the chi-square (χ²) test for categorical variables.

ANOVA = analysis of variance, BMI = body mass index, PF = plantar fascia, SWE = shear-wave elastography.

BMI differed significantly across groups (P = .042). Middle-distance (19.41 ± 2.53 kg/m²) and long-distance runners (19.65 ± 1.69 kg/m²) exhibited lower BMI than sedentary controls (21.78 ± 3.18 kg/m²). Short-distance runners showed intermediate values (20.05 ± 2.08 kg/m²) without a significant difference.

On B-mode ultrasound, no focal hypoechoic areas, border irregularities, or calcifications of the PF were observed in any participant; all examined fascial segments demonstrated homogeneous echotexture and regular margins, in line with the predefined inclusion criteria. PF thickness: Measurements demonstrated high repeatability at both proximal and distal levels (ICC proximal: r = 0.788–0.814; distal: r = 0.765–0.802; all P < .001). No differences between groups were found for right-proximal (P = .221) or right-distal (P = .456) thickness. At the left-proximal site, a significant difference was observed (P = .043): the highest values occurred in short-distance runners (3.09 ± 0.38 mm) and the lowest in middle-distance runners (2.64 ± 0.43 mm), whereas long-distance runners (2.75 ± 0.47 mm) and sedentary controls (2.92 ± 0.35 mm) showed intermediate values. No difference was detected at the left-distal site (P = .951). When elite runners (all subgroups pooled) were compared with sedentary controls, PF thickness did not differ (Welch t test and Mann–Whitney U, all P > .10). Right–left comparisons were also nonsignificant (proximal: P = .64; distal: P = .48). Across all participants, proximal thickness was significantly greater than distal thickness (P < .001). Group-wise PF thickness (mm) at 4 measurement sites (left proximal, left distal, right proximal, and right distal) is visualized with boxplots in Figure 2.

Figure 2.

Boxplots of PF thickness (mm) in short-, middle-, and long-distance runners and sedentary controls, stratified by site (left proximal, left distal, right proximal, right distal). PF = plantar fascia.

Gender comparisons: PF thickness was higher in males across all sites. Differences at the left-proximal (males: 2.91 ± 0.43 mm; females: 2.75 ± 0.40 mm; P > .10) and left-distal sites (males: 1.76 ± 0.26 mm; females: 1.65 ± 0.17 mm; P ≈ .06–.08) were not significant. By contrast, right-proximal (males: 2.99 ± 0.38 mm; females: 2.68 ± 0.43 mm; P = .006–.010) and right-distal (males: 1.86 ± 0.25 mm; females: 1.66 ± 0.19 mm; P = .002–.006) measurements showed statistically significant differences. Within runner subgroups, males generally exhibited higher means, although most contrasts were not significant. Among sedentary participants, males had distinctly higher values at the right-distal site (P < .05). At the distal level, only in males was the right side thicker than the left (P < .05), whereas no right–left difference was observed proximally.

SWE: repeatability varied by site – excellent at the distal level (ICC, r = 0.965–0.979, P < .001), good at the right-proximal level (ICC, r = 0.755, P < .001), and moderate at the left-proximallevel (ICC, r = 0.456, P = .057). No significant between-group differences in SWE were observed (ANOVA, all P > .45). Left-proximal values were 6.68 ± 1.08 m/s (short-distance), 7.30 ± 1.34 m/s (middle-distance), 6.93 ± 1.71 m/s (long-distance), and 7.06 ± 1.06 m/s (sedentary) (P = .718). Right-proximal values were 6.70 ± 1.39, 6.96 ± 1.52, 6.42 ± 1.74, and 7.01 ± 1.34 m/s (P = .685), respectively. Left-distal values were 6.21 ± 1.06, 5.92 ± 1.15, 6.19 ± 1.10, and 6.41 ± 1.14 m/s (P = .720), and right-distal values were 5.73 ± 0.99, 6.21 ± 1.00, 5.83 ± 0.85, and 6.19 ± 1.01 m/s (P = .458). Comparisons of pooled elite runners versus sedentary controls were likewise nonsignificant; effect sizes were small (Hedges g −0.07 to −0.29). Post hoc power analyses indicated low statistical power (≈6%–16%) to detect effects of this magnitude. Group-wise PF SWE (m/s) at 4 measurement sites (left proximal, left distal, right proximal, and right distal) is visualized in Figure 3 using boxplots (median and interquartile range) with superimposed individual values.

Figure 3.

Boxplots of PF SWE values (m/s) in short-, middle-, and long-distance runners and sedentary controls, stratified by site (left proximal, left distal, right proximal, right distal). Medians, interquartile ranges, and individual data points are shown. PF = plantar fascia, SWE = shear-wave elastography.

Gender-stratified analyses: Among elite runners, female–male differences in PF SWE values were not significant. By contrast, in sedentary participants, proximal values were higher in females than in males: at the left-proximal site, females 7.75 ± 0.97 m/s versus males 6.71 ± 1.35 m/s (P = .014–.041); at the right-proximal site, females 7.93 ± 1.26 m/s versus males 6.55 ± 1.50 m/s (P ≈ .031–.053).

Comparisons of the right/left sides: No difference was detected at the proximal level (P > .10). At the distal level, left-sided values were slightly higher (6.21 ± 1.10 vs 6.00 ± 0.96 m/s); this difference was not significant by parametric analysis (P = .125) but reached significance with the Wilcoxon test (P = .032). In females, proximal values were significantly higher than distal values in both feet (P < .001), whereas this contrast was not significant in males. Overall, proximal SWE values were significantly greater than distal values in both feet (P < .001). Effect sizes were in the medium range (Cohen, dz ≈ 0.47–0.51), suggesting that the finding may be not only be statistically significant but also clinically meaningful. Figure 4 shows boxplots of PF thickness (mm) and SWE (m/s) at proximal and distal sites.

Figure 4.

Boxplots of PF thickness (mm) and SWE values (m/s) at proximal and distal sites. Circles indicate thickness measurements; squares indicate SWE values. Medians, interquartile ranges, and individual data points are displayed. PF = plantar fascia, SWE = shear-wave elastography.

Correlation analyses: At the left-proximal site, PF thickness and SWE exhibited a moderate negative correlation (Spearman, ρ = −0.432, P = .001), whereas at the left-distal site a moderate positive correlation was observed (Pearson, r = 0.347, P = .009). On the right side, correlations were weak and not statistically significant.

4. Discussion

Using B-mode ultrasonography and longitudinal VTTIQ-based 2D SWE, this study evaluated the training-related adaptive response of PF thickness and elasticity in elite runners versus sedentary individuals. Our findings indicate that biomechanical adaptation processes are more complex and multifactorial than initially presumed. The principal result is that participation at an elite athletic level, in asymptomatic individuals, is not associated with significant changes in PF thickness or elasticity.

Careful consideration of participant demographics is essential when interpreting our findings. The absence of a statistically significant difference in gender distribution between groups minimizes potential confounding by gender.^[18,19] Although age differed across groups (P = .011), this contrast was driven primarily by the younger age of the middle-distance runners relative to the other subgroups. A key distinguishing feature of our study is that all participants were young, asymptomatic elite athletes within a narrow age range (≈18–22 years). This design reduces the likelihood that age-related chronic changes act as confounders and sets our work apart from much of the literature, which typically focuses on middle-aged, symptomatic patients with plantar fasciitis.^[13,20,21] Consequently, the homogeneity and specificity of our cohort support the interpretation that our results reflect a physiological adaptation process rather than disease-related degenerative change.

The significant between-group difference in BMI (P = .042) is an expected finding consistent with athlete physiology. The lower BMI in middle- and long-distance runners compared with sedentary controls accords with prior evidence that endurance sports are characterized by reduced body fat.^[22] Given that elevated BMI is a recognized risk factor for greater PF thickness, the lower BMI of our athlete cohorts minimizes confounding by adiposity and constitutes a methodological strength, permitting more direct attribution of observed patterns to training effects.^[23–25]

PF thickness: Our findings further elucidate this complex relationship. Notably, no significant overall difference in PF thickness was detected between elite runners and sedentary controls. This contrasts with several influential reports describing a thicker fascia in athletes.^[26,27] Taken together, these observations suggest that adaptation in elite athletes may occur primarily at the level of tissue quality and stiffness, and that overt hypertrophy is not an obligatory outcome. In subgroup analyses, the highest thickness values were observed in short-distance runners, whose events require explosive power, supporting the hypothesis that these athletes may develop a hypertrophic response to high, abrupt tensile stresses.^[27]

The most salient – and most debated relative to the existing literature – finding of our study derives from SWE measurements. No statistically significant differences in PF elasticity were detected either between elite runners and sedentary controls or among runner subgroups (all P > .45). This suggests that chronic, high-intensity mechanical loading does not produce a discernible change in macroscopic fascial stiffness in asymptomatic athletes. Instead, PF adaptation to repetitive loading may reflect subtler microstructural adjustments that optimize energy storage and return rather than a marked increase in tissue stiffness. Supporting the distinct behavior under acute loading, Costello et al reported that even 5 minutes of walking significantly increased SWV (≈14%) in healthy individuals,^[28] implying that while the fascia may stiffen immediately in response to acute load, chronic adaptation – as in our cohort – may proceed via different mechanisms.

Studies comparing 10 long-distance runners with 10 sedentary individuals and 10 track runners with 10 sedentary individuals have reported findings concordant with ours.^[29,30] Addressing a literature largely constrained by small sample sizes, our investigation contributes a larger and relatively more homogeneous sample. Statistically, with measurements obtained from 112 ft, this study comprises one of the larger sample sizes reported for this specific population. Our results corroborate prior smaller studies indicating no appreciable between-group differences.

In our study, comparisons of PF SWE between runners and sedentary controls – and among runner subgroups – showed no difference at the proximal level (P > .10), whereas at the distal level the left side was significantly higher (P = .032). These results contrast with prior reports noting left-sided and proximal elevations.^[30]

In our study, proximal SWE values in the sedentary group (7.03 ± 1.04 m/s) were higher than those reported for healthy controls in prior literature.^[31] This suggests that the 2D-SWE VTTIQ platform we employed may yield systematically higher measurements than point-SWE or alternative vendor systems, highlighting how limited cross-system standardization complicates interstudy comparisons. Moreover, by acquiring measurements parallel to fascial fibers (longitudinal) and averaging across an entire predefined segment (~3.8 cm), our protocol likely produced more stable estimates than single-point sampling, while potentially attenuating subtle between-group differences through spatial averaging. Given the anisotropic behavior of the PF, aligning the measurement plane with fiber orientation is critical for capturing true mechanical properties, which we regard as a methodological strength of the present study.

In our study, SWE values were significantly higher at proximal sites than at distal sites across all participant groups – an important finding consistent with the literature. Similarly, Shiotani et al reported that proximal segments of the fascia are both thicker and stiffer than distal segments.^[29]

Gender-related differences in SWE: In the sedentary group, females exhibited significantly higher proximal fascial stiffness than males (P < .05). This parallels the findings of Shiotani et al, who reported that despite having a thinner fascia, females demonstrated significantly greater stiffness than males.^[29] These observations suggest that gender-related biomechanical differences are not solely attributable to anthropometric factors but may also reflect hormonal milieu and tissue compositional differences. Among runners, the absence of a significant female–male difference may indicate that the mechanical loading imposed by intensive training elicits an adaptive response that attenuates or obscures these baseline gender effects.

This study has several limitations. First, its cross-sectional design precludes causal inference; thus, it is not possible to determine whether differences between athletes and sedentary controls are consequences of training or reflect preexisting characteristics that predispose individuals to sport participation. Second, the sample size – particularly at the subgroup level – may have been too few to detect small effects; indeed, our post hoc power analyses corroborated this limitation. Third, potential confounders such as training intensity and volume, training surface, and footwear were not analyzed in detail. Future prospective studies with larger, well-characterized cohorts and longitudinal follow-up, incorporating standardized exposure metrics, are needed to clarify the impact of these factors on PF biomechanics.

Such prospective work, particularly in larger samples with standardized exposure metrics, will also be important to validate these findings and to elucidate the PF adaptive mechanisms in greater depth.

5. Conclusion

Based on B-mode ultrasonography and SWE assessments in young, asymptomatic elite runners versus age-matched sedentary peers, chronic mechanical loading was not associated with statistically significant changes in PF thickness or stiffness. This finding suggests that the PF physiological adaptation to repetitive loading may involve more complex – and likely microstructural – mechanisms that optimize energy storage and return, rather than the simple hypertrophy or generalized stiffening frequently reported in prior literature.

Author contributions

Conceptualization: Zülküf Akdemir, Suat Orak.

Data curation: Zülküf Akdemir, Harun Arslan, Hüseyin Akdeniz, Şeyhmus Kaplan.

Formal analysis: Zülküf Akdemir, Zekiye Özkan, Suat Orak.

Investigation: Zülküf Akdemir, Zekiye Özkan, Suat Orak, Harun Arslan, Şeyhmus Kaplan.

Methodology: Zülküf Akdemir, Suat Orak, Harun Arslan, Hüseyin Akdeniz.

Project administration: Zülküf Akdemir, Zekiye Özkan, Harun Arslan, Hüseyin Akdeniz, Şeyhmus Kaplan.

Resources: Zülküf Akdemir, Zekiye Özkan, Suat Orak, Harun Arslan, Şeyhmus Kaplan.

Supervision: Zülküf Akdemir, Hüseyin Akdeniz, Şeyhmus Kaplan.

Visualization: Zülküf Akdemir, Harun Arslan, Hüseyin Akdeniz, Şeyhmus Kaplan.

Writing – original draft: Zülküf Akdemir, Suat Orak, Harun Arslan, Şeyhmus Kaplan.

Writing – review & editing: Zülküf Akdemir, Zekiye Özkan, Suat Orak, Harun Arslan, Şeyhmus Kaplan.