Foot bone structure and plantar soft tissue deformation behavior in different functional positions

Introduction

Foot injuries often occur in the support phase of the gait cycle during exercise. The foot arches play a key role in supporting body weight and cushioning, acting as a shock absorber to push the body forward during movement, thus reducing the impact on the body and the risk of injury when walking (1,2). The foot arches stretch and collapse with weight increase during exercise and return to their original shape when the weight decreases (3). However, when the arch structure and soft tissues deteriorate, it is not possible to transfer body weight to the anterior part of the foot during walking. In turn, this tilts the ankle joint toward the midline of the body and increases the pressure on the knees, hips, and lower back (4), which can lead to a range of diseases and injuries (5-8). Therefore, a flexible arch is necessary to ensure that the foot adapts to weight transfer and stress dispersion (2). A comprehensive assessment of the morphology of the arch in different states is essential to understanding the function of the foot.

However, research on arch motion is still limited, and the academic community lacks a systematic understanding of skeletal and morphological changes in the arch. Although there have been systematic reviews conducted on foot measurement methods indicating that foot impression analysis methods such as the Foot Posture Index-6, Staheli arch index, or Chippaux-Smirak index can adequately diagnose the foot (9), these methods are unable to provide a reference for the morphology of the arch. Furthermore, the navicular drop test is employed to assess the mobility of the arch (10), yet it does not fully represent the mobility of the bones due to the relative motion of the skin. X-rays are widely used in clinical practice to accurately analyze the status of bony foot arches; for example, Ryu et al. (11) used weight-bearing X-rays to diagnose foot type. Unfortunately, however, the assessment of the foot arch is static in terms of clinical anthropometry and imaging standards (11-14). During motion, the bones and soft tissues of the foot arch change rapidly and dynamically (15). However, none of the existing assessment methods reflect the dynamics of the arch bones during functional activities such as walking and running (12). Therefore, the absence of dynamic detection of the foot arch both affects diagnostic accuracy and may misinform treatment decisions. For instance, current orthosis designs for the conservative treatment of flatfoot primarily rely on static positional measurements (3,16), an approach that fails to account for dynamic arch variations and may consequently induce abnormal plantar pressure patterns during activities. In order to accurately obtain the changes in the bones and soft tissues of the foot during functional activities, dynamic information on the skeletal and morphological changes in the arch needs to be extracted in conjunction with gait analysis.

There are no mature techniques for measuring the dynamic motion of the arch. Although some recent studies have used markers on the skin or artificial intelligence algorithms to automatically label the skin to estimate the position of skeletal landmarks in vivo (17), the skin impedes accurate kinematic estimation of motion. Benoit et al. (17) showed that although skin marker-derived kinematics can provide reproducible results, this is not representative of the movement of the underlying skeleton. This error is further exacerbated by the fact that the movement inside the foot is relatively small. Although computed tomography provides the possibility of dynamic foot detection, it is limited by high radiation and difficulty in acquisition and cannot be widely used. Given that foot radiographs are the standard for evaluating foot pathology (18), combining imaging and three-dimensional (3D) foot morphology may provide more comprehensive information for ascertaining dynamic skeletal and morphological changes in the arch.

The aim of this study was thus to investigate the changes in the foot bones and plantar soft tissues at three critical nodes (non-weight-bearing, weight-bearing, and tiptoe positions) during the support phase of the gait cycle in adults and to determine the sagittal plane motion of the foot arches in different postures so as to provide a more in-depth understanding of foot motion. This quantitative analysis may not only aid in the assessment of normal foot function but may also serve as a comprehensive reference for the diagnosis of foot diseases or injuries and the design of orthoses. We present this article in accordance with the STROBE reporting checklist (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-869/rc).

Methods

Participants

Sixty-nine adult males were recruited between May 2022 and July 2023. Young adults who cooperated with the test were included. The exclusion criteria were an inability to walk normally; a history of trauma to the lower limbs, surgery, skeletal deformity, or neuromuscular dysfunction; and incomplete skeletal display on radiological images. Accordingly, 67 participants were included and 2 were excluded. This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments and was approved by the Ethics Committee of Chongqing Orthopaedic Hospital of Traditional Chinese Medicine (batch No. 20210149). Informed consent was obtained from all patients.

Procedure and data collection

All participants were examined with a 3D plantar scanner and foot radiographs in three functional positions. The non-weight-bearing position served as a baseline, and the arch was examined in a relaxed state free from load-induced deformations. The weight-bearing position simulated the midstance phase of gait, where the arch bears substantial pressure, allowing for measurement of significant structural variations under load. The tiptoe position represented the terminal support phase (push-off), where the windlass mechanism actively contributes to arch reconstruction. Specifically, 3D scans and radiographs were performed in the non-weight-bearing position, with participants positioned in a seat with hips, knees, and ankles at 90º and feet in a neutral position (Figure 1A). For the 3D scans and radiographs performed in the weight-bearing position, participants were asked to keep one foot fully bearing weight and with the center of body weight on that foot, while the other foot was in a tiptoe position at the end of the propulsion phase; the scans and X-rays of the fully weight-bearing foot were then obtained (Figure 1B). To capture a 3D foot model in the tiptoe position, participants were seated with their hips, knees, and ankles at 90º; their feet in a neutral position; and toes cocked 65º off the ground (Figure 1C). To ensure the completeness of the 3D foot model, 3D scanning was performed when the toe-off angle reached 65º. This posture was designed to simulate the key kinematic state of the propulsion phase terminal, based on Root’s description of normal gait requirements (19). Although this static capture allows for the assessment of windlass mechanism engagement critical to arch reconstruction, it does not replicate dynamic forces—an inherent limitation of static imaging methods. Additionally, for the tiptoe position, participants’ tiptoe positions were captured on X-ray film at the end of the propulsion phase (Figure 1D). Both 3D scanning and X-ray measurements in the natural horizontal plane during standing served as the global reference. During data acquisition, the foot was positioned to point straight ahead, parallel to the 3D scanner’s coordinate frame and the X-ray receptor, respectively.

Figure 1 The test positions: (A) non-weight-bearing position for radiography and 3D scanning; (B) weight-bearing position for radiography and 3D scanning; (C) tiptoe position for 3D scanning; (D) tiptoe position for radiography.

The 3D foot model was scanned with Foot Secret 3D Foot Plantar Scanner (Chongqing Liangdao Medical Equipment Co., Ltd., Chongqing, China). The scanner uses active stereoscopic 3D technology with white-light pattern projection. Thus, it is safe for participants’ eyes and can be used without protective eyewear. Each foot was scanned by with the Foot Secret 3D Foot Plantar Scanner at the non-weight-bearing, weight-bearing, and tiptoe positions. The arch index and arch volume were obtained after scanning. The arch index was calculated as follows: the software automatically identified and segmented the entire footprint area. Subsequently, a line parallel to and perpendicular to the midline of the foot divided the footprint area (the area enclosed by the outer contour of the footprint formed by the sole of the foot, excluding the toes, in contact with the reference surface into three equal parts by length. The arch index is the ratio of the middle footprint area to the total footprint area (20). Meanwhile, the arch volume is the volume of the arch surface projected toward the support surface to form the arch (21,22). This volume was derived from the 3D foot model by vertically projecting the footprint boundary onto the plantar surface. Its upper boundary was defined by the plantar skin surface, while the lower boundary was defined by a horizontal reference plane delineated by the outermost points of the footprint. Finally, arch height was measured in the non-weight-bearing and weight-bearing positions according to Zifchock’s criteria (23). Participants stood with their weight evenly distributed over their feet, and to maintain a common reference, the back height measured standing was 50% of the total sitting foot length. Arch heights in the non-weight-bearing and weight-bearing positions were used to calculate arch height flexibility (AHF) according to the following formula: AHF=AHsitting−AHstanding0.4×BW×100(mm/kN), where AH is arch height from the floor to the dorsal surface of the foot at half the total foot length, and BW is body weight. The assessment ranges for AHF in the flexible, neutral, and stiff groups are defined as AHF ≥16.00, AHF ≥13.54 but <16.00, and AHF <13.54, respectively.

Participants were asked to undergo lateral radiographs, which were projected uniformly parallel to the lateral aspect of the foot to show the complete bony structure of the calcaneus, talus, navicular, first metatarsal, and fifth metatarsal, ensuring that the bone structure was clearly visible. The radiology department’s experienced technicians conducted all measurements using the same standardized procedure. The radiation dose was set to 65 kV and 5 mAs. During each exposure, participants’ thyroid and gonads were shielded with a lead apron to minimize radiation exposure.

Trials were reviewed for image quality, which included determining if the images clearly showed the cortical margins of the calcaneus, talus, navicular, first metatarsal, and fifth metatarsal. Radiographic images were transferred to the RadiAnt DICOM Viewer (Medixant, Poznań, Poland), and measurements were taken with the program’s tools for the most accurate angle readings. All angles measured in the sagittal plane were defined based on the established anatomical planes and bony landmarks. The angles measured included the talo-first metatarsal angle, calcaneal pitch angle, medial arch angle, lateral arch angle, calcaneus-fifth metatarsal angle, anterior arch angle, and posterior arch angle. These measurements were all obtained through conventional methods (Figure 2) (24-27).

Figure 2 Radiographs showed the talo-first metatarsal angle, calcaneal pitch angle, medial arch angle, lateral arch angle, calcaneus-fifth metatarsal angle, anterior arch angle and posterior arch angle, and the lines drawn to determine these angles.

Statistical analysis

Data analysis was conducted with SPSS 26.0 software (IBM Corp., Armonk, NY, USA). For all statistical tests, the threshold for statistical significance was P<0.05. Data are presented as the mean and standard deviation or as the median and interquartile range (IQR). A one-way analysis of variance (ANOVA) was used to compare the radiographic angles and foot parameters among participants in different positions (non-weight-bearing, weight-bearing, and tiptoe positions) and to determine the intergroup differences in parameter differences between functional positions across different arch flexibility groups (flexible, neutral, and rigid). The Levene test was used to test the homogeneity of the variances. The Shapiro-Wilk test was used to evaluate the normality of the data, and the Kruskal-Wallis test was used when the data were not normally distributed. Bonferroni post hoc tests were used to conduct pairwise comparisons, compare differences in angular and morphological parameters between different functional positions, and compare intergroup variability in the parameter differences between functional positions across the arch flexibility groups. The functional position parameter difference was considered to be the difference between the non-weight-bearing position and the weight-bearing position and between the tiptoe position and the weight-bearing position.

Results

Initially, 69 adult males were recruited for the study, among whom 67 were ultimately enrolled according to the inclusion and exclusion criteria. The demographics of the participants are shown in Table 1.

The angle changes of the radiographs in the three positions in the study are shown in Table 2. The statistical analysis revealed significant differences in all measured angles for the three positions (P<0.001). Analysis of the data showed that the talo-first metatarsal angle, calcaneal pitch angle, anterior arch angle, and posterior arch angle were significantly higher in the tiptoe position, while they were lowest in the weight-bearing position. The medial arch angle, lateral arch angle, and calcaneus-fifth metatarsal angle were significantly lower in the tiptoe position and higher in the weight-bearing position (Table 2). All data suggest that the arch structure decreases from the non-weight-bearing to weight-bearing position but then rises and shortens in the tiptoe position.

As shown in Table 3, the difference in arch index between the three positions was statistically significant (H=62.145; P<0.001). Analysis of the data indicated that the arch index was significantly lower in the tiptoe position and highest in the weight-bearing position. In addition, the difference in arch volume among the three positions was also statistically significant (H=121.383; P<0.001). Furthermore, the arch volume was significantly higher in the tiptoe position than in both the non-weight-bearing and weight-bearing positions, and the variation of arch volume accounted for 15.98% (32719−2821128211×100%) of the non-weight-bearing position and 48.71% (32719−2200222002×100%) of the weight-bearing position. It was further found that the soft tissue of the arch also decreases from the non-weight-bearing to weight-bearing position but then rises and shortens in the tiptoe position.

To further evaluate the patterns of changes associated with different arch flexibility, feet were further categorized into flexible, neutral, and stiff groups based on the AHF criteria. The values of posture differences from the three groups were compared to gain further insight into structural variation and its potential clinical significance. The differences are reported as the mean ± standard deviation or as the median with IQR in Table 4. For the medial arch angle [non-weight-bearing measurements minus weight-bearing measurements (NWB-WB)], the stiff group showed a significantly smaller change (−2.33±2.22) compared to the flexible (−4.62±2.08) and neutral (−3.82±1.87) groups (P<0.001). Significant differences were also observed in the lateral arch angle (NWB-WB) (P=0.042). For the anterior arch angle (NWB-WB), the stiff group showed a significantly smaller change (median =1.20) compared to the flexible (median =2.93) group (P<0.05). For the posterior arch angle (NWB-WB), the stiff group showed a significantly smaller change (median =1.69) compared to the flexible (median =2.15) group (P<0.05). The results of all statistically significant radiological parameters indicated that the stiff group had a smaller magnitude of change than did the flexible group, and in the case of the medial arch angle (NWB-WB), the change in the flexible group was calculated to be approximately 1.98 times that of the stiff group (4.62/2.33≈1.98). This result suggests that the flexible foot arch exhibits greater deformability under dynamic conditions, while the stiff foot arch remains relatively stable with less change. Moreover, differences in arch flexibility may lead to changes in arch cushioning patterns.

Discussion

The average human walks more than 6,000 steps per day (28), subjecting the foot arch—the primary load-absorbing structure—to substantial cumulative loads. A large body of literature has thoroughly investigated foot arch structure and its possible relationship with musculoskeletal injuries (4-8). However, previous research has predominantly focused on static arch structure, leaving its dynamic 3D deformation across key functional positions inadequately characterized. In this study, we quantified the sagittal plane motion of the foot arch in gait by measuring radiological angles and 3D foot parameters in three positions.

First, this study confirmed that there were significant differences in radiographic angles and foot morphology parameters between the three key nodes in gait. Consistent with the findings of Kimura et al. (29) and Kitaoka et al. (30), who reported that foot radiographic angles varied with the magnitude of the load, we also found variations in foot arches across the three positions. We further identified features of arch reconstruction, which involves the biomechanical process of medial longitudinal arch restoration and functional transition during the terminal stance phase, primarily driven by the windlass mechanism. This process is characterized by an elevation in arch height and shortening of the midfoot, enabling a functional shift from a “shock-absorbing” structure in the earlier stance phase to a “rigid lever” for effective force transmission during push-off, in agreement with the findings of Griffin et al. (31). The clinical relevance of this arch change is underscored by its link to pathologies of the propulsive mechanism; for instance, a recent study by Colò et al. (32) demonstrated the correlation between hindfoot valgus and first-ray insufficiency, conditions often associated with a dysfunctional windlass mechanism. This connection highlights the importance of our findings for understanding pathological gait adaptations and for developing individualized treatment strategies based on biomechanical principles. Specifically, radiological angles representing the skeletal structure of the medial longitudinal arch indicated elevation of the arch to a maximum throughout the terminal phase of the gait support phase, while the arch was reduced to a minimum during weight-bearing. The differences in calcaneal pitch angle and medial arch angle were most significant. The changes in arch bone structure were accompanied by similar changes in arch morphology, with arch volume elevated by 48.71% at tiptoe position compared to weight-bearing position. The skeletal and morphological changes in the foot reflect the mechanical leverage of the windlass mechanism, and therefore the calcaneal pitch angle, medial arch angle, and arch volume are important for the assessment and description of foot motion.

The results indicate that the arch changes significantly during simulated gait positions, which is the reason for the existence of the windlass mechanism as well as the physiological basis for arch cushioning. In addition, there were significant interindividual differences, suggesting that arch flexibility can have a relatively large impact on gait patterns in different individuals. Previous studies have reported that arch flexibility may be a key contributor to injury. For example, a study by Kudo and Hatanaka (33) revealed an association between reduced transverse arch flexibility and medial tibial stress syndrome. Takabayashi et al. (34) concluded that with a high AHF (i.e., flexible foot), the plantar fascia is expected to stretch, which may lead to increased tension. However, it is worth noting that all of the abovementioned studies were based on measurements in a single body position and therefore could not fully capture the changes of the foot arch during the gait cycle, making it difficult to accurately assess the mechanical properties of the arch and the specific characteristics of flexibility. To compensate for this, we divided foot types into flexible, neutral, and stiff groups according to AHF criteria and found that the change in the flexible group was approximately 1.98 times that of the stiff group. This finding suggests that differences in arch flexibility can directly contribute to changes in arch buffering patterns during weight-bearing conditions. More interestingly, the values of the difference between non-weight-bearing and weight-bearing were statistically significant for four angles between the flexible and stiff groups, whereas the values of the difference between tiptoe position and weight-bearing position were not significantly different for any of the angles except for the posterior arch angle. This phenomenon may suggest that arch flexibility affects the degree of arch buffering during the support phase but not arch reconstruction at the end of the gait support phase. This finding provides a novel perspective on the site of action of the windlass mechanism, but further in-depth studies are needed to confirm its veracity.

Finally, there are morphological changes and mechanical tension in the foot during movement, and the design of orthosis needs to incorporate both dynamic and biomechanical adaptability. Orthoses made under weight-bearing conditions cannot adapt to the dynamic changes of the arch, and, meanwhile, the rigidity of fixation cannot meet the support required for different arch flexibility. Therefore, we believe that the parameters of foot morphology changes and arch flexibility obtained by testing the radiological and morphological indicators of the three key positions can be a powerful guide to foot treatment and orthotic design specifications. In the case of flatfoot, orthosis, as a common method for conservative management of flatfoot deformity, is able to improve foot deformity by limiting arch collapse. However, the current practice of evaluation before orthosis production is mostly limited to a single position (3), which ignores the dynamic changes of the foot arch during gait. If the orthosis cannot match the arch changes during normal gait, it may increase additional pressure and shear forces (35). The key to the fabrication of orthosis is to understand the morphological changes and flexibility of the foot arch and to design and manufacture the appropriate orthosis with appropriate materials and techniques. It should be noted that this study was conducted on adult males, and the findings may not be directly generalizable to females or other age demographics. Therefore, future research should further clarify the impact of arch flexibility on gait, motor performance, and injury risk across diverse populations and develop treatment strategies for flexible flatfoot and rigid foot.

Conclusions

This study revealed significant differences in radiographic angles and foot morphometric parameters in three key positions during the gait cycle, highlighting the changing characteristics of the arch. These findings confirm the role of the windlass mechanism in arch reconstruction at the end of the support phase and suggest that arch flexibility does not affect arch reconstruction at the end of the support phase. The changes in arch flexibility have important clinical implications, especially in orthopedic design. The use of static assessments may not capture arch changes, which may increase foot pressure and shear forces. Given the unique biomechanical properties of different foot types, individualized treatment strategies for flexible and stiff feet are essential. Future studies should examine how arch flexibility affects gait, sports performance, and injury risk to provide an objective basis for conservative foot treatment and orthotic specification design.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-869/rc

Data Sharing Statement: Available at https://qims.amegroups.com/article/view/10.21037/qims-2025-869/dss

Funding: This study was supported in part by a grant from the Chongqing Yuzhong District Science and Technology Bureau Project (grant No. 20210149).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://qims.amegroups.com/article/view/10.21037/qims-2025-869/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. This study was approved by the Ethics Committee of Chongqing Orthopaedic Hospital of Traditional Chinese Medicine (batch No. 20210149) and informed consent was obtained from all patients. This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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