Introduction

Pediatric flexible flatfoot (FFT) is a common musculoskeletal disorder of the lower extremities. It is primarily characterized by the collapse of the medial longitudinal arch and excessive subtalar joint eversion [1,2]. These structural abnormalities significantly alter the mechanical characteristics of the lower extremities. Although many children remain asymptomatic in the short term, clinical observations indicate that persistent structural deformities can lead to altered stress distribution, lower extremity fatigue, and an elevated risk of injury [3–5] Current evidence suggests that the biomechanical consequences of FFT involve complex structural and sensorimotor adaptations that extend well beyond local foot and ankle complex mechanics [6]. Specifically, within the closed kinetic chain of the lower extremities, distal instability is transmitted proximally, thereby altering the kinematics and dynamic motor control of proximal joints [7–9].

According to biomechanical principles, static alignment significantly influences dynamic function [10] Clinically, adaptive changes within the closed kinetic chain of the lower extremities often manifest as altered neuromuscular recruitment patterns [11]and an increased demand on proximal stabilizing muscle groups to maintain posture and distal control [12]. These muscle groups must counteract the abnormal joint moments (e.g., excessive or misdirected inversion/eversion and rotational moments) generated by the flatfoot deformity to prevent further biomechanical dysfunction.Although these functional compensations persist to maintain overall walking efficiency, this motor strategy—rooted in an initial structural malalignment—may induce an occult loss of postural stability. Furthermore, prolonged and unmanaged proximal compensation can lead to adverse consequences, including accelerated joint wear [13], patellofemoral pain syndrome [14], and altered force transmission across the articular surfaces of the hip and knee [15].

However, research concerning the biomechanical consequences of FFT—particularly regarding its systematic impact on overall postural control and proximal joints of the lower extremities—remains relatively limited. First, previous studies have predominantly focused on joint angle alterations during dynamic gait cycles, frequently overlooking potential three-dimensional (3D) kinematic abnormalities during static stance. Consequently, few studies have systematically explored how static 3D joint malalignments translate into dynamic gait deviations. Second, when evaluating dynamic gait quality, isolated spatiotemporal parameters often mask multi-planar kinematic deficits [16]. The incorporation of objective, comprehensive gait indices—such as the Gait Profile Score (GPS) and Gait Variable Scores (GVS)—can more sensitively quantify neuromuscular adaptability to distal deformities [17]. More importantly, regarding research design and statistical methodology, prior studies have largely utilized the subject as a single analytical unit. This approach overlooks the fact that the closed kinetic chain of the lower extremities is fundamentally a limb-specific mechanical transmission process. Clinically, FFT presents with either unilateral or bilateral involvement; grouping solely by subject may obscure critical biomechanical differences between an unaffected limb and a collapsed arch within the same patient. Furthermore, the uncontrolled pooling of bilateral limb data violates the assumption of statistical independence. Finally, because pediatric gait is significantly influenced by growth and development, studies failing to strictly control for confounding factors, such as age and body mass index (BMI), often yield inconsistent and less robust conclusions [18].

To address these limitations, this study employed a high-precision infrared optoelectronic motion capture system and a dual-level analytical strategy integrating both the limb and subject levels. By utilizing linear mixed-effects models (LMMs) to rigorously control for within-subject correlations and developmental covariates, we compared the biomechanical characteristics of normal feet and flatfeet, treating the individual lower extremity as the primary unit of analysis. Specifically, this study aims to investigate the following two primary objectives: (1) Assessing static differences: To quantitatively compare the 3D spatial alignment of the lower extremity joints between the two groups during static stance; (2) Quantifying dynamic function and gait deviations: To determine whether static structural alterations influence the spatiotemporal parameters of dynamic walking, and to reveal underlying multi-planar kinematic deviations using comprehensive gait indices (i.e., GDI, GPS, and GVS).

Materials and methods

Results

Baseline characteristics

A total of 153 subjects were included in this study, comprising 25 in the unilateral flatfoot group and 128 in the bilateral flatfoot group. All statistical tests for baseline comparisons were two-tailed. No significant between-group differences were observed regarding age (Z = −0.206, P = 0.837), height (Z = −0.417, P = 0.677), weight (Z = −0.111, P = 0.912), or BMI (Z = −0.600, P = 0.549). Additionally, gender distribution did not differ significantly between the groups (χ2 = 0.041, P = 0.839). Overall, the baseline demographic and anthropometric characteristics of the two groups were highly comparable (Table 1).

Download:

• PNG
  larger image
• TIFF
  original image

Table 1. Demographic characteristics of the participants.

https://doi.org/10.1371/journal.pone.0358183.t001

Global spatiotemporal parameters at the subject level

Regarding macroscopic dynamic function, this study compared the overall walking performance of patients with unilateral versus bilateral involvement, utilizing the subject as the analytical unit. Since the data were not normally distributed, a nonparametric Mann-Whitney U test was employed to compare the unilateral and bilateral flatfoot subgroups. The results indicated that overall walking efficiency was highly consistent between children with unilateral and bilateral flatfoot. During natural walking, no statistically significant between-group differences were observed in Mean Velocity (Z = −0.384, P = 0.701), Frequency (Z = −0.400, P = 0.689), or Step Width (Z = −0.812, P = 0.417). This suggests that bilateral, as opposed to unilateral, structural deformities do not lead to a significant deterioration of macroscopic spatiotemporal parameters during dynamic walking (Table 2, S1 Fig).

Download:

• PNG
  larger image
• TIFF
  original image

Table 2. Comparison of gait parameters between unilateral flatfoot and bilateral flatfoot.

https://doi.org/10.1371/journal.pone.0358183.t002

S1 Fig shows the comparison results of mean velocity, stride frequency, and step width in sequence using violin plots, with no statistically significant differences.

Spatiotemporal parameters at the limb level

At the limb level, the individual limb was utilized as the analytical unit to compare gait performance between normal feet and flatfeet, following adjustment for relevant covariates using linear mixed-effects models (LMMs). No statistically significant differences were observed in any spatiotemporal parameters across temporal, spatial, or gait cycle phase proportions (P > 0.05). This indicates that during level walking, the flatfoot deformity does not directly alter the temporal rhythm or spatial span of the ipsilateral limb within the fundamental gait cycle. Consequently, the macroscopic spatiotemporal execution capacity of the affected lower extremity remains preserved (Table 3, S2 Fig).

Download:

• PNG
  larger image
• TIFF
  original image

Table 3. Comparison of spatio-temporal parameters between normal foot and flat foot.

https://doi.org/10.1371/journal.pone.0358183.t003

S2 Fig shows the comparison results of temporal parameters, spatial parameters, and gait cycle in sequence using bar charts, with no statistically significant differences.

Static joint alignment at the limb level

During static stance, the three-dimensional (3D) kinematic characteristics of the lower extremity joints were compared between the normal and flatfoot sides. After adjustment via linear mixed-effects models (LMMs), both groups exhibited an external rotation posture in the transverse plane (with internal rotation defined as positive and external rotation as negative). Specifically, the static Hip Rotation angle on the flatfoot side (−2.11° ± 0.67°) demonstrated significantly less external rotation than that on the normal side (−5.79° ± 1.79°), representing a statistically significant between-group difference (P = 0.043). No significant differences were observed between the two groups in the static joint angles of the hip, knee, or ankle in other planes (P > 0.05; Table 4, S3 Fig).

Download:

• PNG
  larger image
• TIFF
  original image

Table 4. Comparison of static joint angles between normal foot and flat foot.

https://doi.org/10.1371/journal.pone.0358183.t004

S3 Fig shows the comparison results of static joint angles using bar charts, and significant differences were found in Hip Rotation angle (P < 0.05).

Gait deviation and variability analysis

LMM analysis revealed no significant differences in comprehensive gait indices (i.e., GDI and GPS) between the unilateral and bilateral involvement groups (P > 0.05). However, the Hip Abduction/Adduction GVS was significantly higher on the flatfoot side (5.40° ± 0.14°) compared to the normal side (4.45° ± 0.43°; P = 0.032). No other significant between-group differences were observed in the GVS for kinematic variables across the hip, knee, or ankle joints (P > 0.05; Table 5, S4 Fig).

Download:

• PNG
  larger image
• TIFF
  original image

Table 5. Comparison of gait variability between normal foot and flat foot.

https://doi.org/10.1371/journal.pone.0358183.t005

S4 Fig shows the comparison results of GPS, GDI, and GVS of each joint. Significant differences were observed in Hip Abduction/Adduction GVS (P < 0.05).

Discussion

This study employed a dual-level analytical strategy integrating subject- and limb-specific data to characterize the relationship between pediatric FFT and lower-extremity biomechanics while controlling for developmental covariates. The principal finding was not a generalized impairment of walking performance, but rather a dissociation between preserved macroscopic gait characteristics and selective proximal kinematic alterations. Major spatiotemporal parameters remained comparable between groups and between limbs, whereas relative hip internal rotation during static stance and increased Hip Abduction/Adduction GVS during walking were identified on the affected side. This distinction is clinically relevant because apparently normal walking speed, cadence, and stride characteristics may coexist with more localized changes in lower-extremity alignment and movement.

All core spatiotemporal parameters, including Mean Velocity, Frequency, Stride Length, and gait-cycle phase proportions, showed no significant differences (P > 0.05) at either the subject or limb level. Rather than indicating an absence of biomechanical influence, these findings suggest that pediatric FFT does not necessarily produce detectable changes in global walking performance during self-selected gait. Previous studies have shown that proximal joint kinematics and inter-articular coordination may be redistributed when peripheral muscle function or arch support mechanisms are altered [22–24]. The present kinematic findings are compatible with such an adaptive response; however, because muscle activation and joint kinetics were not measured, the underlying neuromuscular control strategy cannot be determined directly from the current data. Therefore, preservation of macroscopic gait should be interpreted as a functional observation rather than as evidence of a specific central compensatory mechanism.

The static analysis provides further evidence that the biomechanical influence of FFT may extend beyond the foot and ankle. During standing, the affected hip showed a significant shift toward relative internal rotation (−2.11°) compared with the unaffected limb (−5.79°). Souza et al. [25] previously reported temporal coupling between hindfoot pronation and hip internal rotation during gait. In this context, the present findings extend the observation of distal–proximal coupling to static alignment and suggest that altered foot posture may coexist with changes in proximal joint orientation even before dynamic gait demands are considered. Importantly, the absolute angular difference observed in this study was modest. It should therefore not be interpreted in isolation as evidence of pathological hip dysfunction or as an indication for intervention. Rather, it may represent a subtle alteration in lower-extremity alignment that warrants consideration together with dynamic gait findings.

During walking, the affected limb demonstrated a significantly increased Hip Abduction/Adduction GVS (P = 0.032), indicating greater deviation in frontal-plane hip kinematics. Static structural malalignment has previously been associated with dynamic functional alterations [10], while femoral internal rotation and hip adduction have been biomechanically linked to dynamic knee valgus patterns [26,27]. Hip abductors such as the gluteus medius play an important role in frontal-plane pelvic and hip stability during single-limb support [28]. Accordingly, the combination of altered static hip orientation and increased frontal-plane GVS observed in this study provides evidence of a localized proximal kinematic alteration associated with FFT. However, GVS quantifies kinematic deviation and does not directly measure muscle activation, joint loading, or neuromuscular effort. Therefore, the present findings cannot establish that greater hip-abductor activation or increased mechanical loading actually occurred.

The relatively small magnitude of the observed angular changes also requires careful clinical interpretation. In growing children, a statistically detectable difference does not necessarily represent clinically important dysfunction, particularly when conventional spatiotemporal gait parameters remain unaffected. Nevertheless, the coexistence of altered static hip alignment and increased dynamic hip kinematic deviation suggests that localized biomechanical changes may be detectable before obvious deterioration in gross walking performance becomes apparent. The clinical implication is therefore not that children with FFT require intervention solely on the basis of these small angular differences, but that assessment limited to foot morphology or global gait parameters may overlook proximal movement characteristics. Incorporating static lower-extremity alignment and hip kinematic assessment may provide complementary information when evaluating children with FFT, particularly during follow-up.

Despite altered static hip alignment and elevated dynamic frontal-plane GVS, the absence of significant differences in macroscopic spatiotemporal parameters is consistent with the substantial adaptability of pediatric locomotion described previously [29]. However, the present cross-sectional kinematic data cannot determine whether the observed pattern represents an active neuromuscular compensatory strategy, nor can they establish whether these alterations persist, progress, or contribute to future musculoskeletal injury. Previous studies have associated persistent lower-extremity malalignment and altered joint mechanics with cumulative musculoskeletal loading and degenerative changes [13,30,31], providing a rationale for further investigation. These studies should not, however, be taken as evidence that the children examined here are at increased long-term injury or degenerative risk. Longitudinal follow-up incorporating kinetic and EMG measurements will be necessary to determine the functional consequences and prognostic relevance of the kinematic differences observed in the present study.

Overall, the value of the present study lies less in demonstrating gross gait dysfunction than in identifying a pattern in which pediatric FFT is associated with selective proximal alignment and kinematic differences despite preserved macroscopic walking characteristics. This provides a complementary perspective to previous studies of flatfoot biomechanics by jointly considering static alignment, dynamic gait, and affected–unaffected limb differences within the same analytical framework. Clinically, these findings support a broader kinetic-chain assessment of pediatric FFT while emphasizing that small kinematic deviations should be interpreted cautiously and in conjunction with symptoms, function, and longitudinal changes.

Conclusion

This study showed that pediatric FFT was associated with selective proximal biomechanical alterations, including relative hip internal rotation during static stance and increased frontal-plane hip kinematic deviation during walking, despite largely preserved macroscopic spatiotemporal gait parameters. These findings suggest that the biomechanical influence of FFT may extend beyond the foot and ankle and may not be fully captured by conventional gait measures alone. However, the observed angular differences were modest, and the present kinematic data do not establish specific neuromuscular compensatory mechanisms or long-term injury risk. Static lower-extremity alignment and dynamic hip kinematics may therefore provide complementary information in the clinical assessment and follow-up of pediatric FFT. Longitudinal studies incorporating kinetic and EMG measurements are needed to determine the clinical and prognostic significance of these alterations.

Supporting information

Acknowledgments

We would like to thank all the patients who kindly gave their time and cooperation for their participation in this research. Their valuable involvement was essential to the completion of this work and the analysis of relevant clinical outcomes.

Generative AI statement: The author(s) declared that generative AI was not used in the creation of this manuscript.