Abstract
Objective
Adolescent Idiopathic Scoliosis (AIS) is an idiopathic three-dimensional spinal deformity that predominantly affects adolescents during their peak growth period. This study aimed to understand the foot plantar pressure characteristics of individuals with Lenke 3 and 6 type AIS during standing and walking.
Method
This case-control study enrolled 84 adolescent females aged 10 to 18 years. Based on the coronal plane imaging findings of standing full-spine radiographs, the individuals were classified into three groups according to the Lenke classification system: the Lenke 3 AIS group (n = 28, right thoracic-left lumbar double curves, primary thoracic), the Lenke 6 AIS group (n = 28, left thoracic-right lumbar double curves, primary thoracolumbar), and the control group (n = 28, healthy adolescents free from AIS). The MatScan™ pressure mat was employed to collect the plantar pressure and center of pressure (COP) characteristics of the subjects during bipedal standing, as well as the temporal characteristics of ground reaction forces and foot impulse characteristics during walking. Subsequently, we analyzed the inter - group differences in plantar pressure parameters across the three cohorts.
Results
The Lenke 3 AIS group showed a significant decrease in forefoot pressure compared to the control group. In contrast, rearfoot pressure, peak pressure, and average pressure in the left foot increased significantly. Additionally, the COP characteristic exhibits significant differences (P < 0.05). The Lenke 6 AIS group showed significant differences compared to the control group in the rear foot pressure, peak pressure, and average pressure of the left and right feet (P < 0.05). Compared with the Lenke 3 AIS group, the Lenke 6 AIS group exhibited significantly increased pressure in the right foot and forefoot. During gait, the Lenke 6 AIS group heel impulse decreased significantly while the midfoot impulse increased significantly (P < 0.05).
Conclusion
The Lenke 6 group exhibited more complex changes in plantar pressure. Both groups showed a decline in balance ability, but the Lenke 6 group showed a more severe decline, likely attributable to the larger lumbar curvature angle. Furthermore, both groups exhibited significant alterations in key biomechanical parameters, including energy absorption efficiency during ground contact and propulsive efficiency during push-off.
Keywords: Adolescent idiopathic scoliosis, Plantar pressure, Center of pressure, Foot impulse
Introduction
Adolescent idiopathic scoliosis (AIS) is a common three-dimensional deformity of the spinal column characterized by lateral curvature of the spine (Cobb angle ≥ 10°) and vertebral rotation in the absence of identifiable etiologies such as neuromuscular disorders, congenital anomalies, or syndromic associations [1]. It predominantly affects adolescents, accounting for approximately 80% of all scoliosis cases, with a higher prevalence in females, especially during the pubertal growth spurt [1]. The progression of spinal curvature in AIS not only leads to cosmetic deformities that may cause psychological distress and social withdrawal in affected individuals but also poses potential risks of impaired pulmonary function and chronic back pain in severe cases [2–4]. Given its high incidence and enduring impacts on physical and mental health, AIS remains a primary focus of clinical research and rehabilitation, with the academic community continuously striving to elucidate its pathogenic mechanisms to facilitate targeted rehabilitation treatments [5].
Studies have shown that individuals with AIS exhibit abnormal physiological positions of the spine and pelvis [6]. To maintain postural balance, this may lead to increased compensatory loads on the lower limb joints, which in turn induce alterations in the Mechanical Axis of the Lower Limb and the Anatomical Axis of the Lower Limb, ultimately resulting in abnormal plantar pressure distribution in AIS individuals [7]. Furthermore, long-term foot pressure imbalance and uneven distribution, via the body’s negative feedback regulatory mechanism, alter the spine’s original relative position, induce abnormal sensory input, and consequently exacerbate scoliosis, forming a vicious cycle [8, 9]. Consequently, comprehending the plantar pressure distribution characteristics in AIS individuals during standing and walking is critical for guiding their screening, treatment, and rehabilitation assessment [10].
Walking is one of the most prevalent motor behaviors in human daily activities. Through the course of evolution, humans have developed a functional and efficient pattern of bipedal locomotion to facilitate ambulation [11]. Studies have demonstrated that AIS can impair spinal mobility and trunk balance, alter the body’s center of gravity during gait, and consequently induce gait abnormalities [12]. Kinetic parameters during gait have thus attracted substantial research attention. Current studies have indicated that no significant differences exist between healthy individuals and AIS individuals in terms of the numerical characteristics (i.e., vertical and anteroposterior peak values) and curve symmetry of ground reaction forces (GRF) [13–15]. However, Chu et al. reported that the gait asymmetry associated with both magnitude and time variables of GRF was correlated with pelvic tilt and the severity of spinal deformities [16]. Therefore, further investigations are required to elucidate the temporal characteristics of GRF in AIS individuals.
However, current AIS research has limitations. Despite the growing body of literature exploring the pathogenesis and clinical management of AIS, three critical research gaps remain to be addressed, forming the core rationale for the present study. First, there exists a marked imbalance in research focus: most studies have predominantly focused on Lenke types 1 and 5 (single curves) [9, 17–19], with fewer addressing types 3 and 6 (double curves) [20]. Lenke 3 and 6 AIS often arise from other scoliosis types and represent relatively stable spinal deformities, consisting of primary and secondary curves: the primary curve is the most severe spinal curvature, while the smaller secondary curve primarily compensates for it, establishing a new bodily balance [21]. Precisely due to this “pseudo-balance” state, this unique biomechanical characteristic renders such double-curve cases easily overlooked in clinical and basic research, leaving the underlying mechanisms governing their formation and progression largely unelucidated. Second, the poor homogeneity of research subjects undermines the reliability of study outcomes: many existing studies enroll AIS individuals across different Lenke classifications without strict stratification [20, 22]. This study design leads to substantial heterogeneity within the research cohort, as single-curve and double-curve deformities differ fundamentally in terms of spinal biomechanical characteristics, progression patterns, and clinical phenotypes. The lack of subject homogeneity inevitably impairs the internal validity of research findings, making it difficult to draw precise conclusions about specific AIS subtypes. Finally, insufficient consideration of gender factors and deficiencies in research quality control represent additional limitations: AIS is a condition with a significant gender predisposition, yet some relevant studies fail to incorporate gender as a key stratification variable in their experimental design [23, 24]. These limitations restrict the external validity and generalizability of existing research results, and also impede the development of personalized intervention strategies for AIS individuals.
Against this backdrop, the present study focuses on the under-investigated Lenke type 3 and 6 AIS subtypes. We strictly control subject homogeneity by enrolling only female individuals with double-curve deformities. Our study enrolls adolescent females with Lenke type 3 (right thoracic-left lumbar double curves, primary thoracic) and type 6 (left thoracic-right lumbar double curves, primary thoracolumbar) AIS as subjects. A plantar pressure testing system is utilized to collect plantar pressure distribution characteristics during standing and walking. The research questions addressed are: (1) Do plantar pressure characteristics differ between Lenke 3 AIS individuals and healthy adolescents? (2) Do they differ between Lenke 6 AIS individuals and healthy adolescents? (3) Do they differ between Lenke 3 and 6 AIS individuals?
Subjects and methods
Subjects
Based on previous studies [6], the required sample size for this study was calculated using G*Power software (Version 3.1.9.7). The parameter settings were configured as follows: the Test family was specified as F tests; the Statistical test was set to ANOVA: Fixed effects, omnibus, one-way; and the Type of power analysis was defined as A priori: Compute required sample size - given α, power, and effect size. For the input parameters, an effect size f of 0.40, a Type I error rate (α err prob) of 0.05, a statistical power (1 − β err prob) of 0.80, and a total of 3 study groups were designated. Based on these settings, the minimum total sample size required for the study was calculated to be 66 individuals. Considering a potential 15% sample dropout rate commonly reported in AIS. Accordingly, a total of 78 individuals (26 individuals per group) were planned to be enrolled in the present study to ensure that the effective sample size met the statistical requirements after accounting for possible attrition. The recruitment period for this study commenced on 01/03/2024 and concluded on 01/06/2025. Ultimately, this case-control study enrolled 84 adolescent females. This study enrolled 28 Lenke 3 AIS individuals(n = 30; age:13.90 ± 2.58years; height:160.10 ± 10.20 cm; weight:45.87 ± 5.53 kg), 28 Lenke 6 AIS individuals(n = 30;age:13.83 ± 2.59years; height:159.84 ± 10.14 cm; weight:45.70 ± 6.55 kg), and 28 healthy adolescents(n = 30; age:14.10 ± 2.52years; height:160.07 ± 10.02 cm; weight:46.03 ± 7.16 kg). Inclusion criteria for AIS: female; 10–18 years old; right-footed; Lenke 3/6 scoliosis; Cobb angle 20°–45°; no limb impairment/deformity; normal foot type. Inclusion criteria for healthy adolescents: female; 10–18 years old; right-footed; normal growth/development; no scoliosis; normal foot type. All individuals had not received any brace treatment or exercise therapy. The study was approved by the Ethics Committee of Shanxi University (IRB Approval Number: SXULL2024081) prior to initiation. All adolescents and their legal guardians signed informed consent forms prepared in accordance with the Declaration of Helsinki and relevant regulations.
Methods
Pre-experiment screening procedures
To avoid interference from abnormal foot morphology [9], the subjects’ foot arch index was acquired using a 3D foot scanner (iFoot 3D Foot Scanner, China). The arch index (AI), which is closely associated with foot posture and morphology, is used to assess whether the foot presents an abnormal posture and is defined as the ratio of the midfoot contact area to the total foot area when the subject is in a relaxed standing position; specifically, an AI value of less than 0.21 indicates a high arch, with 0.15 < AI < 0.20 denoting a mild high arch and AI < 0.15 denoting a severe high arch, an AI value ranging from 0.21 to 0.26 indicates a normal arch, and an AI value greater than 0.26 indicates a flat foot, with 0.26 < AI < 0.35 denoting a mild flat foot and AI > 0.35 denoting a severe flat foot [25]. Only subjects with normal foot types were included after screening.
Based on the screening protocol for idiopathic scoliosis established by the International Society on Scoliosis Orthopaedic and Rehabilitation Treatment (SOSORT), the process is as follows:
Step 1: Initial Physical Examination
The examiner observes the standing participant from the front, back, and side. Key indicators for assessment include uneven shoulder heights, abnormal scapular positioning, asymmetry of the waistline, and truncal shift. This is followed by the Adam’s Forward Bend Test. For this test, the participant stands with feet together, knees fully extended, and bends forward at the hips to approximately 90 degrees, allowing the arms to hang freely. The examiner observes the participant’s back from behind. A positive test is noted if a “rib hump” is observed, where one side of the back appears conspicuously higher than the other. This finding suggests spinal scoliosis with vertebral rotation. Individuals who present with postural asymmetries or a positive Adam’s test are categorized into the potential AIS group, while those without these findings are assigned to the control group. To minimize systematic error, all physical tests were performed by the same investigator.
Step 2: Instrument Measurement and Radiographic Confirmation
For subjects in the potential AIS group, a scoliometer is used to quantify the Angle of Trunk Rotation (ATR) in the forward-bending position. If the ATR measurement is less than 5°, the screening for the individual is terminated, no radiographical examination is required, and the individual is assigned to the control group. If the ATR measurement exceeds 5°, individuals should undergo radiographical examination, including four views: Upright Coronal Radiograph, Upright Sagittal Radiograph, Left Supine Bending Radiograph, and Right Supine Bending Radiograph. Based on the coronal plane findings of radiographs, to effectively reduce individual subjective errors, independent blinded measurements were performed by three orthopedic surgeons, and the mean value of the three measurements was finally adopted as the Cobb angle of the individual.
Step 3: Group Classification
Three orthopedic surgeons with physical therapy qualifications identified the primary and compensatory curves in individuals with a Cobb angle ranging from 20° to 45°, completed the Lenke classification of AIS by integrating indicators including vertebral rotation and sagittal plane parameters, assigned double curves (right thoracic and left lumbar, with a primary thoracic curve) to the Lenke type 3 AIS group, and categorized double curves (left thoracic and right lumbar, with a primary thoracolumbar curve) into the Lenke type 6 AIS group.
Plantar pressure testing
Plantar pressure data were collected using a 500 mm × 500 mm MatScan™ pressure mat (Tekscan, USA) with semiconductor thin-film sensors (density: 4 sensors/cm²) and a 100 Hz sampling frequency. Prior to formal testing, all subjects familiarized themselves with the procedure. Tests included: (1) Bipedal static standing: subjects stood naturally on the platform facing forward to collect left/right foot pressure distribution; (2) Bilateral COP Measurement: Participants stood on the platform for 15 s, and COP data were collected to assess balance ability; (3) Gait Test: Participants walked naturally and placed their feet on the platform sequentially. Temporal characteristics of the ground reaction force curve and foot impulse data are collected. Each test was repeated 3 times, and averages were used for analysis.
Testing indicators
Plantar pressure distribution characteristics in bipedal standing
Left foot pressure distribution percentage; right forefoot pressure distribution percentage; pressure distribution percentages of 10 foot subregions; average plantar pressure; peak plantar pressure. Collected data were normalized to body weight to eliminate inter-subject variability. See Fig. 1 for the plantar pressure distribution zoning diagram of both feet.
Fig. 1.
Zoning diagram of the pressure of both plantar feet. Note: The red dot divides the left and right feet; the yellow dot divides the forefoot and hindfoot. A1: Medial heel region; A2: Lateral heel region; A3: Medial midfoot region; A4: Lateral midfoot region; A5: First metatarsal region; A6: Second metatarsal region; A7: Third, fourth, and fifth metatarsal regions; A8: First toe region; A9: Second toe region; A10: Third, fourth, and fifth toe regions
COP characteristics
Trajectory length of COP; Swing area of 95% confidence ellipse of COP trajectory.; Maximum displacement of the COP in the anterior-posterior direction; Maximum displacement of the COP in the medio-lateral direction, see Fig. 2.
Fig. 2.
Trajectory of the center of pressure (COP) during 15 s of static standing. The gray dots represent the sequential COP positions over 15 s. Key kinematic parameters are labeled as follows: COP-TL (A→B): Trajectory length of the COP. COP-SA: Swing area of 95% confidence ellipse of COP trajectory. COP-APMD: Maximum displacement of the COP in the anterior-posterior direction. COP-MLMD: Maximum displacement of the COP in the medio-lateral direction
Temporal characteristics of the ground reaction force
The percentage of the first peak of ground reaction force in the stance phase (GRF First Peak % of Stance Phase); The percentage of the trough of ground reaction force in the stance phase (GRF Trough % of Stance Phase); The percentage of the second peak of ground reaction force in the stance phase (GRF Second Peak % of Stance Phase).
Foot impulse
The plantar pressure plate divides the plantar area into several discrete pressure units, and the discrete integral method is adopted for calculation, with the formula as follows.
 |
Total impulse of the target plantar region.
The 
-th discrete pressure unit of the plantar pressure distribution system.
Total number of sampling points during the gait contact phase.
Average pressure of the 
-th pressure unit within the sampling interval Δt.
Effective contact area of the 
-th pressure unit.
: Sampling time interval of the pressure sensor
In the present study, we collected heel impulse, midfoot impulse and metatarsal impulse.
Statistical analysis
Bilateral foot data of the subjects were collected in this study. Raw data were entered using Microsoft Excel 2019, and statistical analyses were performed with SPSS 26.0 software. A significance level of α = 0.05 was set, indicating that a P-value < 0.05 was considered statistically significant.
The Shapiro-Wilk test was first used to verify data normality, and the Levene test was employed for homogeneity of variance. If the data met both normality and variance homogeneity assumptions, one-way analysis of variance (ANOVA) was applied to compare the overall means among different groups. When the one-way ANOVA results showed a statistically significant difference between groups (P < 0.05), the Bonferroni test was further conducted for post-hoc multiple comparisons.
All statistical results were expressed as “mean ± standard deviation (x̄±s)”. The results of one-way ANOVA were presented with F-value, P-value, and eta-squared (η²). The results of multiple comparisons were labeled using the letter marking method: under the same indicator, the same letter marked on two groups of data indicates no significant difference (P > 0.05), while different letters indicate a statistically significant difference (P < 0.05).
Paired-samples t-tests were used for intra-group comparisons between the left and right feet, and the analysis results were reported with t-value, P-value, and Cohen’s d.
Result
Individual characteristics
A total of 84 female individuals were recruited in this study and divided into three groups with 28 individuals per group, namely the moderate Lenke type 3 AIS group, the moderate Lenke type 6 AIS group, and the control group. The basic demographic indicators and foot morphological indicators of the individuals across the three groups were generally well-balanced. All individuals exhibited balanced baseline characteristics, with good comparability among the groups(P>0.05). Results are presented in Table 1.
Table 1.
Comparative analysis of general data of subjects
| Indicators | Moderate Lenke 3-type AIS Group | Moderate Lenke 6-type AIS Group | Control Group |
|---|---|---|---|
| Number of individuals (n) | 28 | 28 | 28 |
| Gender (M/F) | F | F | F |
| Age (years) | 13.90 ± 2.58 | 13.83 ± 2.59 | 14.10 ± 2.52 |
| Height (cm) | 160.10 ± 10.20 | 159.84 ± 10.14 | 160.07 ± 10.02 |
| Weight (kg) | 45.87 ± 5.53 | 45.70 ± 6.55 | 46.03 ± 7.16 |
| BMI | 18.52 ± 0.84 | 18.57 ± 0.76 | 18.41 ± 0.85 |
| Cobb angle (°) | 27.20 ± 4.43 | 25.20 ± 4.76 | None |
| Left foot length (cm) | 24.19 ± 1.65 | 23.95 ± 1.54 | 24.45 ± 1.43 |
| Right foot length (cm) | 24.19 ± 1.65 | 23.85 ± 1.45 | 24.34 ± 1.65 |
Abbreviations: AIS Adolescent idiopathic scoliosis, BMI Body mass index
Plantar pressure characteristics
The results of the plantar pressure characteristics are presented in Tables 2 and 3.
Table 2.
Intra-group comparisons of plantar pressure characteristics
| Left/Right Foot | ||||||
|---|---|---|---|---|---|---|
| Indicators | Group | Left | Right | t | P | Cohen’s d |
| Total foot pressure (%) | Control | 51.50 ± 8.25 | 49.18 ± 8.57 | 1.950 | 0.058 | 0.30 |
| Moderate Lenke 3-type AIS | 53.02 ± 8.18 | 47.02 ± 7.88 | 2.906 | 0.005 | 0.32 | |
| Moderate Lenke 6-type AIS | 50.69 ± 8.42 | 51.01 ± 8.67 | -0.371 | 0.711 | 0.03 | |
| Forefoot pressure (%) | Control | 22.60 ± 3.04 | 20.49 ± 8.20 | 0.107 | 0.916 | 0.02 |
| Moderate Lenke 3-type AIS | 17.45 ± 9.07 | 15.10 ± 8.83 | 3.032 | 0.003 | 0.35 | |
| Moderate Lenke 6-type AIS | 19.21 ± 9.56 | 19.43 ± 6.58 | -1.024 | 0.310 | 0.12 | |
| Hindfoot pressure(%) | Control | 77.20 ± 3.04 | 77.72 ± 12.86 | -0.107 | 0.916 | 0.02 |
| Moderate Lenke 3-type AIS | 80.77 ± 9.63 | 84.37 ± 8.67 | -2.245 | 0.028 | 0.26 | |
| Moderate Lenke 6-type AIS | 82.65 ± 9.10 | 84.80 ± 8.52 | -1.596 | 0.113 | 0.15 | |
| Peak plantar pressure (kg/cm2) | Control | 2.55 ± 0.26 | 2.62 ± 0.60 | -0.403 | 0.690 | 0.07 |
| Moderate Lenke 3-type AIS | 2.89 ± 0.69 | 2.38 ± 0.62 | 3.332 | 0.001 | 0.4 | |
| Moderate Lenke 6-type AIS | 2.85 ± 0.72 | 2.49 ± 0.78 | 2.832 | 0.006 | 0.27 | |
| Average plantar pressure (kg/cm2) | Control | 0.63 ± 0.10 | 0.66 ± 0.19 | -1.069 | 0.291 | 0.17 |
| Moderate Lenke 3-type AIS | 0.76 ± 0.18 | 0.70 ± 0.21 | 1.376 | 0.173 | 0.16 | |
| Moderate Lenke 6-type AIS | 0.70 ± 0.18 | 0.65 ± 0.21 | 2.024 | 0.045 | 0.18 | |
| Medial heel region(%) | Control | 49.03 ± 17.75 | 40.82 ± 14.71 | 3.084 | 0.004 | 0.48 |
| Moderate Lenke 3-type AIS | 42.56 ± 15.28 | 37.13 ± 15.64 | 3.522 | 0.001 | 0.39 | |
| Moderate Lenke 6-type AIS | 43.42 ± 18.07 | 39.13 ± 14.49 | 3.124 | 0.002 | 0.28 | |
| Lateral heel region(%) | Control | 16.75 ± 11.14 | 29.56 ± 12.23 | -5.690 | 0.000 | 0.87 |
| Moderate Lenke 3-type AIS | 25.77 ± 8.64 | 25.52 ± 11.38 | 0.500 | 0.619 | 0.06 | |
| Moderate Lenke 6-type AIS | 25.28 ± 8.44 | 28.43 ± 11.84 | -2.819 | 0.006 | 0.26 | |
| Medial midfoot region(%) | Control | 0.92 ± 0.44 | 0.74 ± 0.33 | 2.317 | 0.103 | 0.19 |
| Moderate Lenke 3-type AIS | 1.68 ± 0.24 | 0.91 ± 0.41 | 8.903 | 0.000 | 1.63 | |
| Moderate Lenke 6-type AIS | 1.15 ± 0.71 | 0.86 ± 0.56 | 0.069 | 0.946 | 0.02 | |
| Lateral midfoot region(%) | Control | 2.46 ± 1.24 | 3.00 ± 2.37 | -0.668 | 0.518 | 0.19 |
| Moderate Lenke 3-type AIS | 3.72 ± 1.57 | 2.68 ± 1.42 | 1.985 | 0.058 | 0.39 | |
| Moderate Lenke 6-type AIS | 3.59 ± 1.07 | 1.33 ± 0.88 | 5.858 | 0.000 | 1.03 | |
| First metatarsal region(%) | Control | 5.69 ± 1.23 | 4.62 ± 1.85 | 2.069 | 0.046 | 0.34 |
| Moderate Lenke 3-type AIS | 5.04 ± 2.16 | 4.52 ± 2.54 | 0.938 | 0.352 | 0.12 | |
| Moderate Lenke 6-type AIS | 3.86 ± 1.68 | 3.22 ± 0.92 | 2.224 | 0.030 | 0.29 | |
| Second metatarsal region(%) | Control | 6.68 ± 1.43 | 5.24 ± 1.13 | 0.913 | 0.371 | 0.20 |
| Moderate Lenke 3-type AIS | 5.25 ± 2.17 | 5.78 ± 2.41 | -0.726 | 0.470 | 0.09 | |
| Moderate Lenke 6-type AIS | 4.28 ± 1.03 | 5.53 ± 2.44 | -2.447 | 0.016 | 0.25 | |
| Third, fourth, and fifth metatarsal regions(%) | Control | 7.98 ± 3.06 | 7.12 ± 3.77 | 1.549 | 0.131 | 0.26 |
| Moderate Lenke 3-type AIS | 4.42 ± 1.85 | 4.71 ± 2.83 | -0.582 | 0.563 | 0.07 | |
| Moderate Lenke 6-type AIS | 3.50 ± 1.35 | 4.63 ± 2.68 | -3.036 | 0.003 | 0.33 | |
| First toe region(%) | Control | 0.74 ± 0.67 | 1.77 ± 0.03 | -1.147 | 0.370 | 0.23 |
| Moderate Lenke 3-type AIS | 0.79 ± 0.39 | 1.83 ± 1.4 | -3.197 | 0.006 | 0.82 | |
| Moderate Lenke 6-type AIS | 1.09 ± 2.93 | 1.95 ± 1.42 | -0.008 | 0.994 | 0.02 | |
| Second toe region(%) | Control | 1.32 ± 0.76 | 1.52 ± 0.59 | -1.273 | 0.424 | 0.72 |
| Moderate Lenke 3-type AIS | 1.33 ± 0.53 | 1.23 ± 0.88 | -0.258 | 0.799 | 0.06 | |
| Moderate Lenke 6-type AIS | 1.36 ± 0.42 | 1.30 ± 0.52 | 0.971 | 0.340 | 0.18 | |
| Third, fourth, and fifth toe regions(%) | Control | 2.94 ± 1.14 | 1.18 ± 0.70 | 6.730 | 0.000 | 1.14 |
| Moderate Lenke 3-type AIS | 3.21 ± 1.31 | 0.76 ± 0.35 | 6.855 | 0.000 | 1.1 | |
| Moderate Lenke 6-type AIS | 2.90 ± 1.95 | 0.85 ± 0.50 | 6.458 | 0.000 | 0.9 | |
Table 3.
Intre-group comparisons of plantar pressure characteristics
| Indicators | Left/Right Foot | Group | Mean ± SD | F | P | η2 | Bonferroni |
|---|---|---|---|---|---|---|---|
| Total foot pressure (%) | Left | Control | 51.50 ± 8.25 | 1.253 | 0.288 | 0.010 | |
| Moderate Lenke 3-type AIS | 53.02 ± 8.18 | ||||||
| Moderate Lenke 6-type AIS | 50.69 ± 8.42 | ||||||
| Right | Control | 49.18 ± 8.57 | 6.115 | 0.003 | 0.050 | b | |
| Moderate Lenke 3-type AIS | 47.02 ± 7.88 | b | |||||
| Moderate Lenke 6-type AIS | 51.01 ± 8.67 | a | |||||
| Forefoot pressure (%) | Left | Control | 22.60 ± 3.04 | 4.181 | 0.017 | 0.037 | a |
| Moderate Lenke 3-type AIS | 17.45 ± 9.07 | ab | |||||
| Moderate Lenke 6-type AIS | 19.21 ± 9.56 | b | |||||
| Right | Control | 20.49 ± 8.20 | 8.180 | 0.000 | 0.081 | a | |
| Moderate Lenke 3-type AIS | 15.10 ± 8.83 | b | |||||
| Moderate Lenke 6-type AIS | 19.43 ± 6.58 | a | |||||
| Hindfoot pressure(%) | Left | Control | 77.20 ± 3.04 | 4.238 | 0.016 | 0.037 | b |
| Moderate Lenke 3-type AIS | 80.77 ± 9.63 | ab | |||||
| Moderate Lenke 6-type AIS | 82.65 ± 9.10 | a | |||||
| Right | Control | 77.72 ± 12.86 | 8.844 | 0.000 | 0.071 | b | |
| Moderate Lenke 3-type AIS | 84.37 ± 8.67 | a | |||||
| Moderate Lenke 6-type AIS | 84.80 ± 8.52 | a | |||||
| Peak plantar pressure (kg/cm2) | Left | Control | 2.55 ± 0.26 | 2.556 | 0.080 | 0.022 | |
| Moderate Lenke 3-type AIS | 2.89 ± 0.69 | ||||||
| Moderate Lenke 6-type AIS | 2.85 ± 0.72 | ||||||
| Right | Control | 2.62 ± 0.60 | 1.125 | 0.327 | 0.010 | ||
| Moderate Lenke 3-type AIS | 2.38 ± 0.62 | ||||||
| Moderate Lenke 6-type AIS | 2.49 ± 0.78 | ||||||
| Average plantar pressure (kg/cm2) | Left | Control | 0.63 ± 0.10 | 4.736 | 0.010 | 0.038 | b |
| Moderate Lenke 3-type AIS | 0.76 ± 0.18 | a | |||||
| Moderate Lenke 6-type AIS | 0.70 ± 0.18 | ab | |||||
| Right | Control | 0.66 ± 0.19 | 1.552 | 0.214 | 0.013 | ||
| Moderate Lenke 3-type AIS | 0.70 ± 0.21 | ||||||
| Moderate Lenke 6-type AIS | 0.65 ± 0.21 | ||||||
| Medial heel region(%) | Left | Control | 49.03 ± 17.75 | 2.112 | 0.123 | 0.017 | |
| Moderate Lenke 3-type AIS | 42.56 ± 15.28 | ||||||
| Moderate Lenke 6-type AIS | 43.42 ± 18.07 | ||||||
| Right | Control | 40.82 ± 14.71 | 0.888 | 0.413 | 0.007 | ||
| Moderate Lenke 3-type AIS | 37.13 ± 15.64 | ||||||
| Moderate Lenke 6-type AIS | 39.13 ± 14.49 | ||||||
| Lateral heel region(%) | Left | Control | 16.75 ± 11.14 | 16.361 | 0.000 | 0.119 | b |
| Moderate Lenke 3-type AIS | 25.77 ± 8.64 | a | |||||
| Moderate Lenke 6-type AIS | 25.28 ± 8.44 | a | |||||
| Right | Control | 29.56 ± 12.23 | 2.165 | 0.117 | 0.017 | ||
| Moderate Lenke 3-type AIS | 25.52 ± 11.38 | ||||||
| Moderate Lenke 6-type AIS | 28.43 ± 11.84 | ||||||
| Medial midfoot region(%) | Left | Control | 0.92 ± 0.44 | 6.191 | 0.005 | 0.241 | b |
| Moderate Lenke 3-type AIS | 1.68 ± 0.24 | a | |||||
| Moderate Lenke 6-type AIS | 1.15 ± 0.71 | ab | |||||
| Right | Control | 0.74 ± 0.33 | 1.939 | 0.156 | 0.085 | ||
| Moderate Lenke 3-type AIS | 0.91 ± 0.41 | ||||||
| Moderate Lenke 6-type AIS | 0.86 ± 0.56 | ||||||
| Lateral midfoot region(%) | Left | Control | 2.46 ± 1.24 | 2.832 | 0.065 | 0.067 | |
| Moderate Lenke 3-type AIS | 3.72 ± 1.57 | ||||||
| Moderate Lenke 6-type AIS | 3.59 ± 1.07 | ||||||
| Right | Control | 3.00 ± 2.37 | 10.965 | 0.000 | 0.184 | a | |
| Moderate Lenke 3-type AIS | 2.68 ± 1.42 | a | |||||
| Moderate Lenke 6-type AIS | 1.33 ± 0.88 | b | |||||
| First metatarsal region(%) | Left | Control | 5.69 ± 1.23 | 5.404 | 0.005 | 0.046 | a |
| Moderate Lenke 3-type AIS | 5.04 ± 2.16 | a | |||||
| Moderate Lenke 6-type AIS | 3.86 ± 1.68 | b | |||||
| Right | Control | 4.62 ± 1.85 | 8.356 | 0.000 | 0.089 | a | |
| Moderate Lenke 3-type AIS | 4.52 ± 2.54 | a | |||||
| Moderate Lenke 6-type AIS | 3.22 ± 0.92 | b | |||||
| Second metatarsal region(%) | Left | Control | 6.68 ± 1.43 | 10.527 | 0.000 | 0.092 | a |
| Moderate Lenke 3-type AIS | 5.25 ± 2.17 | b | |||||
| Moderate Lenke 6-type AIS | 4.28 ± 1.03 | c | |||||
| Right | Control | 5.24 ± 1.13 | 0.329 | 0.720 | 0.003 | ||
| Moderate Lenke 3-type AIS | 5.78 ± 2.41 | ||||||
| Moderate Lenke 6-type AIS | 5.53 ± 2.44 | ||||||
| Third, fourth, and fifth metatarsal regions(%) | Left | Control | 7.98 ± 3.06 | 35.195 | 0.000 | 0.265 | a |
| Moderate Lenke 3-type AIS | 4.42 ± 1.85 | b | |||||
| Moderate Lenke 6-type AIS | 3.50 ± 1.35 | b | |||||
| Right | Control | 7.12 ± 3.77 | 10.397 | 0.000 | 0.090 | a | |
| Moderate Lenke 3-type AIS | 4.71 ± 2.83 | b | |||||
| Moderate Lenke 6-type AIS | 4.63 ± 2.68 | b | |||||
| First toe region(%) | Left | Control | 0.74 ± 0.67 | 7.232 | 0.001 | 0.178 | ab |
| Moderate Lenke 3-type AIS | 0.79 ± 0.39 | b | |||||
| Moderate Lenke 6-type AIS | 1.09 ± 2.93 | a | |||||
| Right | Control | 1.77 ± 0.03 | 0.712 | 0.496 | 0.031 | ||
| Moderate Lenke 3-type AIS | 1.83 ± 1.4 | ||||||
| Moderate Lenke 6-type AIS | 1.95 ± 1.42 | ||||||
| Second toe region(%) | Left | Control | 1.32 ± 0.76 | 0.644 | 0.530 | 0.025 | |
| Moderate Lenke 3-type AIS | 1.33 ± 0.53 | ||||||
| Moderate Lenke 6-type AIS | 1.36 ± 0.42 | ||||||
| Right | Control | 1.52 ± 0.59 | 0.739 | 0.482 | 0.026 | ||
| Moderate Lenke 3-type AIS | 1.23 ± 0.88 | ||||||
| Moderate Lenke 6-type AIS | 1.30 ± 0.52 | ||||||
| Third, fourth, and fifth toe regions(%) | Left | Control | 2.94 ± 1.14 | 0.554 | 0.575 | 0.005 | |
| Moderate Lenke 3-type AIS | 3.21 ± 1.31 | ||||||
| Moderate Lenke 6-type AIS | 2.90 ± 1.95 | ||||||
| Right | Control | 1.18 ± 0.70 | 6.528 | 0.002 | 0.091 | a | |
| Moderate Lenke 3-type AIS | 0.76 ± 0.35 | b | |||||
| Moderate Lenke 6-type AIS | 0.85 ± 0.50 | b |
Table note: Same letters indicate no significant intergroup difference (P>0.05); different letters indicate a significant difference(P<0.05)
Intra-group comparisons showed that: Significant differences were observed in total foot pressure, forefoot pressure, hindfoot pressure, peak plantar pressure, medial heel region, medial midfoot region, first toe region and third, fourth, and fifth toe regions in the Lenke 3-type AIS group; in peak plantar pressure, average plantar pressure, medial heel region, lateral heel region, lateral midfoot region, first metatarsal region, second metatarsal region, third, fourth, and fifth metatarsal regions and third, fourth, and fifth toe regions in the Lenke 6-type AIS group; and in medial heel region, lateral heel region, first metatarsal region and third, fourth, and fifth toe regions in the control group.
Inter-group comparisons revealed that Compared with the control group, the Lenke 3-type AIS group showed significant differences in forefoot pressure, hindfoot pressure, average plantar pressure, lateral heel region, medial midfoot region, second metatarsal region, third, fourth, and fifth metatarsal regions and third, fourth, and fifth toe regions; the Lenke 6-type AIS group exhibited significant differences in total foot pressure, forefoot pressure, hindfoot pressure, lateral heel region, lateral midfoot region, first metatarsal region, second metatarsal region, third, fourth, and fifth metatarsal regions and third, fourth, and fifth toe regions. Compared with the Lenke 3-type AIS group, the Lenke 6-type AIS group had significant differences in total foot pressure, forefoot pressure, lateral midfoot region, first metatarsal region, second metatarsal region, and first toe region.
COP characteristics
Inter-group comparisons showed that, compared with the control group, the COP-TL, COP-SA, COP-APMD and COP-MLMD in the Lenke 3 type AIS group were all significantly increased. In the Lenke 6 type AIS group, the COP-TL, COP-SA, COP-APMD and COP-MLMD were also significantly increased. Compared with the Lenke 6 type AIS group, the COP-APMD and COP-MLMD in the Lenke 3 type AIS group were all significantly increased. The results are presented in Table 4.
Table 4.
Intre-group comparisons of COP characteristics
| Indicators | Group | Mean ± SD | F | P | η2 | Bonferroni |
|---|---|---|---|---|---|---|
| COP-TL (mm) | Control | 74.33 ± 6.29 | 3.451 | 0.034 | 0.037 | b |
| Moderate Lenke 3-type AIS | 76.81 ± 10.68 | a | ||||
| Moderate Lenke 6-type AIS | 79.58 ± 11.39 | a | ||||
| COP-SA (mm²) | Control | 34.15 ± 21.33 | 9.784 | 0.000 | 0.123 | b |
| Moderate Lenke 3-type AIS | 59.26 ± 21.38 | a | ||||
| Moderate Lenke 6-type AIS | 50.70 ± 30.54 | a | ||||
| COP-APMD (mm) | Control | 5.49 ± 1.56 | 5.292 | 0.014 | 0.022 | c |
| Moderate Lenke 3-type AIS | 6.83 ± 4.12 | a | ||||
| Moderate Lenke 6-type AIS | 6.31 ± 3.51 | b | ||||
| COP-MLMD (mm) | Control | 6.98 ± 2.12 | 5.815 | 0.004 | 0.059 | c |
| Moderate Lenke 3-type AIS | 8.65 ± 2.89 | a | ||||
| Moderate Lenke 6-type AIS | 8.09 ± 2.96 | b |
Table note: Same letters indicate no significant intergroup difference (P>0.05); different letters indicate a significant difference(P<0.05)
Temporal characteristics of the ground reaction force
Intra-group comparisons showed that the Lenke 6-type AIS group showed significant difference in the GRF Second Peak % of Stance Phase. Inter-group comparisons showed that Compared with the control group, the Lenke 3-type AIS group exhibited significant differences in the GRF Trough % of Stance Phase; the Lenke 6-type AIS group showed significant difference in the GRF Trough % of Stance Phase. The results are presented in Tables 5 and 6.
Table 5.
Intra-group comparisons of Temporal characteristics of the ground reaction force
| Indicators | Group | Left/Right Foot | t | P | Cohen’s d | |
|---|---|---|---|---|---|---|
| Left | Right | |||||
| GRF First Peak % of Stance Phase(%) | Control | 20.51 ± 6.42 | 20.47 ± 9.79 | 0.139 | 0.662 | 0.06 |
| Moderate Lenke 3-type AIS | 20.78 ± 6.82 | 21.58 ± 7.24 | -0.068 | 0.946 | 0.01 | |
| Moderate Lenke 6-type AIS | 21.90 ± 8.57 | 22.03 ± 7.78 | -0.071 | 0.944 | 0.01 | |
| GRF Trough % of Stance Phase(%) | Control | 40.74 ± 11.35 | 40.47 ± 12.52 | 0.298 | 0.922 | 0.01 |
| Moderate Lenke 3-type AIS | 45.44 ± 12.08 | 42.54 ± 13.76 | 1.574 | 0.121 | 0.20 | |
| Moderate Lenke 6-type AIS | 46.18 ± 12.32 | 45.02 ± 14.21 | 0.562 | 0.575 | 0.06 | |
| GRF Second Peak % of Stance Phase (%) | Control | 72.43 ± 9.48 | 73.77 ± 12.12 | -0.245 | 0.807 | 0.03 |
| Moderate Lenke 3-type AIS | 74.47 ± 9.56 | 76.46 ± 10.67 | -1.947 | 0.346 | 0.11 | |
| Moderate Lenke 6-type AIS | 73.45 ± 8.14 | 76.42 ± 9.59 | -2.655 | 0.009 | 0.25 | |
Table 6.
Intre-group comparisons of Temporal characteristics of the ground reaction force
| Indicators | Left/Right Foot | Group | Mean ± SD | F | P | η2 | Bonferroni |
|---|---|---|---|---|---|---|---|
| GRF First Peak % of Stance Phase(%) | Left | Control | 20.51 ± 6.42 | 0.763 | 0.468 | 0.007 | |
| Moderate Lenke 3-type AIS | 20.78 ± 6.82 | ||||||
| Moderate Lenke 6-type AIS | 21.89 ± 8.57 | ||||||
| Right | Control | 20.47 ± 9.79 | 0.610 | 0.544 | 0.006 | ||
| Moderate Lenke 3-type AIS | 21.58 ± 7.24 | ||||||
| Moderate Lenke 6-type AIS | 22.03 ± 7.78 | ||||||
| GRF Trough % of Stance Phase(%) | Left | Control | 40.74 ± 11.35 | 3.527 | 0.031 | 0.029 | b |
| Moderate Lenke 3-type AIS | 45.44 ± 12.08 | a | |||||
| Moderate Lenke 6-type AIS | 46.18 ± 12.32 | a | |||||
| Right | Control | 40.47 ± 12.52 | 2.090 | 0.126 | 0.018 | ||
| Moderate Lenke 3-type AIS | 42.54 ± 13.76 | ||||||
| Moderate Lenke 6-type AIS | 45.02 ± 14.21 | ||||||
| GRF Second Peak % of Stance Phase (%) | Left | Control | 72.43 ± 9.48 | 1.032 | 0.358 | 0.009 | |
| Moderate Lenke 3-type AIS | 74.47 ± 9.56 | ||||||
| Moderate Lenke 6-type AIS | 73.45 ± 8.14 | ||||||
| Right | Control | 73.77 ± 12.12 | 1.276 | 0.281 | 0.010 | ||
| Moderate Lenke 3-type AIS | 76.46 ± 10.67 | ||||||
| Moderate Lenke 6-type AIS | 76.42 ± 9.59 |
Table note: Same letters indicate no significant intergroup difference (P>0.05); different letters indicate a significant difference (P<0.05)
Foot impulse
Inter-group comparisons revealed that, compared to the control group, the heel impulse, midfoot impulse and metatarsal impulse of both feet in the Lenke 3 type AIS group were significantly decreased. In contrast, the midfoot impulse of both feet in the Lenke 6 type AIS group was significantly increased, while the heel impulse and metatarsal impulse decreased considerably. Compared with the Lenke 3 type AIS group, the heel impulse of both feet in the Lenke 6 type AIS group were both significantly increased. The results are presented in Tables 7 and 8.
Table 7.
Intra-group comparisons of foot impulse
| Left/Right Foot | ||||||
|---|---|---|---|---|---|---|
| Indicators | Group | Left | Right | t | P | Cohen’s d |
| Heel impulse (N•s) | Control | 228.18 ± 91.09 | 238.91 ± 75.26 | -0.034 | 0.973 | 0.04 |
| Moderate Lenke 3-type AIS | 174.93 ± 70.27 | 185.27 ± 96.93 | -0.245 | 0.807 | 0.03 | |
| Moderate Lenke 6-type AIS | 214.69 ± 95.24 | 225.21 ± 85.71 | -0.915 | 0.363 | 0.10 | |
| Midfoot impulse (N•s) | Control | 30.62 ± 19.51 | 25.68 ± 12.95 | 0.039 | 0.969 | 0.06 |
| Moderate Lenke 3-type AIS | 39.58 ± 28.17 | 28.88 ± 25.32 | 1.227 | 0.226 | 0.18 | |
| Moderate Lenke 6-type AIS | 50.65 ± 24.29 | 42.48 ± 25.71 | 1.298 | 0.204 | 0.23 | |
| Metatarsal impulse (N•s) | Control | 487.25 ± 139.58 | 489.81 ± 144.28 | -1.299 | 0.200 | 0.19 |
| Moderate Lenke 3-type AIS | 343.62 ± 74.35 | 338.03 ± 118.35 | 0.653 | 0.516 | 0.08 | |
| Moderate Lenke 6-type AIS | 355.24 ± 77.22 | 339.96 ± 125.57 | 0.661 | 0.510 | 0.07 | |
Table 8.
Intre-group comparisons of foot impulse
| Indicators | Left/Right Foot | Group | Mean ± SD | F | P | η2 | Bonferroni |
|---|---|---|---|---|---|---|---|
| Heel impulse (N•s) | Left | Control | 228.18 ± 91.09 | 6.257 | 0.002 | 0.050 | a |
| Moderate Lenke 3-type AIS | 174.93 ± 70.27 | c | |||||
| Moderate Lenke 6-type AIS | 214.69 ± 95.24 | b | |||||
| Right | Control | 238.91 ± 75.26 | 8.914 | 0.000 | 0.078 | a | |
| Moderate Lenke 3-type AIS | 185.27 ± 96.93 | c | |||||
| Moderate Lenke 6-type AIS | 225.21 ± 85.71 | b | |||||
| Midfoot impulse (N•s) | Left | Control | 30.62 ± 19.51 | 6.670 | 0.002 | 0.071 | b |
| Moderate Lenke 3-type AIS | 39.58 ± 28.17 | a | |||||
| Moderate Lenke 6-type AIS | 50.65 ± 24.29 | a | |||||
| Right | Control | 25.68 ± 12.95 | 6.544 | 0.002 | 0.081 | b | |
| Moderate Lenke 3-type AIS | 28.88 ± 25.32 | a | |||||
| Moderate Lenke 6-type AIS | 42.48 ± 25.71 | a | |||||
| Metatarsal impulse (N•s) | Left | Control | 487.25 ± 139.58 | 26.724 | 0.0000 | 0.196 | a |
| Moderate Lenke 3-type AIS | 343.62 ± 74.35 | b | |||||
| Moderate Lenke 6-type AIS | 355.24 ± 77.22 | b | |||||
| Right | Control | 489.81 ± 144.28 | 15.000 | 0.000 | 0.116 | a | |
| Moderate Lenke 3-type AIS | 338.03 ± 118.35 | b | |||||
| Moderate Lenke 6-type AIS | 339.96 ± 125.57 | b |
Table note: Same letters indicate no significant intergroup difference(P>0.05); different letters indicate a significant difference(P<0.05)
Discussion
Plantar pressure characteristics
Since the spinal structures responsible for postural regulation are impaired in AIS individuals, this results in asymmetric plantar pressure distribution between the two feet and concomitant balance dysfunction [9]. From a human biomechanical perspective, maintenance of upright bipedal stance requires the spine to remain free of scoliotic deformities in all anatomical planes; under such normal conditions, plantar pressure is theoretically distributed symmetrically across both feet [26]. In the present study, total plantar pressure was symmetrically distributed between the left and right feet across all three study groups, which is consistent with findings reported by Malfair et al. [21]. Notably, both the Lenke type 3 AIS group and the control group exhibited marginally higher plantar pressure on the left foot than on the right. In contrast, the Lenke type 6 AIS group displayed the opposite pressure profile. Combined with intergroup comparative analyses, our data reveal elevated right-foot pressure in the Lenke type 6 AIS cohort. We therefore infer that Lenke type 6 AIS is associated with spinal and lumbopelvic morphological alterations, which prompt pressure redistribution toward the concave side of the scoliotic curve as a compensatory biomechanical adaptation. The core biomechanical mechanism underlying this discrepancy lies in the distinct anatomical locations of the primary scoliotic curves: Lenke type 3 AIS is predominantly characterized by thoracic scoliosis. As the mechanical core of the upper trunk, thoracic deformity exerts its influence on plantar pressure distribution indirectly through shifts in the trunk’s center of gravity, thereby preserving the basic pressure pattern typical of right-handed individuals. In contrast, Lenke type 6 AIS is mainly associated with lumbar scoliosis. Adjacent to the lumbosacral joint, the lumbar spine directly connects the pelvis to the lower extremities; thus, its deformity directly disrupts lumbosacral force line transmission, compelling the body to elevate contralateral (right foot) pressure to counteract pelvic rotational imbalance induced by lumbar scoliosis.
Regarding forefoot and hindfoot pressure, all three groups exhibited a shared biomechanical characteristic: the hindfoot sustained a greater proportion of weight-bearing pressure, a pattern consistent with the biomechanics of human upright stance [27]. Relative to the control group, both Lenke type 3 and type 6 AIS individuals demonstrated reduced forefoot pressure alongside elevated hindfoot pressure. This pressure redistribution aligns with the findings of prior studies [28–30], indicating a tendency toward trunk backward tilt in AIS individuals. However, the present study further identified subtype-specific differences: both the Lenke type 3 AIS group and the control group exhibited elevated pressure in the left forefoot and right hindfoot, a characteristic pattern that was absent in the Lenke type 6 AIS group. This discrepancy indicates that thoracic scoliosis exerts a minimal impact on the interlimb pressure coordination pattern, whereas lumbar scoliosis completely disrupts this coordination. Specifically, the elevated pressure in the left forefoot and right hindfoot represents a physiological coordination pattern during upright stance in right-handed individuals, which facilitates center-of-gravity adjustment during upper limb movements. While thoracic scoliosis in Lenke type 3 AIS preserves this coordination, the pelvic mechanical disturbance induced by lumbar scoliosis in Lenke type 6 AIS forces the body to restructure the bipedal pressure coordination mode, resulting in the disappearance of this pattern. Collectively, these findings suggest that the plantar pressure distribution in Lenke type 3 AIS individuals—who present with predominant thoracic scoliosis—is more strongly associated with structural abnormalities of the thoracic spine than with those of the thoracolumbar junction.
Plantar peak pressure reflects the maximum load exerted on localized plantar regions [31]. whereas plantar mean pressure characterizes the overall pressure distribution patterns across distinct plantar regions during both static stance and dynamic locomotion [32]. Intergroup comparisons demonstrated that all AIS cohorts exhibited significantly higher plantar peak and mean pressures in the left foot relative to the control group; additionally, intragroup analyses revealed a consistent pattern of significantly elevated left-foot pressure compared with the right foot across all AIS subgroups. Given that this left-foot pressure elevation was uniformly observed in both Lenke type 3 and type 6 individuals—two classic double-curve AIS subtypes—these findings suggest that plantar peak and mean pressures are not contingent on the anatomical location of the primary scoliotic curve in double-curve AIS. Notably, abnormally elevated plantar peak and mean pressures in AIS individuals are likely to induce increased compensatory loading on lower limb joints [33], thereby ultimately contributing to gait abnormalities [34] and impaired balance function [35, 36]. Collectively, these observations underscore the necessity of routine monitoring for abnormal elevations in non-dominant foot plantar peak and mean pressures during AIS screening protocols, and highlight the value of incorporating this biomechanical parameter as a key reference for evaluating rehabilitation treatment efficacy.
In the present study, the AIS groups exhibited significant differences from the control group, with such differences characterized by Lenke subtype-specificity. During normal standing, the plantar region maintains balance relying on a stable triangular structure composed of the first metatarsal region, fifth metatarsal region, and heel [37]. While the control group showed left-right foot pressure asymmetry in specific regions, the Lenke type 6 AIS group displayed partial adjustments in this asymmetric pattern, manifested as the disappearance of pressure difference in the lateral heel region and the emergence of pressure difference in the first metatarsal region. This phenomenon may be associated with compensatory adjustments formed by the body to maintain spinal-pelvic mechanical balance, which is consistent with the common biomechanical compensatory characteristics of AIS individuals [9]. From the perspective of subtype differences, pressure abnormalities in the Lenke type 3 AIS group are mainly concentrated in the left foot, and pressure changes in the medial and lateral heel regions may be associated with the risk of ankle eversion [38], Elevated pressure in the medial and lateral midfoot regions may imply a potential association with pes planus (flatfoot), while reduced pressure in the second metatarsal region as well as the third, fourth, and fifth metatarsal regions may compromise the stability of the plantar triangular support structure, thereby impairing overall balance capacity. This aligns with the conclusions of existing studies that link plantar pressure abnormalities to trunk balance dysfunction in AIS individuals [19, 36, 39]. By contrast, the Lenke type 6 AIS group exhibited bilateral symmetric plantar pressure abnormalities: reduced pressure in the first, second metatarsal regions, and the third, fourth, and fifth metatarsal regions correlates with impaired static balance function, while elevated pressure in the first toe region may elevate the risk of hallux rigidus [40, 41], suggesting that double-curve AIS may be accompanied by more complex bilateral foot mechanical transmission abnormalities.
Collectively, our findings suggest that abnormal plantar pressure distribution in AIS individuals is not a mere local phenomenon, but more likely a manifestation of systemic mechanical imbalance caused by scoliosis in the foot [42]. It participates in regulating postural control and mechanical balance through compensatory or abnormal adjustments of regional plantar pressure [6]. This aligns with existing studies indicating a close association between plantar pressure abnormalities, compensatory spinal scoliosis, and lower limb mechanical transmission disorders [6, 43, 44]. However, the specific direction of action (i.e., causality) and core regulatory mechanisms remain to be explored, which can be validated via large-sample longitudinal studies and 3D biomechanical analyses in the future.
COP characteristics
Intergroup comparisons reveal that, relative to the control group, the Lenke type 3 AIS group exhibits a significant increase in COP-TL, COP-SA, COP-APMD, and COP-MLMD. Notably, a larger COP-SA reflects an elevated demand for neuromuscular control [17, 18, 20, 22]. The present study thus confirms that postural control is impaired in the Lenke type 3 AIS group, which is consistent with the findings of prior studies [20, 45]. The COP characteristics of the Lenke type 6 AIS group are comparable to those observed in the Lenke type 3 group [17]. Of note, compared with the Lenke type 3 AIS group, the Lenke type 6 AIS group shows significantly lower levels of COP-APMD and COP-MLMD. Regarding this manifestation, we hypothesize it arises from two potential mechanisms. First, this can be attributed to the distinct curve patterns of the two subtypes: Lenke type 3 AIS is primarily characterized by a thoracic curve, whereas Lenke type 6 AIS is dominated by a lumbar curve. As a core component of trunk stability, the lumbosacral musculoskeletal complex plays a pivotal role in maintaining static balance, and adaptive adjustments in this region may contribute to enhanced postural control [46]; Second, a reduced COP trajectory does not necessarily indicate improved balance; instead, it may reflect a rigid or restricted postural strategy. Therefore, further in-depth investigations are warranted in future studies to clarify the underlying mechanisms.
Temporal characteristics of the ground reaction force
Intergroup comparisons revealed that both AIS groups exhibited a significant increase in the GRF Trough % of Stance Phase for the left foot. This observation indicates a delay in the energy transition point from the cushioning to the propulsion phase, as well as a later onset of relative balance. Relative to the Lenke type 3 AIS group, the Lenke type 6 AIS group demonstrated a significantly higher level of the GRF Trough % of Stance Phase for the right foot—a characteristic that may serve as a key discriminator for individuals with Lenke type 6 AIS.
We hypothesize that this change is attributable to abnormal spinal curvature in AIS, which necessitates increased neuromuscular recruitment during the left foot stance phase to maintain relative bodily balance [47].
Foot impulse
Impulse is a physical quantity representing the cumulative effect of force acting on an object over time; during human walking, the foot is affected by ground reaction forces [48]. In this study, we find that compared with the control group, the Lenke type 3 AIS group shows significantly reduced heel impulse in both feet, indicating decreased energy absorption power during the cushioning phase [49]. The impact of ground reaction forces may be buffered by lower limb joints, thereby leading to injury. Significantly reduced metatarsal impulse indicates decreased force generation efficiency of lower limb muscles during forward propulsion of the body [50]. Furthermore, compared with the control group, the Lenke type 6 AIS group resembles type 3 individuals, but is distinguished by a significant increase in midfoot impulse. As a key balance structure during the stance phase of walking, increased impulse here may lead to flatfoot [51]. We hypothesize this change is a compensatory response to lumbar spinal imbalance. Moreover, compared with the Lenke type 3 AIS group, increased heel impulse in the Lenke type 6 AIS group indicates better landing cushioning capacity in these individuals [49], whereas increased midfoot impulse in both feet suggests a higher risk of flatfoot [51].
Summary
The impairment of postural control in AIS individuals stems primarily from systemic mechanical imbalance induced by spinal deformity. This imbalance is not an isolated phenomenon, but rather manifested through the coordinated changes in four key biomechanical parameters: plantar pressure, COP, temporal characteristics of GRF, and foot impulse.
The disruption of force transmission caused by spinal deformity constitutes the fundamental driver of postural control abnormalities in AIS. The Lenke subtype-specific patterns of plantar pressure distribution directly verify the dominant role of curve location in shaping postural compensatory strategies. Specifically, Lenke type 3 AIS is predominantly characterized by thoracic scoliosis. As the mechanical core of the upper trunk, thoracic deformity exerts an indirect impact on plantar pressure via trunk center-of-gravity shift, only leading to unilateral local pressure abnormalities. In contrast, Lenke type 6 AIS is dominated by lumbar scoliosis. The lumbar spine directly connects the pelvis and lower extremities, and its deformity directly disrupts lumbosacral force transmission, compelling the body to elevate right-foot pressure to compensate for pelvic rotational imbalance, which results in bilaterally symmetrical pressure abnormalities and an opposite pressure distribution pattern.
This subtype-specific difference is further validated by COP parameters, with Lenke type 3 and type 6 AIS individuals exhibiting distinct patterns in COP-APMD and COP-MLMD. Analysis of GRF temporal characteristics reveals that both AIS subgroups present with a delay in energy transition from the cushioning to the propulsion phase, accompanied by a delayed onset of relative balance. This finding is highly consistent with the trunk backward tilt tendency indicated by reduced forefoot pressure and elevated hindfoot pressure in plantar pressure measurements: trunk backward tilt shifts the dynamic force line posteriorly, thereby retarding the energy transition process.
For foot impulse, the significantly reduced heel impulse in both feet of Lenke type 3 AIS individuals correlates with the delayed GRF energy transition, reflecting diminished energy absorption capacity during the cushioning phase and decreased force generation efficiency during the propulsion phase. These observations collectively demonstrate that although thoracic scoliosis exerts a limited effect on interlimb pressure coordination patterns, it impairs the energy regulation capacity required for dynamic postural control. On the other hand, the markedly elevated midfoot impulse in Lenke type 6 AIS individuals serves as a compensatory response to lumbar imbalance, which is also consistent with the potential flatfoot risk indicated by plantar pressure data and the postural strategy reorganization reflected by COP parameters.
Moreover, the paradox of Lenke type 6 AIS individuals exhibiting higher heel impulse than that of their Lenke type 3 counterparts yet presenting abnormally elevated midfoot impulse exemplifies the complexity of postural compensation in AIS: the improvement of local cushioning capacity comes at the expense of midfoot structural stability, which further verifies that lumbar scoliosis induces more complex bilateral lower limb force transmission abnormalities.
Conclusion
Lenke type 3 and type 6 adolescent idiopathic scoliosis (AIS) groups both exhibit distinct plantar pressure profiles relative to the control group: Type 3 cases are marked by heelward pressure shift, elevated peak and average pressures in the left foot, local pressure anomalies, postural control impairment, and reduced walking efficiency; Type 6 cases feature left-foot heelward pressure shift, increased peak and average pressures, bilateral local pressure abnormalities, balance deficits, insufficient landing cushioning, weakened propulsive force generation. Furthermore, compared with Lenke type 3 AIS, type 6 AIS individuals display more complex plantar pressure abnormalities, greater right-foot impact, compromised static stability during stance phase.
These subtype-specific plantar pressure abnormalities provide critical biomechanical support for individualized AIS interventions, requiring targeted strategies (e.g., customized orthoses, subtype-specific rehabilitation) to correct lower limb load asymmetry. For Lenke type 6 AIS with severe bilateral pressure perturbations, intensive balance and shock absorption training is warranted to improve gait stability and long-term prognosis.
Limitations
However, this study has several limitations. First, the small sample size reduces the statistical power of the findings. Second, only participants with moderate spinal curvature were included, limiting extrapolation to mild or severe cases. Third, the exclusion of male participants impairs the generalizability of conclusions. Fourth, potential learning and fatigue effects were not controlled for during repeated plantar pressure measurements, which may have biased the data.
Future studies should expand the sample size and stratify participants by curvature severity to clarify the linear relationship between curvature progression and plantar pressure. Additionally, male participants should be recruited to explore gender - specific effects, and rigorous protocols (e.g., adequate rest intervals, randomized testing orders) should be adopted to minimize learning and fatigue - related biases.
Acknowledgements
None.
Acknowledgment of presentation
No report has been made.
Acknowledgment of financial support
This work was supported by the Key Project of Youth Development in Shanxi Province, China. [grant numbers QSNFZ25016].
Explanation of conflicts of interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Abbreviations
- AIS
Adolescent Idiopathic Scoliosis
- AI
Arch index
- ATR
Angle of Trunk Rotation
- COP
Center of pressure
- COP-TL
Trajectory length of the COP
- COP-SA
Swing area of 95% confidence ellipse of COP trajectory
- COP-APMD
Maximum displacement of the COP in the anterior-posterior direction
- COP-MLMD
Maximum displacement of the COP in the medio-lateral direction
- IRB
Institution review board
- GRF
Ground reaction force
- GRF First Peak % of Stance Phase
The percentage of the first peak of ground reaction force in the stance phase
- GRF Trough % of Stance Phase
The percentage of the trough of ground reaction force in the stance phase
- GRF Second Peak % of Stance Phase
The percentage of the second peak of ground reaction force in the stance phase
- SOSORT
Scoliosis Orthopaedic and Rehabilitation Treatment
Authors’ contributions
Xu Y-M: Study conception, design, and manuscript drafting. Wang Z and Ding J-Q: Experiment execution and data collection. Wang H-Land Lu M-J: Data analysis and visualization. Meng L-S: Critical revision and final approval of the manuscript.
Funding
This work was supported by the Key Project of Youth Development in Shanxi Province, China. [grant numbers QSNFZ25016].
Data availability
All data generated or analyzed during this study are presented in the manuscript. Please contact the corresponding author for access to data presented in this study.
Declarations
Ethics approval and consent to participate
This study was conducted in accordance with the Declaration of Helsinki. This study was approved by the Shanxi University Medical Ethics Committee in China (the Ethics Committee’s ID: SXULL2024081). All participants were fully informed of the study’s purpose, procedures, potential risks and benefits, and provided written informed consent; for all participants under the age of 16, informed consent was additionally obtained from their parents or legal guardians. In accordance with the Declaration of Helsinki, all aspects of this study were conducted ethically.
Consent for publication
Not Applicable.
Competing interests
The authors declare no competing interests.
Corresponding author & reprint contact information
Linsheng Meng, Shanxi University, Tai Yuan, 030006, China, 15935612223, 389522854@qq.com.
Footnotes
Publisher’s note
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Yamin Xu and Zheng Wang contributed equally to this work.
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