Wearable accelerometers reveal objective assessment of walking symmetry and regularity in idiopathic scoliosis patients

Introduction

Scoliosis is a three-dimensional deformity involving the skeletal and joint systems, typically accompanied by vertebral rotation in the transverse plane, alterations in the physiological curvature of the spine in the sagittal plane, and lateral curvature deformities in the coronal plane (Labrom et al., 2021). The Scoliosis Research Society (SRS) defines scoliosis as the measurement of the Cobb angle in the coronal plane X-ray imaging of the entire spine. If the Cobb angle is greater than 10 degrees and there is evident vertebral rotation, it is diagnosed as scoliosis (Negrini et al., 2018). Idiopathic scoliosis is a complex, multifactorial, and etiologically unclear neuro-musculo-skeletal disorder (Trobisch, Suess & Schwab, 2010).

Previous studies have indicated that patients with scoliosis experience impaired balance function, with asymmetry in the spine of scoliotic individuals not only altering the position of the center of mass in the trunk but also affecting the patient’s balance control strategy (Daryabor et al., 2017). Even in cases of mild spinal deformity (Cobb angle less than 25°), individuals with scoliosis demonstrate certain balance control impairments in simple postural control tasks (Haumont et al., 2011). As the severity of spinal deformity increases, the impairment in balance control becomes more pronounced, and individuals tend to adopt a more conservative method of balance control (Wu et al., 2020). Current research attributes the insufficient postural balance control in individuals with scoliosis to two hypotheses: the biomechanical hypothesis and the sensory integration hypothesis (Dufvenberg et al., 2018). The former suggests that spatial displacement of the spinal deformity and the positions of the shoulders, pelvis, and head contribute to the decline in postural control. The latter proposes that inaccurate sensory input weights lead to compromised dynamic regulation of sensorimotor integration, causing a decline in balance control abilities.

Impairments in postural balance control capacity caused by idiopathic scoliosis directly influence the spatiotemporal parameters of patients’ gait, resulting in abnormal gait patterns. Studies indicate that, compared to healthy subjects, individuals with idiopathic scoliosis exhibit a shorter stance phase and a longer swing phase (Liu et al., 2022). Correspondingly, pelvic joint activity and hip joint activity in the horizontal and sagittal planes during walking are reduced (Daryabor et al., 2017). However, previous research has primarily focused on the kinematics of the trunk and pelvis in scoliosis patients and changes in gait spatiotemporal parameters. Limited research exists on the symmetry of bilateral gait, and there is no consensus on the symmetry and regularity of bilateral lower limb gait (Daryabor et al., 2017). Moreover, most previous studies have analyzed gait spatiotemporal parameters and the motion of the trunk and pelvis to determine gait symmetry in individuals with scoliosis, using complex equipment such as plantar pressure plates or infrared three-dimensional motion capture systems (Sklensky et al., 2022; Wu et al., 2020; Yang et al., 2013; Zhu et al., 2021). However, direct measurements of gait symmetry and regularity were not commonly performed.

Spinal curvature in the sagittal, coronal, and horizontal planes alters the balance stability of patients in three-dimensional space. Neurosensory regulatory function maintains the trunk’s static and dynamic stable upright posture, while abnormal head positioning affects both the head and trunk posture, ultimately leading to decreased trunk stability (Beaulieu et al., 2009; Herman et al., 1985; Wen et al., 2022). Furthermore, maintaining head stability is essential for optimizing visual conditions, and maintaining head stability may be one of the fundamental tasks of the body posture control system during walking (Bril & Ledebt, 1998; Cromwell, Newton & Carlton, 2001).

Studies have shown that, based on imaging measurements and pressure analysis of the plantar surface, patients with scoliosis and spinal imbalance exhibit greater head offset distances than scoliosis patients in a balanced group. Additionally, the percentage difference in load between left and right feet and between front and rear feet is greater than in the balanced group. The increased instability of the head and body during static standing exacerbates trunk imbalance (Wen et al., 2022). However, there is currently a lack of research on the three-dimensional dynamic balance control strategies of the head in scoliosis patients during walking and whether they affect the symmetry and regularity of bilateral walking.

Gait symmetry and regularity are paramount in evaluating the walking stability and postural control of patients with idiopathic scoliosis. Conventional assessment techniques, which often employ expensive equipment like three-dimensional motion capture systems and force plates (Ji et al., 2024; Sklensky et al., 2022; Zhu et al., 2021), are confined to laboratory settings, presenting time-consuming and limited real-world applicability. In contrast, three-dimensional accelerometers, with their inherent portability, low cost, and suitability for long-term use, facilitate real-time gait monitoring. They are a preferred clinical instrument for assessing the gait of scoliosis patients, offering a practical and efficient alternative to sophisticated devices such as three-dimensional motion capture systems and force plates.

Therefore, this study aims to comprehensively investigate the symmetry and regularity of bilateral lower limb gait in patients with scoliosis using data collected through three-dimensional accelerometers. Simultaneously, the study investigates the balance control strategies of the head during the walking process. Through this research, we hope to provide a convenient measurement method for assessing the abnormal gait in patients with scoliosis, and simultaneously offer new insights for future clinical practices.

Materials & Methods

Data processing

The acceleration signals from the subjects were kept blinded to the data analysis researchers, and the data results were grouped only after the completion of the entire data analysis. The raw acceleration data, from which the gravity component had been removed, underwent filtering and smoothing using MATLAB (R2014a; the MathWorks, Inc., Natick, MA, USA). A second-order Butterworth low-pass filter with a cutoff frequency of 10 Hz was applied to ensure the representativeness and statistical significance of the results. Specifically, 420 points (7 s) from the middle of the acceleration signal were randomly selected as the data for calculating variables (Fig. 1). Five variables reflecting walking symmetry and regularity were selected and calculated in MATLAB.

Walking symmetry and regularity variables

The variables for each walking symmetry and regularity parameter are derived from calculations based on the X (AP), Y (ML), and Z (VT) axis data of the acceleration signal. The specific parameters include:

Acceleration root mean square (RMS): RMS measures the dispersion of the measured acceleration signal relative to zero. It’s a commonly used parameter to evaluate the dynamic stability of measured segments during walking, reflecting the fluctuation of measured segments during walking (Menz, Lord & Fitzpatrick, 2003).

Harmonic ratio (HR): The HR represents the smoothness and rhythmicity of the acceleration pattern. The HR on the anterior-posterior axis (X) and the sagittal axis (Z) is calculated as the ratio of the sum of even harmonic values to the sum of odd harmonic values. While for the acceleration on the medial-lateral axis (Y), the HR is calculated as the ratio of the sum of odd harmonic values to the sum of even harmonic values, contributing to the assessment of acceleration in the medio-lateral direction (Menz, Lord & Fitzpatrick, 2003).

Step regularity (AD1): AD1 represents the amplitude of the first peak in the acceleration autocorrelation signal.

Stride regularity (AD2): AD2 represents the amplitude of the second peak in the acceleration autocorrelation signal.

Higher AD1 and AD2 values closer to 1 indicate better step regularity and stride regularity, respectively (Moe-Nilssen & Helbostad, 2004) (Fig. 2).

Gait symmetry: Gait symmetry is reflected by the closeness of AD1/AD2 to 1.0. A value closer to 1.0 indicates greater symmetry in gait (Moe-Nilssen & Helbostad, 2004).

These parameters are indicative of various aspects of gait stability, smoothness, regularity, and symmetry and are calculated based on different aspects of the acceleration signal across multiple axes.

Results

Walking symmetry and regularity variables

The analysis results of walking symmetry and regularity variables show significant changes in the measurements of the lumbar and head regions for subjects with idiopathic scoliosis compared to healthy subjects (Table 2).

Table 2:

Walking symmetry and regularity variables between idiopathic scoliosis patients and healthy subjects.

Variables	IS	Healthy
Walking speed(m/s)	1.28 ±0.07	1.29 ±0.08
Cadence(steps/min)	115.5 ±6.51	117.97 ±5.7
RMS
Head
AP	0.49 ± 0.22*	0.65 ± 0.22
ML	0.76 ±0.24	0.84 ±0.21
VT	2.02 ± 0.4*	2.45 ± 0.49
Lumber
AP	1.83 ±0.29	1.96 ±0.23
ML	1.35 ±0.35	1.51 ±0.42
VT	1.96 ± 0.41*	2.43 ± 0.47
HR
Head
AP	1.39 ±0.51	1.59 ±0.57
ML	2.98 ±0.72	3.14 ±0.69
VT	3.37 ± 0.83*	4.35 ± 0.84
Lumber
AP	3.72 ±0.87	3.52 ±0.99
ML	2.02 ±0.44	2.22 ±0.67
VT	3.45 ± 0.72*	3.97 ± 0.71
Step regularity
Head
AP	0.39 ± 0.28*	0.58 ± 0.11
ML	−0.74 ±0.15	−0.75 ±0.13
VT	0.88 ± 0.06*	0.93 ± 0.04
Lumber
AP	0.87 ±0.07	0.84 ±0.13
ML	−0.5 ±0.15	−0.57 ±0.19
VT	0.87 ±0.06	0.9 ±0.07
Stride regularity
Head
AP	0.65 ± 0.13*	0.79 ± 0.1
ML	0.82 ± 0.07*	0.87 ± 0.05
VT	0.89 ± 0.06*	0.94 ± 0.03
Lumber
AP	0.89 ±0.06	0.88 ±0.09
ML	0.69 ±0.11	0.76 ±0.11
VT	0.87 ±0.06	0.9 ±0.06
Gait symmetry
Head
AP	0.58 ±0.4	0.75 ±0.15
ML	−0.89 ±0.14	−0.87 ±0.16
VT	0.98 ±0.06	0.99 ±0.02
Lumber
AP	0.98 ±0.05	0.96 ±0.13
ML	−0.73 ±0.22	−0.8 ±0.28
VT	0.998 ±0.03	0.996 ±0.03

DOI: 10.7717/peerj.17739/table-2

Notes:

IS

idiopathic scoliosis

RMS

root mean square

HR

harmonic ratio

AP

antero-posterior direction

ML

medial-lateral direction

VT

vertical direction

*p < 0.05.

Bold indicates statistically significant values.

In comparison to healthy individuals, patients with idiopathic scoliosis exhibit a decrease in RMS in both the lumbar and head regions across all three directions, but only the RMS in the VT exhibits a significant difference (p < 0.05). For the RMS on the ML, there is no significant difference in the head and lumbar regions between the idiopathic scoliosis group and the healthy control group (p > 0.05). In the AP direction, although the head RMS of the idiopathic scoliosis group is significantly lower than that of the healthy control group (0.49 ± 0.22 vs. 0.65 ± 0.22 respectively, p < 0.05), there is no significant difference between the two groups in the lumbar region (p > 0.05) (Fig. 3). In regard to the HR, the overall HR values of idiopathic scoliosis subjects are smaller than those of the healthy control group, but statistical differences are only present in the VT direction (p < 0.05) (Fig. 4).

When evaluating the regularity of gait, the step regularity of the head in idiopathic scoliosis significantly decreases in both the AP and VT directions (p < 0.05) (Fig. 5). Additionally, the stride regularity in all three directions of the head is significantly lower than that of the healthy control group (p < 0.05) (Fig. 6). However, for the lumbar region, both step regularity and stride regularity in all three directions show no significant difference compared to the healthy control group (p > 0.05).

The gait symmetry in the head and lumbar regions between the idiopathic scoliosis group and the healthy control group does not exhibit significant differences across all three directions (p > 0.05). Additionally, no significant distinctions were found in terms of 10-meter walking speed and cadence when comparing the two groups (p > 0.05) (Table 2).

Discussion

To the best of our knowledge, this is the first study employing wearable three-dimensional accelerometer sensors to analyze the gait regularity and symmetry of individuals with idiopathic scoliosis. The results indicate significant issues in both gait regularity and symmetry for patients with Lenk1 and Lenk5 types of idiopathic scoliosis, particularly evident in the analysis of acceleration signals collected from the head.

RMS values represent the average amplitude of acceleration sensors in three-dimensional directions during walking tests. The analysis conducted in this study on the walking regularity and symmetry among subjects with idiopathic scoliosis reveals that the RMS values recorded in the head and lumbar are lower than those observed in the healthy control group. Notably, there are significant differences in the RMS values of the VT direction in the head and lumbar, as well as the AP direction in the head (p < 0.05), when compared to the healthy control group. However, no significant difference is found in the RMS values in the ML direction and the three directions in the lumbar (p > 0.05). In previous studies, an increase in RMS was considered an indication of decreased walking balance control and a manifestation of walking instability (Osaka et al., 2017; Zhang et al., 2021). However, in this study, with no significant difference in the 10 m walking speed between the two groups, RMS values for idiopathic scoliosis decreased. This finding might be attributed to a specific conservative balance control strategy adopted by individuals with idiopathic scoliosis. As previously suggested, RMS in the VT direction reflects the displacement of the center of gravity (COG) during walking (Osaka et al., 2017). The study aligns with previous research, indicating that individuals with idiopathic scoliosis exhibit reduced displacement in the VT direction of the head and lumbar during walking compared to healthy individuals, implying a more cautious walking pattern (Mahaudens et al., 2009). Mahaudens et al. (2018) proposed that this gait pattern in individuals with idiopathic scoliosis represents an economical way to avoid excessive metabolic energy consumption. Consequently, our study suggests that individuals with idiopathic scoliosis employ conservative and cautious balance strategies and maintain a stable visual field in the VT and AP direction during the walking process to avoid gait imbalance.

In this study, the HR values in the VT direction of the head and lumbar region in patients with idiopathic scoliosis are significantly lower than those of healthy subjects. HR is utilized as an indicator to evaluate step-to-step symmetry within a stride, where a higher HR value indicates stronger symmetry between gaits and a more stable walking pattern (Bellanca et al., 2013). Previous research has employed acceleration data from the lumbar and head to compute HR and evaluate gait stability among patients with hemiplegia, the elderly, parkinson’s patients, and individuals with dizziness (Castiglia et al., 2022; Doi et al., 2013; Mitsutake et al., 2022; Zhang et al., 2021). Findings have demonstrated that as the disparity in limb loading conditions increases, the kinematics and dynamics of both lower limbs become more asymmetric during walking, resulting in a smaller HR value (Bellanca et al., 2013). Previous studies have indicated a significant asymmetry in the loading of the lower limbs on both sides during walking in patients with idiopathic scoliosis (Park et al., 2016; Sklensky et al., 2022), with an abnormal lower limb loading pattern ultimately leading to decreased balance (Márkus et al., 2018). In summary, this study suggests that patients with idiopathic scoliosis exhibit step-to-step asymmetry within a stride during walking.

On the other hand, this study also examines changes in walking stability in patients with idiopathic scoliosis from the perspective of gait quality. The step regularity and stride regularity of idiopathic scoliosis are significantly lower than those of healthy subjects, suggesting that patients with idiopathic scoliosis have poorer regulation of repetitive walking speed and control over body rhythm displacement during walking. However, this disparity is observed only in the acceleration signals collected from the head, while the signals obtained from the lumbar exhibit less noticeable variation between the two groups. This discrepancy may be related to the deformities caused by scoliosis. Apart from altering the overall movement control strategy of the trunk (Wu et al., 2020; Wu et al., 2019; Yang et al., 2013), the spinal deformity also directly affects the stability control capability of the head (Kučerová et al., 2023; Wen et al., 2022). Consequently, during walking, with the head being the top of the trunk, its movement strategy is likely to be more significantly impacted by the spinal deformities.

Three-dimensional accelerometers are wearable devices that offer high practicality, low cost, and convenience. Previous studies have demonstrated their clinical application in assessing patients’ walking stability and fall risk, with high reliability (Gillain et al., 2018; Jarchi et al., 2018). This study not only highlights the differences in walking symmetry and regularity between patients with idiopathic scoliosis and healthy controls but also confirms the effectiveness of wearable three-dimensional accelerometers in evaluating these parameters and head balance control in idiopathic scoliosis patients. These findings underscore the extensive applicability of wearable sensors in future research and clinical settings for understanding motor control in idiopathic scoliosis, particularly in the early identification of gait abnormalities linked to the condition.

Conclusion

In summary, the investigation revealed that individuals with idiopathic scoliosis exhibit abnormalities in the symmetry and regularity of their gait during the walking process, discernible across the AP, ML, and VT direction. Furthermore, there is a relatively conservative dynamic balance control strategy for the head in three-dimensional space. The utilization of wearable three-dimensional accelerometers sensors presents a promising and convenient approach to assess the symmetry and regularity of walking in patients with idiopathic scoliosis. These research findings contribute novel insights into the gait symmetry, regularity, and measurement for individuals with idiopathic scoliosis, offering valuable references for prospective diagnostic. Subsequent studies should further investigate the correlation between lower limb gait asymmetry, spinal deformity progression, and relevant mechanical functional characteristics in patients with idiopathic scoliosis.

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