INTRODUCTION
Scoliosis refers to a 3-dimensional deformity of the spine where one or several segments of the spine curve laterally and the vertebrae rotate. The Scoliosis Research Society defines scoliosis as a lateral curvature of the spine measured using the Cobb method on a standing anteroposterior x-ray image, with a Cobb angle greater than 10° indicating scoliosis [1,2]. Adolescent idiopathic scoliosis (AIS) is the most common spinal deformity occurring during the growth period, accounting for 75%–80% of all cases of scoliosis [1]. The 2019 national monitoring and intervention results for common diseases and health impact factors among students, organized by the National Health Commission of China’s Disease Control Bureau, showed that the overall detection rate of spinal deformities among primary and secondary school students was 2.8%, with the number of diagnosed cases gradually increasing each year [3]. AIS impacts not only the structural alignment of the spine but also the balance and postural stability of affected individuals.
Balance, also known as postural stability, is the ability to control the relationship between the body’s center of mass (COM) and the base of support (BOS). Both static and dynamic balance require the coordinated action of the neuromuscular system and the integration of the visual, vestibular, and somatosensory systems [4]. The limits of stability (LOS) refer to the maximum angle at which the body can tilt while standing, or the maximum angle formed with the vertical line when tilting within the range of balance, and is an important indicator for assessing balance function and postural control ability [5], and is one of the test methods for evaluating dynamic balance [6]. Studies have shown that scoliosis affects the coordination between body segments, the anatomical structure of the spine, the symmetry of the trunk, and changes the normal gait pattern [7]. Some researchers have found that patients with AIS have abnormal proprioceptive function [8], and most AIS patients have more severe trunk imbalance in the coronal plane and reduced standing stability [9]. Haumont et al. [10] have shown that as the Cobb angle increases, the balance function of AIS patients gradually decreases. Fortin et al. [11] believe that AIS patients exhibit differences in brain oscillations in the sensorimotor cortex when controlling balance compared to normal people.
Although research has addressed the anatomical changes and spinal function in AIS, fewer studies have explored the core muscle’s role in balance. Recent findings suggest a strong connection between core muscle contraction and balance, though most prior research focused on paraspinal muscles without examining the full activation of core muscles during balance tasks [12-16]. Previous electromyographic studies have shown that the activation patterns of core muscles during balance tasks in AIS patients differ significantly from healthy individuals. These patterns, particularly the over-reliance on superficial muscles such as the internal and external obliques, may impair the efficiency of postural control and lead to compensatory (CPA) mechanisms that worsen spinal alignment and balance. Therefore, this study investigates the differences in balance function and the degree of core muscle group activation between AIS patients and healthy individuals through the LOS test.
Current studies [4-11] have primarily emphasized the superficial muscles, such as the paraspinal muscles, and their role in maintaining balance. However, the detailed mechanisms by which core muscles—comprising both superficial and deep muscle groups—contribute to postural control in AIS patients remain unclear. The interaction between these muscles during dynamic activities, especially under conditions that challenge balance, has not been fully explored. This lack of detailed knowledge hinders the development of targeted therapeutic interventions that could better address the postural instability seen in AIS patients. Given these gaps, this study investigates the differences in balance function and core muscle activation patterns, including both superficial and deep muscle groups, between AIS patients and healthy individuals. By utilizing the LOS test and surface electromyography (sEMG), the study aims to provide a deeper understanding of how these muscles contribute to balance maintenance in AIS, with the goal of informing more effective treatment strategies.
MATERIALS AND METHODS
1. Subjects
The sample size was determined using a power analysis to detect a moderate effect size (Cohen d=0.5) with a significance level of 0.05 and a power of 0.80. From December 2021 to February 2024, 24 AIS patients from the China Rehabilitation Research Center outpatient clinic formed the AIS patient group, and 25 healthy individuals formed the healthy control group. This study was conducted in accordance with the Declaration of Helsinki and approved by the Research Ethics Committee of China Rehabilitation Research Center (approval No. 2021-122-1), and written informed consent was obtained from all participants. All methods were carried out in accordance with relevant guidelines and regulations.
The inclusion criteria for the AIS patient group: (1) diagnosis of AIS, (2) age range of 10–18 years, (3) Cobb angle between 10° and 50°, (4) presence of right thoracic and left lumbar curvature, (5) no history of spinal surgery or trauma. The exclusion criteria for the AIS patient group: (1) presence of congenital spinal deformities, (2) other neuromusculoskeletal conditions.
The inclusion criteria for the healthy control group: (1) age range of 10–18 years; (2) Cobb angle less than 10° based on radiographic examination; (3) no family history of scoliosis or history of spinal surgery. The exclusion criteria for the healthy control group: any history of spinal abnormalities or surgeries (Supplementary Fig. 1).
2. Methods
A rehab therapist records subjects’ age, height, weight, C7 plumb line-center sacral vertical line (C7PL-CSVL) deviation, and axial trunk rotation angle; an orthopedic or spinal surgeon measures AIS group subjects’ full spine Cobb angles from radiographs.
3. Center of Pressure Displacement Distance
In this study, the center of pressure (COP) displacement was measured using a Zebris FDM-T treadmill to assess participants’ stability during different balance postures. COP displacement refers to the movement of the body’s COM relative to the BOS, measured along the x-axis (left-right) and y-axis (frontback). Participants were briefed on the process and practiced the movements before data collection. During the test, participants stood barefoot on the treadmill and performed maximum trunk inclinations in 4 directions without lifting their toes. Each inclination was held for 10 seconds, with a 10-second break in between, resulting in a total data collection period of 70 seconds. The x-axis displacement reflects lateral movements, with negative values indicating leftward shifts and positive values indicating rightward shifts, while the y-axis displacement reflects anterior-posterior shifts, with negative values indicating backward movements and positive values indicating forward movements [17].
4. Muscle Activity via sEMG
sEMG was used to assess core muscle activity during the balance tasks (Supplementary Table 1). The Noraxon Ultium system, with a sampling frequency of 4,000 Hz, recorded the electrical activity of key core muscles during trunk inclinations in 4 directions. Bipolar electrodes were placed according to SENIAM (surface electromyography for the noninvasive assessment of muscles) guidelines, with an 18-mm distance between electrodes on clean, dry skin. The signals were processed using Noraxon MR3.16 software, employing full-wave rectification and a 50-msec RMS filter, and the results were presented as average EMG values. This allowed us to analyze the activation patterns of the core muscles in AIS patients and compare them with those of healthy individuals during dynamic balance tasks [18].
5. Diaphragm Thickness Measurement
The diaphragm, as a key muscle for both respiration and postural control, plays an important role in maintaining balance and core stability. In this study, the thickness of the diaphragm was measured using a Konica Minolta SONIMAGE HS1 ultrasound system with an L18-4 transducer. The transducer was placed between the 8th and 9th ribs on the participant’s right side. Doppler M-mode was used to record diaphragm thickness in both neutral and inclined positions during the balance tasks. The transducer was positioned perpendicular to the chest wall to ensure accurate imaging. The variation in diaphragm thickness provided insight into differences in postural control between AIS patients and healthy controls, particularly during trunk inclinations [19].
6. Trunk Inclination Distance
A Diers formetricIII 4D system was used to measure trunk inclination distances in both the sagittal and coronal planes during the balance tasks [20]. Participants were asked to expose their back, lower their pants to the iliac spine level, and remove any hair or accessories to avoid reflections that could interfere with the measurements. The system recorded the distance between the vertebra prominens and the midpoint of the lumbar dimples to assess the degree of trunk inclination. This data helped to analyze the differences in trunk stability and postural asymmetry between AIS patients and healthy controls (Supplementary Figs. 2 and 3).
7. Statistical Analysis
Data were analyzed using IBM SPSS Statistics ver. 25.0 (IBM Co., Armonk, NY, USA); continuous variables were first tested for normality using the Shapiro-Wilk test. For normally distributed continuous variables, data were presented as mean±standard deviation. For nonnormally distributed continuous variables, data were expressed as median and interquartile range. Categorical variables, such as sex, were described as frequencies and percentages. Comparisons between groups for categorical variables were performed using the chi-square test or Fisher exact test, depending on the expected frequencies. A p-value of less than 0.05 was considered statistically significant.
RESULTS
1. Baseline Characteristics
The baseline characteristics of the study participants, including 24 AIS patients and 25 healthy controls, are presented in Supplementary Table 2. There were no significant differences between the AIS and control groups in terms of sex distribution, age, height, weight, body mass index, Cobb angle, C7PL-CSVL, or ART (p>0.05). This ensures that the groups were comparable for the subsequent analyses.
2. Diaphragmatic Thickness Variation, COP Displacement Distance, and Trunk Inclination Distance
In the neutral position, no significant difference in diaphragm thickness was found between the AIS and control groups (p>0.05). However, during the LOS tests, the control group had a significantly thicker diaphragm than the AIS group in all directions (p<0.05), especially backward (p<0.01). Both groups showed significant differences in diaphragm thickness between the neutral and leaned positions (p<0.01). No significant COP displacement differences were found between groups during tilts in any direction (p>0.05). Although within-group p-values were < 0.05, the differences were not clinically significant; No significant differences in trunk inclination were found between groups in either plane (p>0.05). Within-group, the AIS group showed significant differences in backward, leftward, and rightward inclinations compared to forward (p<0.05), while the control group only had a significant difference in rightward inclination (p<0.05) (Table 1).
3. Intragroup Comparison of sEMG in the Both Group
In the AIS group, left internal oblique sEMG during left inclination differed significantly from forward inclination (p<0.05). Right internal oblique sEMG during right inclination showed significant differences from forward, backward, and left inclinations (p<0.05). The left external oblique muscle’s p-value indicated significance in the Friedman test but not after Bonferroni correction. Left multifidus sEMG during right inclination was significantly different from forward inclination (p<0.05). In the control group, forward lean sEMG showed significant differences for left internal oblique (vs. backward and left leans, p<0.05), right internal oblique (vs. backward and right leans, p<0.05), left external oblique (vs. backward lean, p<0.05), left multifidus (vs. backward and left leans, p<0.05), and right multifidus (vs. all other leans, p<0.05) muscles. Left erector spinae sEMG also differed significantly (vs. backward and left leans, p<0.05), while right erector spinae showed a significant difference only for the right lean (p<0.05) (Table 2).
4. Forward Lean sEMG Analysis Comparison
Significant sEMG differences were observed in left internal oblique muscles during forward lean (p<0.01) and a notable difference in right internal oblique muscles (p<0.05) between groups in Table 3.
5. Backward Lean sEMG Analysis Comparison
Significant differences in sEMG values were found for the left internal and external oblique muscles during backward lean (both p<0.01), and the right external oblique muscle showed a significant difference (p<0.05) between groups in Table 4.
6. Left Lean sEMG Analysis Comparison
Significant sEMG value differences were noted in the left internal oblique and right external oblique muscles during left lean (p<0.01), and the left external oblique showed a significant difference (p<0.05) in Table 5.
7. Right Lean sEMG Analysis Comparison
Significant sEMG differences in the left internal and external oblique muscles were observed during right lean between groups (p<0.01), with a notable difference in the right internal oblique (p<0.05), as seen in Table 6.
DISCUSSION
This study used the LOS test to examine balance and core muscle activation differences between AIS patients and healthy controls. Despite no significant balance function differences, core muscle engagement and activation levels during predictable dynamic tests varied, highlighting the study’s importance. Subjects controlled balance duration, and muscle strength was gauged via EMG amplitude and muscle thickness changes. Findings revealed distinct postural control patterns, with AIS patients exhibiting higher core muscle activation at inclination limits compared to controls.
1. Surface Electromyography
This study’s sEMG intragroup comparison results show that in the AIS group, there were significant differences in the left and right internal oblique muscles, the right external oblique muscle, and the left multifidus muscle during trunk inclination. Intergroup comparisons revealed that when inclining in all directions, compared to the control group, the AIS group relied more on superficial muscle groups (such as the internal and external oblique muscles) to maintain trunk stability. This result is consistent with previous research by Mahaudens et al. [21], who suggested that AIS subjects have a higher muscle activation rate but lower efficiency than normal subjects. They increase muscle work to reduce the trunk imbalance caused by spinal deformity, which is not conducive to maintaining posture. The biomechanical defects caused by scoliosis affect the stability of the vertical axis balance during movement [22]. Kuo et al. [23] found that AIS subjects have asymmetric habitual muscle activity in disturbance experiments, specifically, the left multifidus muscle and the right gastrocnemius are activated earlier and maintained for a longer time. The latency of the multifidus muscle on the convex side of the lumbar spine is earlier than on the concave side, but the concave side muscles are activated later but with a stronger amplitude to maintain bilateral balance.
The results of this study are closely related to the type of scoliosis in the AIS group subjects (i.e., right thoracic convexity and left lumbar convexity): AIS with right thoracic convexity and left lumbar convexity generally have excessive extension of the right external oblique muscle fibers and left internal oblique muscle fibers on 2 parallel diagonals (line a, b). The extension lines of the left external oblique muscle fibers and right internal oblique muscle fibers are excessively shortened, as shown in Supplementary Fig. 4, based on findings from previous studies [24]. Therefore, during the LOS test, AIS subjects further strengthen the activation of asymmetric muscle groups to maintain postural stability and trunk balance. This is consistent with the findings of Wong et al. [25], and Doran et al. [26] found through ultrasound examination that the asymmetry of the rectus abdominis, external oblique, internal oblique, and transversus abdominis muscles in the AIS group was significantly higher than that in the healthy control group. Clinical manifestations also confirm this result, with the AIS group showing an increase in the rotation of the left convex lumbar spine to the left rear and the rotation of the pelvis to the right front when inclining forward, i.e., line cd is more shortened, and line ab is more elongated.
The intragroup comparison of the control group showed that except for the right external oblique muscle, the rest of the core muscle groups had significant contraction differences when the trunk was inclined to the limit. The intragroup comparison of the AIS group showed significant changes in the core muscle groups, including the internal and external oblique muscles and the left multifidus muscle, while there was no difference in the erector spinae muscles on both sides when the trunk was inclined in 4 directions. This indicates that compared with other core muscle groups, the erector spinae muscles in the AIS group did not show more effect in maintaining postural stability.
2. Diaphragm
The diaphragm, as a respiratory muscle, originates from the posterior side of the xiphoid process, the lower 6 ribs, and the third lumbar vertebra, and forms a domed central tendon at the level of the fifth rib. When the spine bends in 3 dimensions, the abnormal torsion of the ribs and vertebral bodies, as well as the abnormal shape of the thoracic cage, can affect the morphology, breathing pattern, and function of the diaphragm [27]. Studies have shown that even in asymptomatic mild scoliosis, without signs of respiratory dysfunction during rest and exercise, incorrect ventilation patterns can be observed. Respiratory muscles (intercostal muscles, abdominal muscles, and the diaphragm) are the most important muscles in mild scoliosis [28].
Yang and Li [29] believes that the diaphragm not only participates in life maintenance as a respiratory muscle but also participates in posture control as part of the core muscle group. From an anatomical perspective, the diaphragm is equivalent to the “roof” of the core muscle group, playing a role in increasing intra-abdominal pressure (IAP) to stabilize the trunk. Since 1969, scholars [30,31] have successively found that when maintaining an upright posture, the diaphragm contracts earlier than the rectus abdominis, thereby indirectly proving that the diaphragm is involved in the postural regulation process. Since 1997 [32] to the present [33], scholars have experimentally confirmed the role of the diaphragm in maintaining posture, and it has been found that when the body’s posture changes, the diaphragm’s EMG signals change earlier than those of the limb muscles. In recent years, researchers have found that diaphragmatic problems can affect spinal stability and induce lower back pain [34].
The intragroup comparison of the AIS group and the control group showed that compared with the neutral position, the thickness of the diaphragm increased significantly when the trunk inclined in the 4 directions of front, back, left, and right. Moreover, the thickness of the diaphragm in the control group in the forward and backward positions was greater than that in the left and right positions. The intergroup comparison between the 2 groups showed that although there was no significant difference in diaphragm thickness in the neutral position, the thickness of the diaphragm in the control group during the LOS test was significantly greater than that in the AIS group. This indicates that the control group’s contraction and activation of the diaphragm during the postural control process are significantly better than those in the AIS group. The reason may be the abnormal respiratory pattern leading to changes in the morphology of the diaphragm, thereby affecting its role in balance regulation.
This is consistent with previous research results. Mohammadi et al. [35] found that compared with healthy people, the muscle endurance of the concave side’s external intercostal muscles and the diaphragm in patients with idiopathic scoliosis is lower. Noh et al. [36] believes that in AIS subjects with right thoracic convexity, the amplitude of diaphragmatic movement on the convex side during tidal breathing is significantly greater than that on the concave side. Yan [37] found that the thickening rate of the diaphragm is positively correlated with the dynamic balance ability of the trunk.
In addition, several studies in recent years have shown a close relationship between the diaphragm and balance function [38-40]. Most conservative treatments for scoliosis, such as the Schroth method, use the “rotation angle” breathing technique to increase the breathing amplitude of the collapsed area on the concave side of the spine, combined with contracting the muscles on the convex side of the spine, guiding respiratory muscles such as the diaphragm, and adjusting abnormal breathing patterns to promote spinal correction [24]. Examination of the diaphragm is one of the effective methods for assessing AIS, and diaphragmatic training is also an important part of clinical rehabilitation training.
3. Balance Function
Recent studies have presented inconsistent conclusions regarding the balance ability of patients with scoliosis, and this diversity in research findings provides us with an opportunity to gain an in-depth understanding of the balance ability of AIS at different developmental stages.
In previous studies, Xuan et al. [41] found that AIS patients have reduced static balance stability and are at moderate risk of falls. The results of Şahin et al. [42] suggest a decline in the static balance function of patients with scoliosis; Cațan et al. [43] found that AIS has poor postural control ability by comparing static plantar pressure and stability measurements. The results of this study show no statistically significant difference in the displacement distance of the COP x-axis and y-axis when the AIS group and the control group lean in the anterior-posterior and medial-lateral directions; there is no significant difference in the inclination distance of the trunk in the coronal and sagittal planes. The research results of the domestic scholar Meng et al. [44] also show no significant difference in the COP x-axis between the AIS group and healthy individuals in the static LOS test. The foreign scholar Piątek found that in simple to moderately difficult tasks, the performance of AIS is not inferior to that of the healthy control group, indicating that the postural stability of AIS is sufficient to meet most daily activities, but the balance performance is poor when vision or proprioception is disturbed [45]. This is different from the results of some other scholars, such as Li et al. [46], who found that whether standing on one foot with eyes open or standing on both feet with eyes open, the static balance stability of AIS patients is worse than that of normal adolescents, and the postural adjustment ability is lower than that of normal adolescents; Xu [47] found that compared with the healthy control group, scoliosis subjects have dynamic balance dysfunction in the anterior-posterior and medial-lateral directions, and the balance function of the stability limit, while there is no difference in the ability of static balance and dynamic balance circular motion and the healthy control group.
The inconsistency of these research results suggests that the balance function of AIS patients may be affected by various factors, such as test methods, test equipment, the severity of scoliosis, the age of the subjects, skeletal maturity, muscle strength, etc. Therefore, more research is needed in the future to further evaluate the balance function of AIS subjects, taking into account these factors to improve the research results.
4. Mechanism
The spine’s stability depends on intervertebral discs, ligaments, and coordinated muscle contractions. Core muscles, including abdominal, paraspinal, and pelvic muscles, maintain trunk posture by balancing opposing muscle groups, contributing to IAP [48,49]. Increased IAP stabilizes the spine, forming a more stable structure than individual vertebrae.
Postural control strategies are classified as CPA or anticipatory (APA), depending on whether movement is predicted. APA, triggered by early muscle activation, plays a crucial role during predictable disturbances, as seen in LOS tests [50,51]. Core muscle activation ensures effective postural control, maintaining balance during movement and respiration.
AIS patients show asymmetric muscle contractions, with greater activity on the convex side of the curve [52]. These CPA actions reflect differences in postural control strategies compared to healthy individuals. Additionally, AIS is often associated with lower muscle tone and increased joint mobility, leading to CPA mechanisms that affect posture and balance [53-55]. Reduced muscle tone impacts postural muscle activity, leading to abnormal compensation [56-59].
Postural control is achieved through proprioceptive and vestibular systems. AIS patients exhibit immature proprioceptive integration and altered vestibular morphology, impairing balance and motor function [60-62]. Myofascial meridians also play a role in posture and movement, with muscles like the erector spinae and obliques contributing to stability [63].
Core muscle training, such as the Schroth method, incorporates exercises that enhance deep and superficial muscle activation, highlighting the importance of core strengthening in AIS rehabilitation [64].
CONCLUSION
This study compared balance function and core muscle activation in AIS patients versus controls using the LOS test, finding: (1) AIS patients had higher superficial muscle activation but less diaphragm engagement in balance, (2) distinct postural control patterns between groups, (3) mild AIS patients showed no balance differences from healthy individuals. The insights highlight the importance of balance in AIS severity and treatment, potentially enhancing clinical interventions and quality of life for patients.
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