Introduction
Non-specific low back pain (NSLBP) is determined for about 85% of low back pain (LBP) [1]. Literature suggested that NSLBP patients demonstrate different components that may result from different mechanisms [2, 3, 4, 5, 6]. To identify patients with chronic NSLBP (CNSLBP), many test procedures have been developed. Among others, observation of kinematics and kinetics variables during standing and forward bending is a valid, reliable, and often applied test [7, 8]. There is evidence to support changes in kinematics and kinetics variables in patients with recurrent NSLBP [9, 10, 11]. Following trunk loading, patients suffering from LBP showed earlier onsets or decreased lumbar muscle activities [12, 13]; in addition, following multi-directional perturbations, these subjects showed a reduction of trunk moments and enhanced the trunk musculature co-activation [14, 15]. Some research studies employed bilateral forward reaching [16, 17], axial trunk rotation [18], sit-to-stand activities [19], and walking and running [11, 20, 21] assessment. These patterns were characterized as decreased muscle moment in lumbar spine and hip altered power patterns limits [19], along with improved coordination of the lumbar spine and pelvis [16] altered proprioception of trunk movements [22] measured at discrete points during the movement.
The angle-angle graph of body segments against each other could provide information on the coordination pattern between limbs [23, 24]. The angle-angle graph does not determine true information about coordinated motion between the segments throughout an entire motion cycle. The coupling angle quantified the relationship between two joint variables derived from converting sagittal angle-angle plots. This plot contains only spatial information derived from positional data (angle-angle plot). It provides insights into segmental movement coordination [25]. Although these investigators demonstrated kinematic and kinetic changes that represent poor neuromuscular control in patients with CNSLBP, none of these investigators studied changes in kinematics and kinetics variables, such as coupling angle, power, and movement of the joint after motor control exercise. Core stabilization exercise is currently used as a form of individual intervention within physiotherapy. Core stabilization exercises are effective and restore appropriate trunk neuromuscular control in patients with movement coordination impairments [26, 27, 28, 29, 30, 31, 32]. However, treatment approaches designed to reduce pain and improve disability did not affect these motor control variables. Therefore, this study aimed to evaluate the effects of an 8-week core stabilization exercise program on kinematic (i.e. coupling angle values) and kinetics variables (hip joint power and moment) in patients with CNSLBP. 

Materials and Methods
Study design
This was a prospective study. A total of 30 CNSLBP patients participated in this study and followed the 8-week program. The participants gave their informed consent to take part in the research. Then, each subject randomly entered the intervention (core stability exercise group) or control groups. The randomization method was known to only one investigator who was not engaged in the recruiting process.


Study participants
The subjects were selected in November 2022 from 3 clinics in Ardabil City, Iran. The inclusion criteria were as follows: I) being in the age range of 25 to 45 years; II) having LBP in between T12 and sacrum region; III) having no recent experience of pain in the upper/lower limbs at least a month before the experiments; and IV) having no impaired function of the spine or lower limbs that could potentially change trunk motion during standing. The clinical examinations for the diagnosis of movement impairment were provided by 2 experienced physiotherapists trained in this protocol. The exclusion criteria were as follows: having pain in the lower back for more than 8 weeks [33], having a previous history of using core stabilization training; having a history of serious pathologies (e.g. fractures, acute trauma, or serious illnesses); having contraindications to the exercise; having psychological and psychiatric problems; having a BMI >30 kg/m2. Two patients in the intervention group were finally excluded.

Core stability exercise program (specific stabilization exercise)
Patients in the core stability exercise group received individual treatment. Each session was around 30 to 45 min twice per week for 8 weeks. 
The subjects with CNSLBP also received daily individualized home exercise programs on a 5-7 days/week basis even if their symptoms resolved during the treatment. The activation of transversus abdominis/internal oblique muscles is accomplished by teaching the subject with CNSLBP the abdominal drawing-in maneuver and will be verified by palpation. This program aims to restore a precise co-contraction pattern to optimize spinal stability during functional tasks and confidence in using the spine. The treatment protocol was provided by two experienced physiotherapists.
The intervention protocol was a modified core stabilization exercise program based on a previous study [34]. The standardized treatment protocol targeted deep stabilizing muscles throughout static, dynamic, and functional tasks. The first instance included isolation and co-contraction of transversus abdominis/internal oblique muscle, lumbar multifidus muscle, and then increased exercise intensity and co-contraction with extremity movement [30] (Table 1).

The intensity of the exercises was increased during treatments with subjects being encouraged to improve their performance. Patients were also instructed to use restore a precise co-contraction pattern to control posture, spinal stability, and breathing during functional tasks [35].

Home exercises were taught to be performed daily along with the sitting, four-point kneeling, and standing exercises. The group without core stability exercise continued with their normal daily activities and pain relief (i.e. no physiotherapy). After 8 weeks of treatment, the patients underwent the same biomechanical testing protocol. After the post-test, for the control group, we suggested home exercise and identified a clinic for their rehabilitation. 

Kkinematics and kinetic analysis
Vicon cameras (6 cameras) and the Nexus software, version 1.7.5 (Oxford Metrics, UK) as a data capture software were used to record the trunk flexion–extension kinematics. A calibration procedure was performed [36] before the experiments started. The markers were located on the landmarks based on the full-body plug-in gait model. In addition, 2 markers were placed on the sacrum (S2) and lumbar spine (L1). The sampling frequency of the Vicon system was 100 Hz. The ground reaction force variables during trunk flexion/extension were measured using two Kistler force platforms (Kistler Instruments, Inc., Amherst, NY) with a sampling rate of 1000 Hz. 
As dependent variables, trunk flexion, and extension were divided into 2 phases as follows: the first phase involved a flexion motion (first 50%) and the remaining 50% an extension motion (Figure 1).

At least 6 accurate trials were captured. Kinematics and ground reaction force data were filtered by the cut-off frequency of 6 Hz and 20 Hz, respectively. Data were exported from the polygon authoring tool to a spreadsheet for patterns (ranges of motion, joint moments and powers, etc.). All moments and powers were normalized using the body weight. An angle–angle diagram was constructed from the successive sampled data points of sagittal lumbar and hip angles (Figure 2).

Thereafter, the coupling angle between the data points vector regarding the horizontal axis was computed as described by Freeman17 as follows (Equation 1):

Where Ɵy and Ɵx represent the lumbar and hip sagittal plane rotation angles, respectively. An angle of 45° indicates a 1:1 motion ratio between two segments (pelvis and lumbar spine). An angle greater than 45°C indicates pelvis dominance, while an angle less than 45°C indicates lumbar dominant movement patterns [25].

Flexion–extension task
The forward bending task was explained and demonstrated by clinicians before any experimental trial was undertaken.
A metronome was used for movement pacing. The flexion and extension lasted 5 s. A total of 6 practice trials were also performed before the actual data collection. Patients stood with their feet open to the width of their shoulder and a footprint was drawn and used for re-test positioning. The patients were instructed to perform a total of 6 trials of forward bending moving as far as they could at their comfortable pace.

Statistical analysis
A multivariate analysis of variance (MANOVA) was used to determine between-group differences in baseline variables. Statistical analysis was done using separate 2 (group: CG, CST)×2 (time: Pre, post) analysis of variance with repeated measures test. The classification of effect sizes (d) was done by calculating partial η2. According to Cohen [37], 0.00≤d≤0.24 demonstrate small, 0.25≤d≤0.39 demonstrate medium, and d≥0.4 demonstrate large effects. The significance level was P<0.05 for all analyses. All analyses were done using the SPSS software, version 21

Results
There were no significant between-group baseline differences (P>0.05) (Tables 2 and Table 3).

Table 3 describes pre- and post-intervention results for all outcome variables during the trunk flexion task. The statistical analysis indicated significant main effects of “time” 
(F(1, 28)=5.265, P=0.029, f=0.84) and “Time×group” interaction (F(1, 28)=5.411, P=0.03, f=0.87) for the hip abductor moment. Our post hoc analysis indicated no statistically significant differences in hip abductor moments for the experimental group (P>0.05).
A tendency was observed toward a significant main effect of the “time×group” interaction (F(1, 28)=5.401, P=0.03, f=0.87) for the external rotator moment. However, no significant main effects of “time” or “group” interactions were found. Our post hoc analysis indicated no statistically significant pre to post-change in hip external rotator moment for the experimental group (P>0.05) (Table 3).

According to the results indicated in the experimental group, the peak hip flexor moment during the post-test was significantly lower compared to the pre-test (∆ 14%, P<0.05, d=0.26). The findings indicated no statistically significant differences between the two groups in adductor and external rotator hip joint moments (P>0.05) (Table 3). In the control group, no significant differences were observed between the pre-test and post-test stages (P>0.05) (Table 3).
A trend toward a significant main effect of “time” was found for the coupling angle during the flexion phase 
(F(1, 28)=25.78, P<0.05, f=1.88) and extension phase (F(1, 28)=6.42, P=0.02, f=0.93). In addition, a significant “time×group” interaction was found for the flexion phase (F(1,28)=27.84, P<0.05, f=1.96) and extension phase (F(1, 28)=6.92, P=0.01, f=0.96). Moreover, the post hoc analysis indicated that in the experimental group, the mean coupling angles during the post-test in both flexions (∆ 5.1%, P<0.05; d=0.94) and extension (∆ 4.1%, P=0.02, d=0.63) phases were significantly lower compared to the pre-test (∆ 14%, P<0.05, d=0.26) (Table 4).
 The findings indicated that in the experimental group, the peak negative hip joint power during the post-test increased by 17% compared to the pre-test (P<0.05, d=0.57) (Table 4). In the control group, both peak positive and negative hip joint powers and coupling angles were not statically altered in the post-test compared to the pre-test (Table 4). 

Discussion 
Our results demonstrated that the changes in the sagittal mean coupling angle between the lumbar and pelvic after the implication of treatment demonstrated significantly decreased during the trunk flexion phase. The changes of roland morris disability questionnaire (RMQ), peak hip extension moment, peak hip extension moment, and peak negative hip power values were significantly larger in the core stabilization exercise group compared to the control group. Higher peak negative hip power in the experimental group after the training protocol may be because of the improvement in core muscular strength that was reported in the previous studies [6, 11, 32, 38, 39]. Shock absorption and load dissipation are related to negative joint power [40]; therefore, increased peak negative hip power may be one of the causes of pain reduction after the training protocol [41]. Furthermore, the back muscles of patients with LBP have shown protective guarding behavior and splinting, as indicated by earlier research [42]. This could affect the lumbar spine’s range of motion and velocity [43]. Additionally, according to the pain-spasm-pain model, pain leads to heightened muscle activation that triggers agonist and antagonist muscle co-contraction; therefore, individuals with back pain are expected to exhibit reduced net muscle moment values because of the antagonistic co-contraction [44]. The present study demonstrated greater peak hip extension moments after the training protocol in the experimental group. The observed significant increase in the hip extensor moments in the experimental group, when compared to the control group, may have resulted from a variety of mechanical factors, including the activation of deep trunk muscles, the use of a motor learning approach to retrain optimal spinal control and coordination, progression of proprioceptive receptors, and an improvement in movement quality, motor skills, and stability. [6, 9, 45, 46, 47, 48, 49]. Furthermore, in the present study, the peak hip external rotator moment was significantly decreased after the treatment. According to Panjabi, inadequate spinal stability can cause excessive segmental rotations which may trigger pain [50]. Therefore, the reduction of peak hip internal rotor moment after core stabilization training protocol may be a possible mechanism in pain reduction.
To the best of our knowledge, existing studies on the effects of core stabilization exercise on movement impairments have focused on measuring changes in muscle activity patterns, specific to transverse abdominis and lumbar multifidus muscles using either electromyography or real-time ultrasound [6, 51, 52, 53, 54]. These changes in muscle activity patterns after core stabilization exercise are also associated with improvement in pain and function [11, 51, 55, 56]. To date, we are unaware of investigations on movement pattern coordination changes after exercise intervention. Our results demonstrated lower sagittal mean coupling angles between the lumbar and pelvic after the implication of training protocol during both trunk flexion and extension phases. This alteration in mean coupling angle after the implication of treatment may lead to improvement in neuromuscular control and postural stability [57, 58]. This is the first study to assess the potential of core stabilization exercise on the trunk and pelvic coordination that represent clinically observed aberrant movement patterns. The understanding of these coordination changes after different treatment methods will help clinicians and researchers to better identify and understand the complexity of aberrant movement patterns. Previous studies demonstrated that kinematics in conjunction with the dynamic systems approach (based on spatial and/or temporal information) can be used to capture trunk and pelvis movement patterns and quantify the amount of deviation from typical movement patterns during standing forward bend tasks [59]. The kinematic variable derived from the lumbopelvic coupling-angle diagram represents the frequency of changes in movement coordination between segments without consideration of spatial and temporal information. Poor movement control of the lumbar segment could disrupt angular velocity and decoupling between the lumbar spine and hip/pelvis [60]. Decoupling in the NSLBP group between the lumbar spine and hip/pelvis and hip/pelvis domination in forward bending would increase the length of the pattern with minimal effect on the area kinematic variables, and might be large enough to increase the difference in the length of the Mean±SD. Our findings demonstrated that changes in movement control (reduced instability catch) were strongly and significantly associated with decreased pain and disability after the implication of treatment. Thus, the reduction in back pain [41] may be the result of an increase in negative hip joint power and a decrease in lumbar-pelvic mean coupling angle that improved the altered sensory and motor organization in NSLBP. Other authors have recommended that patients with CNSLBP may find relief from specific exercises as they have observed an improved range of motion in the lumbar spine and both hips, as well as decreased disability and pain during activities [26, 27, 28]. Even though adherence to the home protocol was not assessed, it was tracked through a log. The treating physiotherapist’s verbal report indicated that compliance was satisfactory overall. These findings will be the first step toward future work that will directly assist in determining the effectiveness of core stabilization exercises and will progress our understanding of the therapeutic mechanism underlying this treatment approach. This will lead to more appropriate exercise prescriptions that may reduce the recurrence of symptoms associated with the lack of resolution of underlying motor impairments in patients with NSLBP [61].

Study limitations 
This study faced a few limitations that should be considered. The sample size was small and the participants were relatively young (in the age range of 25 to 45 years). Although the study had enough statistical power to detect group differences, additional research is required to assess the impact of core stabilization exercises on walking speed, gait cycle, and the incline of walking surfaces. Incorporating electromyographic data from the relevant muscles involved in trunk flexion and extension movements, along with kinematic analysis, could provide further insights into the identification of risk factors associated with movement control impairments in patients with nonspecific low back pain.

Conclusions
A core stabilization exercise program can reduce pain and improve function in patients with CNSLBP [46]. Our findings also revealed that the intervention used in patients with CNSLBP resulted in statistically significant improvement in clinical outcome measures (peak hip joint moments, peak negative hip power, and mean coupling angles). Not all patients may respond to this treatment, and we expected that responders would have greater improvements in movement coordination and control. Therefore, when we further classified patients into responders and non-responders, patients with the presence of deviation away from the sagittal plane or altered lumbopelvic rhythm again demonstrated significant improvements in movement coordination and control with a medium effect size.

Ethical Considerations
Compliance with ethical guidelines
 All ethical principles are considered in this article. The participants were informed of the purpose of the research and its implementation stages. They were also assured about the confidentiality of their information and were free to leave the study whenever they wished, and if desired, the research results would be available to them. A written consent has been obtained from the subjects. principles of the Helsinki Convention were also observed.

Funding
This research did not receive any grant from funding agencies in the public, commercial, or non-profit sectors. 

Authors' contributions
Conceptualization and supervision: Vahid Mohammadi and Amir Letafatkar; Data analysis: Vahid Mohammadi and Amir Ali Jafarnezhadgero; Funding acquisition, methodology, data collection and resources: Vahid Mohammadi; Investigation, writing–original draft, review & editing: All authors.

Conflict of interest
The authors state that they have no competing interests regarding the publication of this manuscript.

Acknowledgments
The authors express their gratitude to Ali Saleh, radiologist, for providing valuable assistance, and to the patients who participated in the study. 

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