Introduction
Nowadays, research on the effectiveness of new methods of treating low back pain is very important due to its high prevalence. Since patients with chronic low back pain have to spend high treatment costs, it should be considered as a cause of functional disability in the patients and a factor causing severe economic losses in society [1]. Back pain has two types of specific and non-specific. Specific low back pain is due to pathophysiological problems, while non-specific low back pain has no any specific cause. Almost 85% of low back pain cases are classified as non-specific [2]. Factors such as individual, psychological, anatomical, and biomechanical structure, weak strength and endurance of trunk muscles, and decreased flexibility may be involved in the occurrence of non-specific low back pain [2, 3]. Chronic low back pain can also affect the quality of life (QoL) due to suffering pain, failed treatment, drug dependence, social isolation, work problems, and severe emotional and psychological distress [4].
In recent studies on the treatment of low back pain, the focus has been on the design and implementation of a type of exercise that aims to maintain and increase lumbar stabilization by retraining the lumbo-pelvic proprioception through affecting the abdominal muscles which are very important in increasing lumbar segmental stability [5]. There is no consensus on a most effective treatment for low back pain, although there are many treatment options. In this study, the we aim to assess and compare the effects of 8 weeks of dynamic neuromuscular stabilization (DNS) and core stability (CS) exercises on pain, hamstring flexibility, balance, abdominal muscle strength, and QoL in men with chronic low back pain.
Methods
This is a quasi-experimental study with a pre-test/post-test design. The study population consists of men aged 21-62 years with chronic low back pain visited the offices of neurologists in Kashan, Iran. They had non-specific low back pain for at least 3 months based on the diagnosis of a specialist, a permit to exercise, and willingness to participate in the study. The samples were randomly divided into two groups of DNS (Age=47.8±9.82 years, BMI=25.8±2.9 kg/m2) and CS (Age=42.4±11.8 years, BMI=25.8±2.9 kg/m2). As baseline, their QoL was measured using the Persian version of 36-item short form survey; the dynamic balance was assessed using the Y-balance test; the pain intensity was measured using the visual analogue scale; the flexibility of the hamstring muscle was measured using a flexibility box, and the strength of the abdominal muscles was measured using half sit-up test. The subjects performed the exercises for 8 weeks, 3 days a week, each session for 45-60 minutes in Kashan Sports House under the supervision of a researcher and a physiotherapist. At the end of the training period, all subjects performed the tests again. 
Shapiro-Wilk test was used to examine the normal distribution of data in each group. Then, paired-sample t-test and analysis of covariance were used to analyze the data and compare the groups. All statistical calculations were performed in IBM SPSS v. 26 software.
Results
As shown in Table 1, comparison of pre-test and post-test scores showed that DNS exercise had significant effect on the QoL (P=0.025) while the effect of CS exercise was not significant.


Both DNS (P =0.002) and CS (P=0.005) exercises could significantly reduce the pain.
They also had a significant effect on abdominal muscle strength (P=0.012 for DNS, P=0.003 for CS) and static balance (P =0.017 for DNS, P =0.001 for CS). While CS exercise improved the hamstring muscle flexibility (P<0.001), the effect of DNS was not significant (P=0.091). The results obtained for dynamic balance did not show significant results in any groups. The analysis of covariance test was used to adjust the pretest scores in the groups. According to the results in Table 1, there was no statistically significant difference between the two types of training methods in any study variables.
Discussion
This interventional study was conducted to compare the effect of 8 weeks of DNS and CS exercises on pain, hamstring muscle flexibility, balance, abdominal muscle strength, and QoL in men with chronic low back pain. The results showed that both training protocols significantly improved the variables of pain, static balance, and abdominal muscle strength, but were not significantly different between the two methods in terms of study variables. Moreover, the DNS exercise significantly improved the QoL and the CS exercise significantly improved hamstring muscle flexibility.
The exercises reduced the low back pain possibly by increasing the pain threshold, strengthening the deep muscles of the back (e.g., multifidus and transverse abdominis), and increasing muscle coordination and lumbar stability [6]. Considering the findings of the present study and other studies, it can be stated that one of the possible mechanisms of DNS exercise is that the pressure of different applied loads, similar to the CS exercise, causes more activity of local metabolism and reduces muscle atrophy, which in turn reduces the sensitivity of the central, peripheral, and spinal pain receptors and changes the person’s perception of pain [7].
Regarding the significant effects of DNS and CS methods on balance, it seems that the physiological mechanisms of balance systems, especially proprioceptors, may have changed and resulted in increased sensitivity of these receptors, reduced pain and improved balance. It should be noted that no single structure covers the balance alone; the balance system includes multiple sensory inputs of the vestibular, visual, proprioceptive and somatosensory systems, which integrates information in the brainstem and cerebellum with the influence of the cerebral cortex, which can reduce back pain both physically and psychologically [8]. To maintain and improve the balance, it is necessary to improve the vestibular and proprioceptive systems, by which the antigravity muscles are activated, and balance is improved. The DNS exercise can be one of the ways to promote sensory stimulation to increase balance [9].
A study showed that both conventional CS and CS suspension exercises reduce pain intensity and increase more tolerance in multifidus muscle, which is one of the stabilizers of lumbar spine in patients with non-specific low back pain [10]. This muscle is one of the paraspinal muscles that plays an important role in strengthening the spine and acts as inhibitory feedback in pain control [10]. A key assumption of the DNS approach is that each joint position depends on the stability of muscle function and the coordination of local and global muscles to ensure the neutral position of the joints in the motor chain. In addition to improving joint function, the quality of this coordination can affect biomechanical and anatomical parameters. The ultimate strategy of this treatment is to control the center of mass, joint stability, and ideal movement quality, which is done automatically by repetition of movements and control of the center of mass. On the other hand, the DNS exercise works by emphasizing the importance of accurate muscle timing and efficient coordination, as well as resistance to compressive forces in static movements [9]. The results of the present study regarding the effect of CS exercise on hamstring muscle flexibility in patients with chronic nonspecific low back pain is consistent with the results of other study [11].
Chronic back pain affect many daily living and social activities; as a result, the patients can no longer play their social role. DNS exercise reduced these problems to some extent; therefore, due to the importance of QoL in the treatment of diseases, especially chronic low back pain, DNS exercise can be used in the current treatment protocols [12]. 
One of the limitations of this study was its small sample size. It is recommended that in future studies, a psychiatrist or psychologist control the patient’s mental state and a larger sample size be used. Another limitation of this study was the lack of control over patients’ physical activities considering their age range; hence, it is recommended that self-management programs should also be given to the patients in future studies.

Ethical Considerations
Compliance with ethical guidelines
The present study obtained ethical approval from the Bioethics Committee of Islamic Azad University of Isfahan (Khorasgan) Branch (Code: IR.IAU.KHUISF.REC.1399.289)

Funding
This article is taken from the master's thesis of Seyyed Mohammad Sadegh Mousavi in the Faculty of Biological Sports Sciences of Islamic Azad University of Isfahan (Khorasgan) Branch.

Authors' contributions
Study concept and design: Mohammad Sadegh Mousavi and Rezvan Mirsafaei; collection, analysis, and interpretation of data, drafting of the manuscript, and funding acquisition: Mohammad Sadegh Mousavi; Critical revision of the manuscript for important intellectual content, statistical analysis, and supervision: Rezvan Mirsafaei.

Conflicts of interest
The authors declare no conflict of interest.

Acknowledgements
The authors would like to thank the Kashan Sports House and all participants for their cooperation in conducting this study.

 

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