1. Introduction
Back pain is one of the most common problems in developing and developed countries that people face problems such as pain and imbalance in daily life. Nurses are considered as high risk groups for low back pain due to their professional work [1, 2]. The aim of this study was to investigate the effect of eight weeks of selected exercises in water on pain and balance of female nurses with chronic low back pain.

2. Methods
This quasi-experimental study was performed on 30 nurses working in Sanandaj city hospitals. The selected exercises in water were performed by the experimental group for eight weeks, three sessions per week, which were progressive and with the approach of central stability exercises. To evaluate the pre-test and post-test of pain and balance of the subjects, the standard Quebec questionnaire (for pain assessment), Stork Test (static balance) and LEAF Test (dynamic balance) were used [3, 4]. Independent and dependent t-tests were used to analyze the data. Significance level was considered P<0.05.

3. Results
The mean scores of pain and static and dynamic balance were significantly different in the experimental and control groups, so that a significant decrease was observed in pain scores (P=0.001). There was a significant increase in static balance (P=0.001) and dynamic balance (P=0.001) scores, while none of these variables were significant for the control group (P≥0.05).
According to the results of independent t-test in Table 1, there is no significant difference in the pre-test of the control and experimental groups in the variables of pain intensity, static balance and dynamic balance, while the differences in the post-test of these variables are significant (P≤0.05).



4. Discussion and conclusion
The results of this study revealed that selected exercises in water improved pain and balance in the experimental group with non-specific chronic low back pain. Due to the effectiveness of these exercises, it is recommended to pay special attention to hydrotherapy exercises (with the approach of strengthening the central muscles) in designing training protocols for patients with chronic back pain. Regarding the possible mechanism of pain relief for nursing women with chronic non-specific low back pain following the selected water exercise program in this study, it should be noted that the water exercise protocol includes a reduction in weight-bearing forces. Patients who exercise in the water feel lighter and easier to move.
The feeling of losing weight in water seems to relieve or significantly reduce muscle cramps, which in turn reduces muscle spasms. Baker believes that non-weight-bearing exercises are a good option for reducing back pain, and that a person can control his or her weight if he or she is submerged in water [14]. On the other hand, in this study, central stability exercises were used as selected exercises in water. Many researchers consider the role of muscles and soft tissues around the spine in causing low back pain [15]. Their argument is that muscle weakness to support inactive structures against overload may destroy these pain-sensitive structures and ultimately cause pain in these individuals [29].
Previous studies have concluded that abnormalities and weaknesses in both the global and local muscular systems contribute to back pain. Researchers have linked these weaknesses to inappropriate postures that people show during various activities. In most cases, weakness and atrophy are observed in the internal parts of the multifidus muscle and other deep muscles of the lower back, and the relationship between back pain and atrophy of these muscles has been proven in many other studies [29]. Regarding the possible mechanism of improving the balance of female patients with chronic low back pain in this study, it can be stated that in people with low back pain, physiological mechanisms in the balance systems are altered and incorrect information about the spatial position of the body is sent to the brainstem [20]. The higher density of water than air helps the resistance force engage the muscles and increase their strength, which in turn improves patients’ balance.
In this study, other causes of improved balance can be attributed to the physical properties of water. Immersion of the body and floating in the water environment increases the input of deep receptors and improves the balance by more adjustment and stability of the body. On the other hand, because water has a higher viscosity than air, it has a higher resistance. Therefore, sensory feedback in the water environment increases, and thus improves the body’s sense of awareness. Other effects of exercise in water on improving balance include stimulating the atrial system and facilitating atrial inlets [22, 23]. Exposure to water can enhance the stimulation of the skin inlets and thus increase the stimulation of the afferent nerves. Therefore, multisensory exercises such as water exercises may stimulate the senses involved in balance to provide conditions for challenging the balance system, and thus be effective in improving balance [15].

Ethical Considerations
Compliance with ethical guidelines
All ethical principles are considered in this article. The participants were informed about 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.

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

Authors' contributions
All authors equally contributed to preparing this article.

Conflicts of interest
The authors declared no conflict of interest.

Refrences

1. Ramazani Badr F, Nikbakht A, Mohammadpour A. Low-back pain prevalence and its risk factors in nurses. Iran J Nursing Res. 2006; 1(2):37-42. http://ijnr.ir/browse.php?a_id=30&sid=1&slc_lang=en
2. Waddell G, Burton AK. Occupational health guidelines for the management of low back pain. Occup Med. 2001; 51(2):124-35. [DOI:10.1093/occmed/51.2.124] [PMID]
3. Agilinejad M, Mostafaei M. [Occupational medicine and professional diseases (Persian)]. Tehran: Arjmand Pub. https://www.gisoom.com/book/1362648/%DA%A9%D88C/
4. Tinubu BMS, Mbada CE, Oyeyemi AL, Fabunmi AA. Work-related musculoskeletal disorders among nurses in Ibadan, south-west Nigeria: A cross-sectional survey. BMC Musculoskelet Disord. 2010; 11:12. [DOI:10.1186/1471-2474-11-12] [PMID] [PMCID]
5. Mohseni-Bandpei MA, Fakhri M, Ahmad-Shirvani M, Bagheri Nessami M, Khalilian AR. [Epidemiological survey of low back pain among nurses (Persian)]. J Babol Uni Med Sci. 2005; 7(26):35-40. http://jbums.org/browse.php?a_id=2635&sid=1&slc_lang=fa
6. Hoy D, Brooks P, Blyth F, Buchbinder R. The epidemiology of low back pain. Best Pract Res Clin Rheumatol. 2010; 24(6):769-81. [DOI:10.1016/j.berh.2010.10.002] [PMID]
7. Ferreira ML, Ferreira PH, Latimer J, Herbert R, Maher C. Dose spinal manipulative therapy help people with chronic low back pain? Australian J Plant Physiol. 2002; 48(4):277-84. [DOI:10.1016/S0004-9514(14)60167-7]
8. Mostafazade F, Rostamzade M, Mashofi M, Afzalifard H. [Assessing quality of life in low back pain admitted in Ardebil Physiography Center 2004 (Persian)]. Aflak. 2005; 2(2-3). https://eprints.arums.ac.ir/631/
9. Harreby M, Kjer J, Hesselsoe G, Neergaard K. Epidemiological aspects and risk factors for low back pain in 38-year-old men and women: A 25-year prospective cohort study of 640 school children. Eur Spine J. 1996; 5(5):312-8. [DOI:10.1007/BF00304346] [PMID]
10. Detmar SB, Aaronson NK. Quality of life assessment in daily clinical oncology practice: A feasibility study. Eur J Cancer. 1998; 34(8):1181-6. [DOI:10.1016/S0959-8049(98)00018-5]
11. Mousavi SJ, Parnianpour M, Mehdian H, Montazeri A, Mobini B. The Oswestry Disability Index, the Roland-Morris Disability Questionnaire, and the Quebec Back Pain Disability Scale: Translation and validation studies of the Iranian versions. Spine. 2006; 31(14): 454-9. [DOI:10.1097/01.brs.0000222141.61424.f7] [PMID]
12. Oleson M. Subjectively perceived quality of life. Image J Nurs Sch. 1990; 22(3):187-90. [DOI:10.1111/j.1547-5069.1990.tb00205.x] [PMID]
13. Bagheri S, Sarmadi A, Dadashi L. Learning effects of the biodex balance system during assessment of postural task with in test-retest measurements. Koomesh. 2012; 13(3):354-63. http://koomeshjournal.semums.ac.ir/article-1-1455-en.html
14. Carpes FP, Reinehr FB, Mota CB. Effects of a program for trunk strength and stability on pain, low back and pelvis kinematics, and body balance: A pilot study. J Bodyw Mov Ther. 2008; 12(1):22-30. [DOI:10.1016/j.jbmt.2007.05.001] [PMID]
15. Resende SM, Rassi CM, Viana FP. Effects of hydrotherapy in balance and prevention of falls among elderly women. Rev Bras Fisioter. 2008; 12(1):57-63. [DOI:10.1590/S1413-35552008000100011]
16. Roth A, Miller MG, Ricard MD, Ritenour D, Chapman BL. Comparisons of static and dynamic balance following training in aquatic and land environments. J Sport Rehabil. 2006; 15(4):299-311. [DOI:10.1123/jsr.15.4.299]
17. Kawasaki JM. The effects of aquatic exercise on balance outcomes in individuals with Parkinson’s disease. Med Sci Sports Exercise. 2009; 41(5):102. [DOI:10.1249/01.MSS.0000354870.87591.d9]
18. Gillespie LD, Robertson MC, Gillespie WJ, Lamb SE, Gates S, Cumming RG, et al. Interventions for preventing falls in older people living in the community. Cochrane Database Syst Rev. 2012; 2012(9):CD007146. [DOI:10.1002/14651858.CD007146.pub3] [PMID] [PMCID]
19. Viliani T, Pasquetti P, Magnolfi S, Lunardelli ML, Giorgi C, Serra P, et al. Effects of physical training on straightening-up processes in patients with Parkinson’s disease. Disabil Rehabil. 1999; 21(2):68-73. [DOI:10.1080/096382899297990] [PMID]
20. Kopec JA, Esdaile JM, Abrahamowicz M, Abenhaim L, Wood-Dauphinee S, Lamping DL, et al. The Quebec Back Pain Disability Scale. Measurement properties. Spine. 1995; 20(3):341-52. [DOI:10.1097/00007632-199502000-00016] [PMID]
21. Waller B, Lambeck J, Daly D. Therapeutic aquatic exercise in the treatment of low back pain: A systematic review. Clin Rehabil. 2009; 23(1):3-14. [DOI:10.1177/0269215508097856] [PMID]
22. Dundar U, Solak O, Yigit I, Evcik D, Kavuncu V. Clinical effectiveness of aquatic exercise to treat chronic low back pain: A randomized controlled trial. Spine. 2009; 34(14):1436-40. [DOI:10.1097/BRS.0b013e3181a79618] [PMID]
23. Sedaghati N, Hematfar A, Behpour N. [The effect of a selected spinal core-muscle stabilization training in water on pain intensity and lumbar lordosis (Persian)]. Feyz. 2013; 17(3):267-74. https://www.sid.ir/fa/journal/ViewPaper.aspx?id=216136
24. Lee JH, Ooi Y, Nakamura K. Measurement of muscle strength of the trunk and the lower extremities in subjects with history of low back pain. Spine. 1995; 20(18):1994-6. [DOI:10.1097/00007632-199509150-00006] [PMID]
25. Rutledge E, Silvers WM, Browder K. Metabolic-cost comparison of submaximal land and aquatic treadmill exercise. Int J Aquat Res Educ. 2007; 1(2):118-33. [DOI:10.25035/ijare.01.02.04]
26. Hernandez-Reif M, Diego M, Field T. Preterm infants show reduced stress behaviors and activity after 5 days of massage therapy. Infant Behav Dev. 2007; 30(4):557-61. [DOI:10.1016/j.infbeh.2007.04.002] [PMID] [PMCID]
27. Ruhe A, Fejer R, Walker B. Center of pressure excursion as a measure of balance performance in patients with non-specific low back pain compared to healthy controls: A systematic review of the literature. Eur Spine J. 2011; 20(3):358-68. [DOI:10.1007/s00586-010-1543-2] [PMID] [PMCID]
28. Arendt-Nielsen L, Graven-Nielsen T. Muscle pain: Sensory implications and interaction with motor control. Clin J Pain. 2008; 24(4):291-8. [DOI:10.1097/AJP.0b013e31815b608f] [PMID]
29. Moseley GL, Hodges PW. Are the changes in postural control associated with low back pain caused by pain interference? Clin J Pain. 2005; 21(4):323-9. [DOI:10.1097/01.ajp.0000131414.84596.99] [PMID]