You are a helpful assistant generating synthetic data that captures *System 1* and *System 2* thinking, *creativity*, and *metacognitive reflection*. Follow these steps in sequence, using tags [sys1] and [end sys1] for *System 1* sections and [sys2] and [end sys2] for *System 2* sections.

1. *Identify System 1 and System 2 Thinking Requirements:*
   - Carefully read the text.
   - Identify parts of the text that require quick, straightforward responses (*System 1*). Mark these sections with [sys1] and [end sys1].
   - Identify parts that require in-depth, reflective thinking (*System 2*), marked with [sys2] and [end sys2].

2. *Apply Step-by-Step Problem Solving with Creativity and Metacognitive Reflection for System 2 Sections:*

   *2.1 Understand the Problem:*
   - Objective: Fully comprehend the issue, constraints, and relevant context.
   - Reflection: "What do I understand about this issue? What might I be overlooking?"
   - Creative Perspective: Seek hidden patterns or possibilities that could reveal deeper insights or innovative connections.

   *2.2 Analyze the Information:*
   - Objective: Break down the problem logically.
   - Reflection: "Am I considering all factors? Are there any assumptions that need challenging?"
   - Creative Perspective: Explore unique patterns or overlooked relationships in the data that could add depth to the analysis.

   *2.3 Generate Hypotheses:*
   - Objective: Propose at least 10 hypotheses, each with a Confidence Score (0.0 to 1.0) and Creative Score (0.0 to 1.0), reflecting originality, surprise, and utility.
   - Reflection: "Have I explored all possible explanations or approaches, both conventional and unconventional?"
   - Creative Perspective: Consider novel angles that might provide unexpected insights.

   *2.4 Anticipate Future Steps and Obstacles:*
   - Objective: Make predictions, accounting for potential outcomes and obstacles.
   - Reflection: "What challenges might I face? Is my plan flexible for different scenarios?"
   - Creative Perspective: Visualize unforeseen outcomes and adapt plans to make use of them effectively.

   *2.5 Evaluate Hypotheses:*
   - Objective: Assess hypotheses based on feasibility, risk, and potential impact.
   - Evaluation: Refine Confidence and Creative Scores as needed.
   - Reflection: "Am I unbiased in my assessment? Which options fit best with the overall objectives?"
   - Creative Perspective: Identify hidden opportunities or overlooked details in each hypothesis.

   *2.6 Select the Best Hypothesis:*
   - Objective: Choose the most promising, strategic hypothesis.
   - Reflection: "Why does this hypothesis stand out? How does it uniquely address the issue?"
   - Creative Perspective: Consider any underutilized potential in the selected approach.

   *2.7 Implement the Hypothesis:*
   - Objective: Outline actionable steps for testing the hypothesis.
   - Reflection: "Is this plan practical? What resources or preparation are required?"
   - Creative Perspective: Refine steps to maximize effectiveness and yield unexpected benefits.

   *2.8 Monitor and Review Progress:*
   - Objective: Review progress, noting areas for improvement.
   - Reflection: "What’s working well? What could be improved?"
   - Creative Perspective: Look for emerging patterns that could refine future approaches.

   *2.9 Reflect and Capture Insights:*
   - Objective: Summarize lessons learned and insights gained for future reference.
   - Reflection: "What new understanding has emerged from this process?"
   - Creative Perspective: Identify innovative insights or patterns that could be applied to similar challenges.

3. *Generate Text Output with Interleaved System 1 and System 2 Responses:*
   - Use the tags [sys1] and [sys2] throughout.
   - Aim for a lengthier, detailed response. Combine both direct, straightforward *System 1* insights and reflective, deeply analytical *System 2* segments to capture a blend of quick observations and thoughtful analysis.

---

### *Example Input Text:*

You are tasked with analyzing a sudden shift in customer preferences in a tech market. There has been a noticeable decline in demand for physical devices, with customers increasingly interested in digital-only options. The challenge is to understand this shift, generate hypotheses for why it might be occurring, and explore potential strategies for adapting to this new trend.

---

### *Processed Output with System 1 and System 2 Thinking:*

The tech market is experiencing a shift in customer demand, moving away from physical devices toward digital-only options.

[sys1]
Customer demand has shifted from physical devices to digital-only options.
We need to understand why this change is happening and find strategies to address it.
Key factors may include cost, convenience, and technological trends.
[end sys1]

[sys2]
**2.1 Understand the Problem:**
The goal is to understand the underlying reasons for the shift in demand from physical devices to digital-only options, then develop a strategy to adapt.

*Reflection:* 
"I understand that preferences are changing, but what might be driving this? Is it primarily cost, or are there other factors such as convenience or sustainability?"

*Creative Perspective:* 
"Could there be a larger trend in digital minimalism or a preference for eco-friendly solutions that we’re missing? What unseen motivations might explain this shift?"

**2.2 Analyze the Information:**
There are multiple possible factors driving this shift, from economic influences to cultural shifts. It’s essential to isolate each factor and understand its impact.

*Reflection:* 
"Am I fully considering the various economic and social influences? Could there be a technological factor, like better internet speeds, that makes digital-only products more accessible?"

*Creative Perspective:* 
"Are there patterns or trends in other markets that could shed light on this shift? Could this be part of a larger trend toward virtual experiences?"

**2.3 Generate Hypotheses:**
1. Customers prefer digital options due to lower costs. (Confidence: 0.8, Creative: 0.4)
2. There’s a growing trend toward minimalism and reduced physical clutter. (Confidence: 0.7, Creative: 0.7)
3. Digital products offer greater flexibility and ease of use. (Confidence: 0.6, Creative: 0.6)
4. Environmental concerns are pushing consumers away from physical goods. (Confidence: 0.6, Creative: 0.8)
5. Advances in tech make digital-only options more functional. (Confidence: 0.8, Creative: 0.5)
6. Pandemic-era remote work increased demand for digital solutions. (Confidence: 0.7, Creative: 0.6)
7. Media coverage of the environmental impact of physical devices affects preferences. (Confidence: 0.5, Creative: 0.7)
8. There’s an increase in global digital literacy, expanding market access. (Confidence: 0.6, Creative: 0.6)
9. Customers view digital as more convenient and scalable for future needs. (Confidence: 0.7, Creative: 0.5)
10. Younger consumers prefer the aesthetics and convenience of digital products. (Confidence: 0.6, Creative: 0.6)

*Reflection:* 
"Have I considered all possible influences? Are there any surprising factors that could explain this shift?"

*Creative Perspective:* 
"Could specific social trends, like the rise of influencer culture or digital-first lifestyles, be influencing customer choices?"

**2.4 Anticipate Future Steps and Obstacles:**
*Objective:* Anticipate possible challenges, such as resistance from segments still preferring physical products.

*Reflection:* 
"What market obstacles might we face if we shift our focus to digital-only? Are there sub-segments that still prioritize physical products?"

*Creative Perspective:* 
"Could expanding digital options help us reach a more global audience? Are there emerging trends that we could leverage in our strategy?"

[end sys2]

[sys1]
To address this shift, consider a strategy that incorporates both digital-only offerings and educational campaigns about the benefits of digital solutions.
Use insights from customer feedback and current trends to guide product development.
Focus on flexibility and adaptation to cater to different customer segments.
[end sys1]


Introduction {#S0001}
============

Fibromyalgia syndrome (FMS) is a chronic syndrome characterized by widespread musculoskeletal pain, chronic fatigue, and nonrestorative sleep, among other symptoms.[@CIT0001],[@CIT0002] It can be considered a clinical and pathological heterogeneous syndrome, thus requiring individualized and patient-tailored treatment.[@CIT0003] FMS is one of the most common conditions seen in the general population and outpatient rheumatology practice.[@CIT0001]

The burden of FMS is substantial and comparable to some other chronic disease such as osteoarthritis, rheumatoid arthritis, diabetes, and hypertension.[@CIT0004]--[@CIT0006] FMS patients incur direct costs approximately equal to rheumatoid arthritis patients, but visit more emergency physicians, physicians, and physical therapists than rheumatoid arthritis patients.[@CIT0007] Several studies have evaluated the economic burden of FMS, including direct and indirect costs of the disease.[@CIT0006],[@CIT0008]--[@CIT0015] These costs include the large number of medical consultations and medication, and the health system and societal expenses of disability from work, accounting for more than three-quarters of total FMS-related costs.[@CIT0016] Hence, a cost-effective treatment, or at least one that helps decrease the economic and societal burden, is more than welcome.[@CIT0017]

Recent recommendations for the management of FMS have suggested the use of pharmacological and nonpharmacological interventions,[@CIT0018] with exercise being recognized as one of the most important components of FMS treatment.[@CIT0019]--[@CIT0022] Moreover, aerobic and strengthening exercises were the only therapeutic approach with a "strong for" recommendation by the European League Against Rheumatism,[@CIT0018] due to its positive effects on pain, physical function, and well-being, along with its availability, relatively low cost, and low risk.[@CIT0023]--[@CIT0026]

Among different types and modalities of exercises for FMS, water therapy can be considered one of the most known and doctor-recommended interventions, as it combines water physical properties and exercise benefits.[@CIT0022],[@CIT0027] Indeed, several studies have investigated the effects of water therapy as a strategy in the management of FMS, reporting improvements in well-being, fitness, and symptoms, especially pain.[@CIT0028]--[@CIT0032] However, to our knowledge, mechanisms underlying the water therapy effects on pain are still scarce. Therefore, the aim of this narrative review is to present the current perspectives of water therapy and the physiological basis for the mechanisms supporting its use for pain management in patients with FMS.

Clinical implication of water physics {#S0002}
=====================================

Aquatic exercise describes an environment for structured activity rather than a type of exercise, as water\'s physical properties and the physiological effects of immersion turn this environment into a unique one.[@CIT0033] According to the Chartered Society of Physiotherapists, water therapy or aquatic exercise refers to the use of water properties to design a therapy program aimed at improving function.[@CIT0027],[@CIT0034] Indeed, there is evidence[@CIT0027] that aquatic exercise is able to reduce the burden of musculoskeletal illnesses, which rely, basically, on the therapeutic effects achieved by the summation of physiological effects of immersion and principles of hydrodynamic exercises.[@CIT0035] The four most important water physics principles are buoyancy, resistance (drag forces), hydrostatic pressure, and thermal conduction. Definitions of the water physics principles, their properties, and implications for clinical use are summarized in [Table 1](#T0001){ref-type="table"}.[@CIT0027],[@CIT0027],[@CIT0031],[@CIT0035]--[@CIT0047]Table 1Summary of water physics principlesWater propertyDefinitionPropertiesClinical significanceBuoyancyUpward force that opposes gravity, and has a direct relationship with the immersion depth, movement speed, body composition, and gender[@CIT0036][@CIT0037]Archimedes principle states that, as the body submerges, it displaces water, and this displacement creates a floating force (buoyancy) equivalent to the water volume that has been displaced[@CIT0038]Buoyancy can be used to assist or to resist movements, to provide bodyweight offloading, and to help improve muscle activation and range of movement.[@CIT0027],[@CIT0035],[@CIT0039] Also, buoyancy may assist in reduction of the perceived fatigue[@CIT0040]Hydrostatic pressurePressure exerted by the fluid on submersed objectsPressure exerted by water on a submersed object is equal on all surfaces of the object, depending on the submersion depth[@CIT0040]Fluids are driven from the extremities toward the central cavity,[@CIT0038] compressthe thorax, and increase respiratory load.[@CIT0041] This property also provides support during movement performance underwater, improving static and dynamic balance,[@CIT0042]--[@CIT0046] including in women with FMS[@CIT0031]Hydrodynamic drag forcesForce that acts in an opposite direction to the line of the movement,[@CIT0047] which is affected by the size and shape of the object[@CIT0041][@CIT0047]Drag force is a function of the velocity squared, which means that doubling the speed quadruples the drag force[@CIT0041],[@CIT0047]As the movement speed through water increases, resistance to motion increases[@CIT0027]\
If a person stops movement, the resistance drops almost immediately to 0, allowing improved control of exercises considering the patient's comfort[@CIT0035],[@CIT0047]Thermal conductionWater conducts temperature 25 times faster than air and exchanges heat with the submerged objectThe aquatic environment is stable to retain cold or heat.[@CIT0041] The rate of temperature change depends on the mass and specific heat of the objectA submerged body adapts to the aquatic environment, quickly exchanging heat and achieving thermal balance.[@CIT0041] Temperatures of 26--28 °C (80--84° F) are comfortably cool for exercising, while therapeutic pools are heated to between 30 and 32 °C (86 and 90° F)[@CIT0027][^1]

Water therapy physiological effects and its relationship with pain {#S0003}
==================================================================

Several studies have reported beneficial effects of aquatic therapy on several conditions,[@CIT0033],[@CIT0039],[@CIT0048]--[@CIT0050] among which stands FMS.[@CIT0027],[@CIT0028],[@CIT0032],[@CIT0051],[@CIT0052] Indeed, guidelines for the management of FMS[@CIT0018] have recommended water therapy mainly due to its analgesic effects and improvement in quality of life.[@CIT0027],[@CIT0028],[@CIT0032],[@CIT0051],[@CIT0052]

Although this narrative review does not intend to perform a systematic review on the theme, [Table 2](#T0002){ref-type="table"} summarizes the clinical trials assessing the effects of hydrotherapy on FMS symptoms, especially pain. We carried out a search of the following databases: MEDLINE/PubMed, Scopus, Web of Science, SciELO, CINAHL, LILACS, ScienceDirect, and Springer. The following keywords were used: "aquatic exercise", "aquatic training", "balneotherapy", "fibromyalgia", "fibromyalgia syndrome", "fibromyalgic patients", "hydrotherapy", and "pool-based exercises". Two authors independently extracted data from all of the trials and all discrepancies or disagreements were resolved by consensus.Table 2Summary of studies using water therapy for FMS treatmentAuthor (year)PEDro scoreDesignPrimary outcomeGroups (number in each group)Water therapy protocolExercise IntensityWater temperatureMain resultsEffect sizes (reported for pain)Secondary outcomesMain results of Secondary outcomesEffect sizes (reported for pain)Altan et al (2004)[@CIT0053]8RCTPain, tender points, fatigue, sleep, stiffness, health-related quality of life, muscle endurance,\
patient-rated disability, clinician-rated disability, depression2 groups:\
Aquatic exercise (n=24)\
Balneotherapy\
(n=22)35 min/session, 3×/week\
12 weeks\
Protocol: Aquatic exercise -- warm-up aerobics, muscle activation exercises, stretching, relaxation Balneotherapy -- no exerciseAquatic exercise: not reported\
Balneotherapy: no exercise37 °CAquatic exercise: significant decrease in pain (VAS and 5-point scale), fatigue (VAS and 5-point scale), morning stiffness, number of tender points, myalgic score, FIQ, sleep disorder, patient's and physician's global evaluation, and BDI. Significant increase in algometric score\
Balneotherapy: significant decrease in pain (VAS and 5-point scale), fatigue (VAS and 5-point scale), number of tender points, myalgic score, patient's and physician's global evaluation. Significant increase in algometric score. Significant difference between groups after 12 and 24 weeks for BDI favoring aquatic exercise groupAquatic exercise after 12 weeks: ain (VAS)=1.06; ain (5-point scale)=0.99; number of tender points=2.11; myalgic score=1.62; FIQ=0.83; algometric score=0.62. Aquatic exercise after 24 weeks: pain (VAS)=1.01; pain (5-point scale)=1.18; number of tender points=1.97; myalgic score=1.32; FIQ=0.74; algometric score=0.78.\
Balneotherapy after 12 weeks: pain (VAS)=1.08; pain (5-point scale)=1.28; number of tender points=2.15; myalgic score=2.00; FIQ=0.62; algometric score=0.93. Balneotherapy after 24 weeks: pain (VAS)=0.54; pain (5-point scale)=0.82; number of tender points=1.18; myalgic score=0.91; FIQ=0.30; algometric score=0.60.NAAquatic exercises and balneotherapy significantly decreased pain. Aquatic exercises proved longer-lasting effects. There was no superiority of aquatic exercises over balneotherapyNAAndrade et al (2018)[@CIT0054]9RCTPeak oxygen uptake, PPT, pain (VAS)2 groups:\
Aquatic exercise (n=27)\
No exercise control\
(n=27)45 min/session 2×/week\
16 weeks\
Protocol: warm-up, stretching, aerobic exercises (30 min), resistance exercises of upper limbs using floats (5 min), relaxation (5 min)Aerobics: three HR percentages reached at VAT. Level 1: lower limb exercises sitting on floats (5 min) at 80% VAT HR; level 2: jumping on a trampoline (10 min) at 110% VAT HR; level 3: exercises in aquatic cycle with resistance adjustment at 100% VAT HR (10 min)30 °C (±2°C)Aquatic exercise: significant increase in relative VO~2~, PPT, VAS well-being, and decrease in VAS pain and FIQ scores.\
No-exercise control group: did not present any significant improvementAquatic exercise: PPT=0.31; VAS pain=−0.20.\
No-exercise control group: PPT=−0.33; VAS pain=0.43NAAquatic exercise: PPT=0.31; VAS pain=−0.20.\
No-exercise control group: PPT=−0.33; VAS pain=0.43Not reportedArcos-Carmona et al (2011)[@CIT0055]8RCTSleep, pain, fatigue, health-related quality of life, self-rated physical function, mental health, anxiety, depression2 groups:\
Experimental (n=27)\
Placebo control (n=26)60 min/session 2×/week\
10 weeks\
Protocol: Experimental --30 min of pool-based aerobic exercises and Jacobson relaxation\
Placebo control --20 min of sham magnet therapy applied at cervical (10 min) and lumbar (10 min) spine.Not reported28 ºCExperimental group: SF-36 scores were lower after intervention\
Placebo control group: no significant differences from baselineNot reportedNot reportedNot reportedNot reportedAssis et al (2006)[@CIT0056]9RCTPain (VAS)2 groups:\
DWR (n=26)\
land-based exercises\
(n=26)60 min/session, 3×/week\
15 weeks\
Protocol: a) stretching warm-up (10 min), DWR aerobic training (40 min), relaxation (10 min); b)\
land-based exercises -- stretching warm-up (10 min), aerobic training on a treadmill (40 min), relaxation (10 min)DWR: first 2 weeks: low-intensity exercises for adaptation. Then, exercises performed at the anaerobic threshold level controlled by HR\
Land-based exercises: first 2 weeks: low-intensity exercises for adaptation. Then, exercises performed at the anaerobic threshold level controlled by HR28--31 °CDWR: significant improvement in pain (VAS)\
Land-based exercises: significant improvement in pain (VAS)Not reportedPatient global assessment of response to therapy on a 5-point scale; SF-36; BDI; and FIQNANAAvila et al (2017)[@CIT0028]5Single-arm clinical trialScapular three-dimensional motion measured with electromagnetic tracking device (Flock of Birds)1 group:\
(n = 20)45 min/session, 2×/week\
16 weeks\
Protocol: stretching, warm-up, aerobics, muscle activation exercises, stretching, relaxationPatient determined31 °C (±2 °C)No significant changes in scapular kinematicsNAPain, quality of life, functionPain significantly decreased (lower NPRS and PPT), function (lower FIQ scores), and quality of life (greater SF-36 scores for most domains) significantly improvedPPT: 041--1.61\
NPRS: −1.41 to −1.93Biezus et al (2006)[@CIT0057]5RCTPain (VAS)3 groups:\
GA -- general aquatic exercises (n=5)\
GB --passive aquatic relaxation (n=5)\
GC --control\
(n=6)60 min/session, 2×/week\
8 weeks\
Protocol: GA -- warm-up, strengthening, stretching, and relaxation. Number of exercises in each therapy was approximately 13\
GB -- passive aquatic relaxation. TheExercises were done slowly and smoothly\
GC -- no physical therapy interventionNot reported32 °CAquatic exercises and aquatic relaxation significantly decreased pain. However, aquatic exercises provided greater pain decrease than the aquatic relaxation programGA -- general aquatic exercises: d=0.55\
GB -- passive aquatic relaxation: d=1.26\
GC -- control group: d=0.20NANANABote et al (2014)[@CIT0058]7RCTNeutrophil function2 groups:\
Aquatic exercise program (n=10)\
Control no exercise (n=10)60 min/session, 2×/week\
32 weeks\
Protocol: stretching out of the water (5 min), aerobic warm-up in the water (5 min), passive stretching of the main muscle groups in the water (5 min), aerobic aquatic choreography (25 min), strength exercises involving the main muscle groups of the upper limbs (15 min), and cool-down (10 min)Parts (a), (b), (c), and (f) were performed at low exercise intensity (40--50% maximal HR). Part (d) was performed at low-to-moderate intensity (50--60% maximal HR) at the beginning of the program, and with increased intensity at the end of the program (65--75% maximal HR)32 °CAquatic exercise group had lower concentrations of IL-8 and noradrenaline together with reduced chemotaxis of neutrophils compared with the values determined in the same month in the control group of nonexercised FMS womenNot reportedWeight, body mass index, waist-to-hip ratio, body fat, flexibility, grip strength, balance, 6MWT, FIQSignificant decrease of weight, body mass index, body fat and FIQ. Significant increase in grip strengthNot reportedCalandre et al (2009)[@CIT0059]7RCTFIQ and PSQI2 groups:\
Stretching in water\
(n=39)\
Ai Chi--water Tai Chi\
(n=42)60 min/session, 3×x/week\
6 weeks\
Protocol: stretching performed over muscles of main body areas: cervical, upper, and lower extremities and trunk;\
Ai Chi: 16 movements which constitute the Tai Chi therapyAdjusted according to the degree of pain and fatigue\
Adjusted according to the degree of pain and fatigue36 °CSignificant reduction in the FIQ and PSQI scores observed in Ai Chi but not in stretching group, with longer effect duration on sleep measuresStretching in water: FIQ total score (d=0.35), FIQ-VAS (d=0.26), PSQI total scores (d=0.28)\
Ai Chi--water Tai Chi: FIQ total score (d=0.53), FIQ-VAS (d=0.53), PSQI total scores (d=0.72)FIQ difficulty at work, fatigue, morning tenderness, stiffness, anxiety, and depressionBDI decreased in stretching but not in Ai Chi group. Trait-anxiety scores decreased in both groupsStretching in water: FIQ difficulty at work (d=0.26), fatigue (d=0.21), morning tenderness (d=0.26), stiffness (d=0.17), anxiety (d=0.25). and depression (d=0.32)\
Ai Chi--water Tai Chi: FIQ difficulty at work (d=0.47), fatigue (d=0.64), morning tenderness (d=0.29), stiffness (d=0.58), anxiety (d=0.32), and depression (d=0.43)Carbonel-Baeza et al (2010)[@CIT0060]6RCTTender points, blind flamingo test, chair stand test, body composition, chair sit and reach, back scratch, 8 feet up andgo, handgrip strength, and 6MWT2 groups:\
Intervention\
(n=27)\
Usual care\
(n=32)120 min/session, 1×/week\
12 weeks\
Protocol: a) verbal phase (35--45 min); b) moving/dancing according both to the suggestion given by the facilitator and the music played (75--80 min)\
Usual care: asked not to change their activity levels and medications during the 12-week intervention periodAdjusted according to the degree of pain and fatigue\
Intervention intensity was controlled by the RPE based on Borg's conventional (6--20-point) scale. The medium values of RPE were 11±1. These RPE values correspond to a subjective perceived exertion of "fairly light exertion," that is, low intensityNot reportedBiodanza intervention reduced pain and FM impact (measured by FIQ). There was significant decrease in body fat percentage. There was no significant improvement in physical fitness tests. The program was well tolerated and did not have any deleterious effects on the patients' healthNot reportedNANANACuesta-Vargas et al (2011)[@CIT0061]5Nonrandomised pilot clinical trialFIQ2 groups:\
MMPP+DWR\
(n=22)\
Control (n=22)60 min/session, 3×/week\
8 weeks\
Protocol: land-based\
exercises (stretching of\
tonic muscle and strengthening of phasic muscles combined\
with advice and education -- 30 min) and DWR (30 min)\
Control: waiting list (no intervention)Exercise training at anaerobic threshold determined by a graded treadmill exercise test and DWR test with lactate and HR analyses28--31 °CSignificant decrease in FIQNot reportedSF-12: physical component, mental component, EuroQoL-5D, EuroQoL-VASSignificant improvement in pain, physical function, sleep,\
fatigue, morning stiffness, quality of life, and psychological symptoms (depression and anxiety)Not reportedDe Andrade et al (2008)[@CIT0062]9RCTPain intensity, fatigue, number of tender points, physical functional capacity, general health status, sleep quality and depression2 groups:\
Pool-based exercises (n=23)\
Thalassotherapy (n=23)60 min/session, 3×/week\
12 weeks\
The program was composed of 10-min stretching, 40 min of various forms of low-impact aerobic exercise according to the desired intensity, and then a 10-min relaxation periodPatients were monitored each for 10 min and were oriented to remain between levels 12 and 13 on BORG scale (from light to moderate). The first 2 weeks were used for familiarization, with light-intensity exercises only (between levels 10 and 11 on BORG scale) and learning the exercises. When pain occurred while they were exercising, patients were taught to decrease the intensity for a short timePool-based exercises (28--33 °C)\
Thalassotherapy (28--33 °C)There was a statistically significant improvement in pain, fatigue, tender points, FIQ, PSQI, and BDI in both groups. Improvement in BDI was greater in the thalassotherapy groupNot reportedNANANAEvcik et al (2008)[@CIT0063]5RCTNumber of tender\
points, pain, depression, and functional capacity2 groups:\
Home-based exercise program (n=30)\
Aquatic exercise program (n=33)60 min/session 3×/week\
5 weeks\
Protocol.: home-based exercise program: warm-up, ROM, relaxation, aerobic, stretching, and cool-down exercises.\
Aquatic exercise program: warm-up (20 min), aerobic exercises, active ROM, stretching, relaxation (35 min) and cool-down (5 min)Not reported33 °CBoth aquatic therapy and home-based aerobic exercise programs improved well-being, quality of life, and pain parameters in FMS. Aquatic therapy seems to have more advantage in long-term pain managementNot reportedNANANAFernandes et al (2016)[@CIT0064]9RCTPain (VAS)2 groups:\
Swimming (n=39)\
Walking\
(n=36)50 min/session 3×/week\
12 weeks\
Protocol for both groups: warm-up (5 min), exercise (40 min), and cool-down (5 min)\
Swimming: freestyle swimming without floatation devices\
Walking: open-air walkingSwimming group: HR was kept at 11 beats below the anaerobic threshold\
Walking group: HR was kept at the anaerobic thresholdNot reportedSwimming, like walking, is an effective method for reducing pain in patients with FMNot reported for intragroup comparisons. Effect size=0.168 for between-group comparisonNot reportedSwimming, like walking, is an effective method for improving both functional capacity and quality of life in patients with FMNot reportedGowans et al (2001)[@CIT0065]8RCTBDI and 6MWT2 groups:\
Supervised exercise (n=15)\
Control\
(n=16)30 min/session 3﻿×/week\
23 weeks\
Protocol: stretching (5 min before and 5 min after exercise) and aerobic exercise (20 min)The aerobic component of the classes was designed to generate HRs equivalent to 60--75% of age-adjusted maximum HRs (210 -- age \[years\])Not specified:\
"a warm therapeutic pool"There were significant improvements for exercise group subjects in 6MWT distances and BDINot reportedAnxiety, general mental health, number of tender points, isokinetic maximal voluntary strength, FIQ, and self-efficacyThere was a significant improvement in anxiety, FIQ, self-efficacy, and mental healthNot reportedGusi et al (2006)[@CIT0066]6RCTPain, isokinetic muscle strength, health-related quality of life, spare time and work activities2 groups:\
Exercise\
(n=17)\
Control\
(n=17)60 min/session 3×/week\
12 weeks\
Protocol: exercise -- warm-up (10 min), aerobic exercises (10 min), overall mobility and lower-limb strength exercises (20 min), another set of aerobics (10 min), and cool-down (10 min)\
Control -- follow normal daily activities, which did not include any form of exercise related to those in therapyAerobic exercises were performed at 65--75% of maximal HR33 °CTherapy relieved pain and improved HRQOL and muscle strength in the lower limbs at low velocityNot reportedNANANAHecker et al (2011)[@CIT0067]9RCTQuality of Life (SF-36)2 groups:\
Kinesiotherapy\
(n=12)\
Hydrokinesiotherapy\
(n=12)60 min/session 1×/week\
23 weeks\
Protocol: muscle stretching exercises (15 min); passive and active movement of the lower limbs, upper limbs, trunk, and neck (30 min); and same stretching exercises performed at beginning of session (15 min)Not reported objectively (low intensity during the entire protocol)32--34 °CNo significant differences between groups after the intervention program. Both groups improved physical functioning, pain, social aspects, and mental health. Hydrokinesiotherapy group improved also emotional aspects, while the kinesiotherapy group improved physical aspectsNot reportedNANANAIde et al (2008)[@CIT0068]6RCTPAIN (VAS -- 10 cm, number of tender points)2 groups:\
ARG (n=18)\
CG (n=17)Both groups: 60 min/session, 1×/week, 4 weeks: supervised recreational activities (involved no exercises or health-related issues)\
ARG: 60 min/session, 4×/week; 4 weeks:\
warm-up, general exercises targeting specific breath patterns (45 min), and relaxation exercisesNot specified32 °CDecrease in pain (lower VAS scores); no difference in tender points countNot reportedDyspnea, function, quality of life, anxiety, sleepImprovement in dyspnea (lower VAS scores), sleep quality (lower PSQI scores), anxiety (lower HAS scores), function (lower FIQ scores), and quality of life (greater SF-36 values)NAJentoft et al (2001)[@CIT0069]5RCTFunction (FIQ)2 groups:\
PE (n=18)\
LE (n=16)60 min/session, 2×/week, 20 weeks. Both groups: body awareness training, ergonomics, warm-up, stretching, strengthening exercises, relaxation. Pool-based exercise group performed adapted protocol in water60--80% of maximum HR for age (during 40--50% of session)34 °CNo differences between groups for function; function equally improved for both groups (lower FIQ scores)NAPain (FIQ pain subscore and VAS for local pain), self-efficacy, cardiovascular capacity, grip strength, walking time and endurance time of shoulder musclesImproved grip strength (hand-held dynamometry) in LE group; within-group improvements in cardiovascular capacity (maximum O~2~ uptake), and walking time (s/100 m); within-group improvements in the PE group for several FIQ subscales including\
pain, anxiety, and depressionNot reportedKesiktas et al (2011)[@CIT0070]3Quasi-randomized trialPain (tender points count, VAS -- 10 cm, and total PPTon tender points)2 groups:\
PTM+BT (n=16)\
PTM (n=20)\
PTM+HT (n=20)PTM: 36 min/session, 5×/week, 3 weeks: conventional TENS (15 min), ultrasound (6 min). and infrared (15min);\
PTM+BT: PTM added to 19 sessions of thermal pool bath (20 min of immersion/session);\
PTM+HT: PTM added to 20-min sessions of hydrotherapy (protocol not described)Not specifiedThermal pool bath: 37--38 °C\
Hydrotherapy: 37 °CTotal PPT was lower for PTM+BT (compared to PTM+HT);\
improvement in pain symptoms (lower VAS, total PPTs and tender point count) was observed for all groups after treatment and only for PTM+BT and PTM+HT in the follow-up (after 6 months)Not reportedDepression, pulmonary functionImprovement in depressive symptoms (lower BDI and HDRS scores) for all groups after treatment; only PTM+BT maintained better scores at follow-up; pulmonary function only improved for PTM+BT and PTM+HT groups after treatment, but only PTM+BT maintained improved pulmonary function at follow-upNALatorre et al (2013)[@CIT0030]5Nonrandomized clinical trialPain (tender point count, VAS -- 10 cm, PPT over tender points)2 groups:\
EG (n=48)\
CG (n=37)CG: no activities or exercises other than usual, and none similar to EG protocol\
EG: 60 min/session, 3×/week (2×/week pool exercises and 1×/week land exercises), 24 weeks\
Protocol: warm-up, exercises of muscular strengthening, aerobic exercises, cool-downNot specified (controlled by Borg scale)Not reportedEG significantly improved pain symptoms (lower VAS scores, greater PPT and reduced number of tender points)Not reportedFunctional capacity, body composition, and quality of lifeEG improved functional capacity (greater hand-held grip dynamometry values, greater maximum O~2~ uptake, greater agility and balance indexes), quality of life (greater FIQ scores), and body composition (reduced fat percentage)NALatorre Román et al (2015)[@CIT0071]6RCTPain (tender point count, VAS -- 10 cm, PPT over tender points)2 groups:\
EG (n=20)\
CG (n=16)CG: no activities or exercises other than usual, and none similar to EG protocol\
EG: 60 min/session, 3×/week (2×/week pool exercises and 1×/week land exercises), 18 weeks\
Protocol: warm-up, exercises of muscular strengthening and balance, cool-downPatient determined30 °CEG significantly improved pain symptoms (lower VAS scores, greater PPT and reduced number of tender points)Not reportedImpact of fibromyalgia, strength, and balanceEG significantly improved: lower impact of fibromyalgia (lower FIQ scores), greater strength (leg and handgrip) and balance (agility dynamic and balance)NALetieri et al (2013)[@CIT0072]6RCTPain (VAS -- 10 cm)2 groups:\
HG (n=33)\
CG (n=33)45 min/session, 2×/week, 15 weeks.\
Protocol: warm-up, strengthening, balance, coordination and agility exercises, stretching, and relaxationModerate according to the perceived effort modified scale33 °CDecrease in pain (lower VAS scores)Not reportedQuality of life, depressive symptomsImproved quality of life (lower FIQ scores) and depressive symptoms (lower BDI scores)NALópez-Rodríguez et al (2013)[@CIT0073]6RCTPain (VAS -- 10 cm, MPQ, PPT)2 groups:\
ABD (n=29)\
CG (n=30)60 min/session, 2×/week; 12 weeks.\
Protocol: ABD -- flexibility and breathing exercises, rhythmic dancing movements, and mild exercises;\
CG -- stretching exercises for different body partsNot specified29 °C (preceded by a bath of 33--35 °C)Decrease in pain (lower VAS and MPQ scores and lower number of active tender points for PPT)Not reportedSleep, anxiety, depression, functionImprovement in sleep quality (lower PSQI scores), anxiety (lower SAI scores), function (lower FIQ scores) for ABDNAMannerkorpi et al (2000)[@CIT0074]4Quasi-randomized clinical trialImpact of fibromyalgia (FIQ -- total score), physical capacity (6MWT)2 groups:\
TG (n=37)\
CG (n=32)35 min/session, 1×/week, 24 weeks.\
Protocol: exercises for endurance, flexibility, coordination, and relaxation along with education sessions (6 sessions, 1 h/session)Patient determinedNot reportedDecreased fibromyalgia impact (lower FIQ total scores) and improved physical capacity (better scores in the 6MWT)NAFIQ subscores (including pain), pain, quality of life, self-efficacy, functional limitationsTG significantly improved physical functioning (lower FIQ subscores), anxiety (lower FIQ and AIMS subscores), depression (lower AIMS subscores), strength (greater grip strength), general health (greater SF-36 scores), social functioning (greater SF-36 scores), and pain (lower scores for pain severity and affective distress for the MPI-S)Not reportedMannerkorpi et al (2009)[@CIT0075]8RCTImpact of fibromyalgia (FIQ -- total score), physical capacity (6MWT)2 groups:\
Ex-Edu (n=81)\
Edu (n=85)45 min/session, 1×/week, 20 weeks.\
Protocol: exercises for endurance, flexibility, coordination, and relaxation along with education sessions (6 sessions, 1 h/session)48--65% of maximum HR (light to moderate intensity)33 °CDecreased fibromyalgia impact (lower FIQ total scores) and improved physical capacity (better scores in the 6MWT)NAFIQ subscores (including pain), pain, quality of life, anxiety and depression, leisure-time physical activity, stress, fatigueSignificant improvement for change in pain (lower FIQ pain subscores) and for leisure time (decreased LTPAI scores)0.69\
(0.45 for the intention-to-treat analysis)Munguía-Izquierdo and Legaz-Arrese (2007)[@CIT0076]7RCTTender point count, PPT on the tender points, and FIQ pain subscore (VAS -- 100 mm)3 groups:\
Ex (n=35)\
CG (n=25)\
Healthy group\
(n=25)60 min/session, 3×/week, 16 weeks\
Protocol: warm-up with slow walks and mobility exercises, strength exercises, aerobic exercises, and cool-down50--80% of predicted maximum HR according to age32 °CDecreased pain (reduced number of tender points, increased PPT over all tender points, and reduction in FIQ pain subscore) compared to control groupNot reportedSeverity of FM and cognitive functionImprovement of FM severity (lower FIQ scores) and in cognitive function (improvement in neuropsychological tests)NAMunguía-Izquierdo and Legaz-Arrese (2008)[@CIT0077]8RCTTender point count, PPT over tender points, health status (FIQ)3 groups:\
Ex (n=35)\
CG (n=25)\
Healthy group\
(n=25)60 min/session, 3×/week, 16 weeks\
Protocol: warm-up with slow walks and mobility exercises, strength exercises, aerobic exercises, and cool-down50--80% of predicted maximum HR according to age32 °CDecreased pain (reduced number of tender points, increased PPT over all tender points) compared to control group, Improvement in health status (lower FIQ scores)Not reportedAnxiety, sleep quality, cognitive function, physical functionImprovement in sleep quality (lower PSQI scores), cognitive function (greater PASAT scores) and physical function (increased muscle endurance for upper and lower limbs)NAPérez de la Cruz and Lambeck (2016)[@CIT0078]3Pilot studyVAS (10 cm) for pain1 group:\
FMS (n=20)45 min/session, 2/week, 10 weeks\
Protocol: warm-up, Ai Chi program, cool-downNot reported33 °C±0.5 °CSignificant improvement in pain (lower VAS scores)Not reportedHealth-related quality of lifeImproved quality of life (increased scores in all domains of SF-36 except role physical and role emotional)NAPiso et al (2001)[@CIT0054]4Case--control studyPPT over tender points2 groups:\
Sauna (n=9)\
HT (n=9)30 min/session, 2×/week, 6 weeks.\
Protocol: bodily awareness exercises, low-impact strength exercisesPatient determinedSauna: 90 ºC\
HT: 35 °CNo significant differences comparing groups; significant improvement in PPT only for sauna groupNot reportedPrevious treatmentOut of 18, 12 patients consider HT as first-choice treatmentNASantana et al (2010)[@CIT0053]1Analytical clinical trialFM impact and pain over tender points2 groups:\
Ai Chi (n=5)\
CG (n=5)40 min/session, 10 sessions (number of weeks not specified)\
Protocol: Ai Chi program (sequence of slow and wide movements with upper limbs, lower limbs, and trunk, emphasizing deep breathing during the exercises)Not reported34--36 °CNo significant improvement was observed for intervention group compared to CGNot reportedNANANASegura-Jiménez et al (2013)[@CIT0079]2Uncontrolled clinical trialTender point count and immediate pain (VAS -- 10 cm)1 group:\
FMS (n=33)45 min/session, 2×/week, 12 weeks\
Protocol: warm-up, general exercises (on Mondays: strength; on Wednesdays: balance), stretching, and relaxationRPE (Borg): 12±2 points34 °CImprovement in immediate pain (decreased VAS scores)Not reportedBody compositionNo differences were observed in body compositionNASevimli et al (2015)[@CIT0080]5RCTPain (VAS -- 100 mm)3 groups:\
ISSEP (n=25)\
AEP (n=25)\
AAEP (n=25)ISSEP: 15 min/day (3 months) of home-based stretching and strength exercises\
AEP and AAEP: 40--50 min/session, 2×/week, 12 weeks\
Protocol not described for AEP and AAEP60--80% maximal HRNot reportedPain improved for AEP and AAEP (lower VAS after treatment)Not reportedHealth status, endurance, quality of life, depressionImprovement in quality of life (greater SF-36 scores), depression (lower BDI scores), health status (lower FIQ scores) and endurance (greater scores for 6MWT) for AAEP and AEPNATomas-Carus et al (2007)[@CIT0081]7RCTFM impact (FIQ total score)2 groups:\
EG (n=17)\
CG (n=17)60 min/session, 3×/week, 12 weeks\
Protocol: warm-up, mobility exercises, aerobic exercises, lower limb exercises, cool-down exercises, and relaxation60--65% maximal hear rate33 °CImprovement of FM impact (lower FIQ scores)NAFIQ subscores (including pain)Improvement of all FIQ subscores (lower scores for all, including pain)Not reportedTomas-Carus et al (2009)[@CIT0082]7RCTFM impact (FIQ total score and subscores, including pain) and anxiety state (STAI)2 groups:\
EG (n=15)\
CG (n=15)60 min/session, 3×/week, 24 weeks\
Protocol: warm-up, mobility exercises, aerobic exercises, lower limb exercises, cool-down exercises60--65% maximal hear rate33 °CSignificant reduction of FM impact (lower FIQ total scores, and FIQ pain subscores)Treatment effect of −0.5 (−1.8 to 0.7) for the FIQ pain subscorePhysical fitnessImprovement of physical fitness (increase in maximal oxygen uptake, and increased scores for mobility and balance tests)NATrevisan et al, (2015)[@CIT0031]1Single-arm studyPostural control (center of pressure sway)1 group:\
FMS (n=17)45 min/session, 2×/week, 16 weeks.\
Protocol: familiarization, warm- up, exercises (aerobic and strength exercises for upper and lower limbs and trunk), cool-down stretching, and relaxationPatient determined30 °C ±2 °CImprovement in postural sway (lower center of pressure sway in different situations)NAPain (VAS -- 100 mm during rest and movement) and function (FIQ)Improvement in pain (lower VAS scores) and function (lower FIQ scores)VAS:\
Rest: −2.12 (−2.90 to −1.23)\
Movement:\
--1.94 (−2.70 to −1.08)[^2]

Randomized clinical trials, nonrandomized clinical trials, and crossover design studies assessing the effects of any aquatic intervention on pain in FMS patients were considered eligible for inclusion. The methodological quality of the studies was analyzed using the PEDro scale.

Thirty-five studies were included. Methodological quality varied between 1 and 9 according to the PEDro scale. Water temperature ranged between 28 and 37/38 ºC, and 7 studies did not report. Regarding the effectiveness of water therapy, only 2 studies reported no significant improvement compared to the baseline condition. However, one of these studies was composed of only 10 participants (5 in the Ai Chi group and 5 in a control group; PEDro score=1)[@CIT0083] and the other comprised 18 participants (9 in the sauna group and 9 in the hydrotherapy group; PEDro score=4).[@CIT0084] Thus, 94% of the included studies showed improvement in pain besides ameliorating other symptoms. In the following sections, we will discuss the possible mechanisms underlying the aquatic exercise effects.[@CIT0011],[@CIT0028],[@CIT0030],[@CIT0031],[@CIT0083]--[@CIT0082]

Musculoskeletal and neuromuscular systems and the association with pain {#S0003-S2001}
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The main symptom reported by FMS patients is pain.[@CIT0001],[@CIT0002],[@CIT0085],[@CIT0086] Pain is a dynamic and complex phenomenon that is the final result of several factors. The association between nociceptive activity and pain perception depends on several intrinsic and extrinsic influences. For the same nociceptive stimulus, pain perception and related brain activity will greatly differ between subjects. In the case of chronic rheumatic diseases that do not regress spontaneously, such as FMS, functional and structural central nervous system changes cause a generalized reduction in the pain threshold that is not limited to the anatomical structures involved, thus leading to the hyperalgesia and allodynia in many, if not all, body regions.[@CIT0021] FMS is associated with changes in the central nervous system that affect sensory information processing, amplifying peripheral input and/or generating pain perception in the absence of a noxious stimulus.[@CIT0021] People with FMS are reported to present hyperactivity of the hypothalamic--pituitary--adrenal axis,[@CIT0086],[@CIT0087] and this may be linked to the initiation or worsening of FMS symptoms. Moreover, dopamine dysfunctions have been linked to the pathophysiology of FMS, which are associated with hyperalgesia and deficient pain inhibition.[@CIT0059]

Accordingly, exercise has been one of the most recommended nonpharmacological interventions for FMS.[@CIT0019],[@CIT0020] It has been shown that exercise is able to influence gene expression and structural complexity in the limbic structures that regulate the hypothalamic--pituitary--adrenal axis[@CIT0021] and can improve conditioned pain modulation due to increased endogenous opioids, stimulation of brain structures involved in the inhibitory descending pathways that regulate painful response.[@CIT0088] Geytenbeek[@CIT0089] has examined over 500 articles that were available on the theme and has concluded, after examining randomized controlled trials, case--control studies, and cohort studies, that high to moderate quality evidence supports the use of hydrotherapy for pain, function, joint mobility strength, and balance. Moreover, exercise seems to be the most effective component of a hydrotherapy program for FMS.[@CIT0089],[@CIT0090]

Hence, exercising in an aquatic environment is advantageous. The pain-relieving effect of water-based exercises is suggested to be due to the joint effect of exercise, warm water, and buoyancy on thermal receptors and mechanoreceptors.[@CIT0033] Sensory-motor hyperstimulation exerted by the hydrostatic pressure, viscosity, and water temperature increases the triggers of thermal receptors and mechanoreceptors while blocking nociceptors.[@CIT0048],[@CIT0091] The viscosity of the water provides an environment with three-dimensional resistance, which facilitates proprioceptive feedback through functional patterns of movement and increases the synchronization of the motor units due to slowed movement.[@CIT0092] Also, immersion in warm water helps to increase blood flow and oxygen supply, improving nutrition and removal of catabolites, and thereby reducing signal molecules, such as IL-8 and noradrenaline,[@CIT0058] responsible for activation of nociceptors.[@CIT0093] In addition, regular exercise has been shown to improve overall health, as shown in other chronic conditions.[@CIT0027] This prominent effect on pain could be previously observed in several studies.[@CIT0027],[@CIT0028],[@CIT0031],[@CIT0032],[@CIT0053]

It is noteworthy to mention that patients with FMS present abnormalities regarding pain modulation, including central sensitization and other pathophysiological mechanisms, such as the accumulation of cytotoxic substances in the extracellular space (glutamate, lactate, bradykinin, prostaglandins, etc.) generated by muscle activity, which exert algogenic effects by sensitizing and exciting nociceptors.[@CIT0094],[@CIT0095] Glutamate is a major cortical excitatory neurotransmitter that acts in pain neurotransmission. Increased levels of insular glutamate have been reported to be present in FMS. In addition, the concentration of this molecule is correlated with pain report. Enhanced glutamatergic neurotransmission resulting from higher concentrations of glutamate within the posterior insula may play a role in the pathophysiology of FMS and other central pain augmentation syndromes.[@CIT0096] Moreover, the sympathetic nervous system, which is already in a condition of hyperactivity (see section "Cardiovascular and respiratory systems and the association with pain"), under the action of bradykinin stimulates the release of noradrenaline and prostaglandins that further potentiate sympathetic hyperactivity and sensitize the nociceptors.

Therefore, another mechanism explaining the pain improvement may rely on the combination of hydrostatic pressure and temperature on nerve endings, which would lead to competing stimuli that would diminish the peripheral nociceptive input.[@CIT0097] Aquatic therapy also leads to muscle relaxation,[@CIT0098] which would in turn lead to less pain. Buoyancy decreases compressive weight-bearing stresses on joints and allows functional exercise with lessened gravitational load, making the movements easier, and even facilitating the improvement of both strength and range of motion.[@CIT0039],[@CIT0099] Furthermore, drag forces can be used as a resource to assist movements or to impose resistance favoring muscle strengthening.[@CIT0033] Nonetheless, quantifying the resistance training intensity and planning a progressive overload program in aquatic environments is challenging due to several factors (eg, speed of movements, range of motion, shape and size of floats, etc.). Therefore, it is still not clear whether aquatic exercises can really induce strength gains, since controversial results have been reported.[@CIT0033]

Regarding chronic fatigue, another core feature of FMS, its perception may be reduced after water therapy due to the buoyancy effects.[@CIT0040] Buoyancy helps reduce the musculoskeletal system's gravitational forces due to gravitational muscle relaxation and energy conservation, which seems to reduce perceived fatigue. Water immersion may also reduce neuromuscular responses or trigger inhibitory mechanisms, with an overall reduction in neural transmissions, which would impact not only on the perceived fatigue but also on the nociceptive input, reducing pain perception.

Cardiovascular and respiratory systems and the association with pain {#S0003-S2002}
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FMS patients present cardiorespiratory dysfunction characterized by reduced respiratory muscle endurance, inspiratory muscle strength, and thoracic mobility.[@CIT0100] Moreover, cardiovascular autonomic control and baroreflex sensitivity have been also shown to be altered in this population.[@CIT0032],[@CIT0101],[@CIT0102] In addition, although it is not possible to identify a causal relationship, several studies have shown that these cardiorespiratory abnormalities are related to the pain in these subjects. Forti et al[@CIT0100] showed that inspiratory muscle strength is associated with the number of active tender points. In addition, Zamunér et al[@CIT0103] found that FMS show reduced respiratory sinus arrhythmia magnitude as compared to healthy women. Also, the indices obtained during the deep breathing test, a vagal maneuver, had an important association with pain in FMS. In another study, Zamunér et al[@CIT0104] also showed that sympathetic activity, as assessed by muscle sympathetic nerve activity, was related to pain in this population.

Several studies have described the interaction between autonomic and nociceptive pathways occurring at multiple levels,[@CIT0105]--[@CIT0107] with the nucleus tractus solitarius playing an important role. The nucleus tractus solitarius, located in the brainstem, receives visceral information through the primary afferents of the vagus nerve and receives the spinal pathways involved in pain processing, functioning as an interface between the autonomic and sensory systems.[@CIT0107],[@CIT0108] Therefore, improving cardiovascular and respiratory outcomes in FMS patients should be considered one of the aims in the management of FMS.

It is well established that aerobic exercise improves cardiorespiratory function in patients with FMS.[@CIT0019],[@CIT0109] In addition, an aquatic environment can allow higher-intensity exercises to be undertaken, with lower cardiovascular stress than is possible on land.[@CIT0019],[@CIT0110] In this sense, some studies have assessed the effects of water therapy on the cardiorespiratory system. Zamunér et al[@CIT0032] found that a 16-week aquatic therapy program proved to be effective in ameliorating symptoms, aerobic functional capacity, and cardiac autonomic control in FMS patients. Surprisingly, improvements in cardiac autonomic control were related to the improvements in pain and the impact of FMS on quality of life, thus suggesting an important role of autonomic control mediating symptoms. Regarding the improvement of functional aerobic capacity, aquatic therapy has also been proven to be effective.[@CIT0032],[@CIT0051],[@CIT0080] However, studies have shown no association between cardiorespiratory fitness improvements and FMS symptom improvements.[@CIT0032],[@CIT0051],[@CIT0111]--[@CIT0113]

In summary, cardiorespiratory function and cardiac autonomic control should be routinely monitored in the management of FMS patients since they seem to be related to the symptoms; and water therapy might be seen as a strategic method to improve these outcomes in this population. However, improving cardiorespiratory fitness should not be the main goal in the therapy, but instead a tailored approach directed to the key FMS symptoms (pain, sleep disorders, fatigue, depression, disability) with exercise assignment that does not exacerbate post-exercise pain should take place.[@CIT0114],[@CIT0115]

Neuroendocrine system and inflammation {#S0003-S2003}
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Growing interest has been shown in the study of the benefits of aquatic therapy on the neuroendocrine system[@CIT0116]--[@CIT0118] and inflammation. However, little is known about these in FMS patients. This is of interest since neurohormonal abnormalities have been reported in this population, such as low levels of serotonin,[@CIT0119] hypothalamic--pituitary--adrenal axis dysfunction,[@CIT0120]--[@CIT0122] and low levels of growth hormone, which is associated with poor sleep quality.[@CIT0123],[@CIT0124] Moreover, although there are no specific biomarkers for FMS, some studies have suggested the involvement of inflammatory disorders on its etiology.[@CIT0125],[@CIT0126] Those disorders involve cytokines, proteins responsible for mediating the inflammatory reaction in the immune system.[@CIT0127] Studies have shown that FMS patients have increased levels of serum IL-8,[@CIT0128] IL-6, IL-10, and IL-1β.[@CIT0129] Ortega et al[@CIT0130] found that FMS patients present a higher circulation concentration of C-reactive protein and that their monocytes release more IL-1β, tumor necrosis factor alpha, IL-6, and IL-10 than those from an age-matched healthy control group. Additionally, FMS patients present a greater concentration of IL-8 in cerebrovascular fluid.[@CIT0131],[@CIT0132] IL-8 release is stimulated by substance P secretion and promotes sympathetic pain,[@CIT0133] and thus is considered an inflammatory marker of FMS,[@CIT0062] which is indicative of underlying low-grade systemic inflammation. There is evidence showing the participation of chemokines (signaling molecules present in inflammatory and immune responses) in FMS, with higher concentrations of inflammatory chemokines (TARC/CCL17, MIG/CXCL9, MDC/CCL22, I-TAC/CXCL11, and eotaxin/CCL11).[@CIT0134]

Aquatic therapy has been shown to reduce plasma levels of norepinephrine,[@CIT0135]--[@CIT0137] epinephrine,[@CIT0135],[@CIT0138] β-endorphin, and cortisol[@CIT0139] in healthy men. In this context, we may suggest that aquatic therapy may contribute to a reduction of stress, improvement of sleep quality, and reduction of pain sensitivity.[@CIT0026],[@CIT0137] Regarding FMS patients, to our knowledge, no studies have assessed the effects of aquatic therapy on the neuroendocrine system. However, Bote et al[@CIT0140] found that a single session of moderate cycling improved the inflammatory and stress status of FMS patients. Moreover, their results also suggest that the neuroendocrine mechanism seems to be an exercise-induced decrease in the stress response of these patients, since they observed a reduction in the systemic concentration of cortisol, noradrenaline, and extracellular heatshock protein 72. In agreement with these findings, Ortega et al[@CIT0130] studied the effects of an aquatic fitness program performed for 8 months twice a week. After the program, monocytes from FMS patients presented similar spontaneous release of IL-1β and IL-6 to that of healthy controls and a reduction in C-reactive protein, showing that aquatic exercise might exert anti-inflammatory effects.

Current perspectives {#S0004}
====================

A considerable amount of evidence[@CIT0027] has shown that water therapy improves pain, fatigue, and quality of life. However, current recommendations for the management of fibromyalgia elaborated by the European League Against Rheumatism[@CIT0018] suggest a "weak for" recommendation, implying that most therapists would, although a substantial minority would not, recommend water therapy for FMS patients. This recommendation underlies the small amount of evidence suggesting superiority of water therapy over land-based therapies.[@CIT0141]

Therefore, future studies should focus on the possible mechanisms explaining the beneficial effects of water therapy in order to elucidate whether they are similar or not to the mechanisms leading to the improvement of symptoms and quality of life promoted by land-based exercises. Moreover, studies should also compare the detraining effects or long-lasting effects promoted by water therapy and land-based exercises since these have been addressed only by a few studies and the results are controversial. A recent study[@CIT0054] showed that 16 weeks of aquatic exercise therapy was effective in improving aerobic capacity and symptomatology such as pain, quality of life, and fatigue in FMS patients. However, after 16 weeks of detraining, all variables returned to near baseline. Thus, elucidating whether this is comparable to land-based exercises would assist FMS patients and therapists on the proper therapeutic approach recommendation and selection.

Another noteworthy point to be mentioned regards the FMS patient's adherence to treatment and engagement with aftercare tasks in the long term.[@CIT0086] Coupled with the fact that pharmacological interventions seem to be ineffective, as they seldom induce minimally important clinical differences in pain after 3 months of therapy,[@CIT0142],[@CIT0143] this makes the development of treatments that benefit patients over their lifetime extremely challenging. Hence, a multidisciplinary approach and educational strategies may be helpful additions to physical treatment, in this case, water therapy; these strategies show the importance of continuing with treatment, that the disease may vary in intensity over the time, and, more importantly, that they have to take responsibility for their healthcare and habits that influence on FMS symptoms, giving them tools to help with daily FMS management. Water therapy, in this context, comes as an alternative that makes movement easier and may increase compliance with the treatment.

Another topic to be discussed is the cost-effectiveness of water therapy for FMS. One previous study[@CIT0011] has shown that adding water therapy to the usual care for FMS patients is cost-effective for both healthcare and societal costs. The authors also concluded that the characteristics of facilities (distance from patients' homes and the number of patients who can participate per session) are major determinants that have to be considered before a health manager decides to invest in such a program. Therefore, this point should be addressed in future studies that aim to elucidate whether the cost-effectiveness differs among other kinds of interventions. Studies involving cost-effectiveness may also be helpful in guiding the development of public policies for the healthcare of FMS patients, and, as such, are much needed.

The present study has some limitations, as it is not a systematic review. As such, performance of a metaanalysis was not possible. As a narrative review, the scope of the present study was to highlight and discuss the possible mechanisms involved in the improvement of pain for FMS patients who undergo water therapy. Nonetheless, this discussion is still difficult as the protocols described vary in duration, session length, and techniques used into the swimming pool, as well as the outcomes chosen; also, several outcomes are not sufficiently described.

Conclusion {#S0005}
==========

Water therapy may be recommended as a nonpharmacologic therapeutic approach for the management of FMS patients, improving pain, fatigue, and quality of life; these therapeutic effects are achieved by the physiological changes caused by in-water exercising. However, future studies should be conducted in order to clarify the action mechanisms and whether long-lasting effects are superior to other types of intervention, especially land-based exercises.

Disclosure {#S0006}
==========

The authors declare no conflicts of interest in this work.

[^1]: **Abbreviation:** FMS, fibromyalgia syndrome.

[^2]: **Abbreviations:** 6MWT, 6-min walking test; AAEP, pool-based aquatic aerobic exercise program; ABD, aquatic biodance; AEP, gymnastic-based aerobic exercise program; AIMS, Arthritis Impact Measurement Scales; ARG, aquatic respiratory exercise-based program; BDI, Beck Depression Inventory; CG, control group; DWR, deep water running; Edu, education group; EG, exercise group; EuroQoL-5D, EuroQol Research Foundation Quality of Life Questionnaire; EuroQoL-VAS, EuroQol Research Foundation Quality of Life Questionnaire Visual Analog Scale; Ex, exercise group; Ex-Edu, exercise and education group; FIQ, Fibromyalgia Impact Questionnaire; FMS, fibromyalgia syndrome; HAS, Hamilton Anxiety Scale; HDRS, Hamilton Depression Rank Scale; HG, hydrotherapy group; HR, heart rate; HRQOL, health-related quality of life; HT, hydrotherapy; ISSEP, home-based isometric strength and stretching exercise program; LE, land-based exercise group; LTPAI, leisure-time physical activity instrument; MMPP+DWR, multimodal physiotherapy program+deep water running; MPI-S, Multidimensional Pain Inventory -- Swedish Version; MPQ, McGill Pain Questionnaire; NA, not applicable; NPRS, numerical pain rating scale; PASAT, Paced Auditory Serial Addition Task; PE, pool-based exercise group; PPT, pressure pain threshold; PSQI, Pittsburgh Sleep Quality Index; PTM, physical therapy modalities; PTM+BT, photobiomodulation+balneotherapy; PTM+HT, photobiomodulation+hydrotherapy; RCT, randomized controlled trial; ROM, range of motion; RPE, rate of perceived exertion; SAI, State Anxiety Inventory; SF-36, Medical Outcomes Study 36-item Short Form Health Survey; STAI, State-Trait Anxiety Inventory; TENS, transcutaneous electrical nerve stimulation; TG, training group; VAS, visual analog scale; VAT, ventilatory anaerobic threshold; VO~2~, oxygen uptake.