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To evaluate the literature on the effectiveness of aquatic exercise in relieving pain in adults with neurologic or musculoskeletal disease.
A systematic literature search of 14 databases was examined for research on aquatic exercise over the period January 1980 to June 2006.
Randomized controlled trials (RCTs) that included adults with neurologic or musculoskeletal disease, pain as an outcome measure, and exercise in water were included.
Information on the participants, interventions, and outcomes was extracted from the included studies. Quality appraisal was assessed using the Scottish Intercollegiate Guidelines Network criteria for RCTs.
Nineteen studies met the inclusion criteria; 8 were of moderate to low risk of bias, and 5 of these had data suitable for meta-analyses. This showed that aquatic exercise has a small posttreatment effect in relieving pain compared with no treatment (P=.04; standardized mean difference [SMD], -.17; 95% confidence interval [CI], -.33 to -.01), but it is not possible to draw a firm conclusion because of the lack of consistency of evidence across studies. Comparable pain-relieving effects were found between aquatic and land-based exercise (P=.56; SMD=.11; 95% CI, -.27 to .50).
There is sound evidence that there are no differences in pain-relieving effects between aquatic and land exercise. Compared with no treatment, aquatic exercise has a small pain-relieving effect; however, the small number of good-quality studies and inconsistency of results means that insufficient evidence limits firm conclusions. Future studies should aim for focused research questions on specific aquatic exercise techniques, using robust methodologic designs and detailed reporting of temperature, depth, and care setting.
EXERCISE IN WARM WATER, usually termed hydrotherapy or aquatic exercise, is a popular treatment for many patients with painful neurologic or musculoskeletal conditions. Pain-relieving effects have been attributed to a wide variety of mechanisms. For example, the warmth and buoyancy of water may block nociception by acting on thermal and mechanoreceptors, thus influencing spinal segmental mechanisms.1, 2 Also, the warmth of the water may enhance blood flow, which is thought to help in dissipating algogenic chemicals, and it may facilitate muscle relaxation. Other mechanisms are based around the effects of hydrostatic pressure, which by virtue of its effect on the cardiovascular system may relieve pain by reducing peripheral edema3 and, centrally, by dampening sympathetic nervous activity.4, 5 Finally, the ease of movement many patients report may activate supraspinal pathways, resulting in a reduction in pain intensity.6 Given the diversity of analgesic pathways it is reasonable to speculate that all patients, irrespective of pain etiology, might benefit from aquatic exercise. Indeed, aquatic exercise is widely recognized as an important modality in the rehabilitation of patients with rheumatologic, orthopedic, and neurologic disorders.7
In the United Kingdom, aquatic exercise for therapeutic purposes is recognized as a physical therapy−led treatment that uses the unique properties of water, “ideally in a purpose built, and suitably heated pool.”8(p5) This definition differs from the European approach in which balneotherapy, the medical application of natural thermal mineral waters, is usually associated with passive bathing, although sometimes exercise may be included.1, 2 Despite the obvious differences between the 2, systematic reviews of balneotherapy have informed aquatic therapy practice.9, 10, 11, 12, 13 Only 1 systematic review, conducted by a single reviewer, has been performed on aquatic exercise.14 It considered all study designs, including those with co-interventions, and provides a detailed qualitative account of the effects of hydrotherapy on all outcomes mentioned. It did not specifically explore the effect of aquatic exercise on pain; nor did it evaluate the raw data on pain.
In this study, we report a systematic review and meta-analysis of published randomized controlled trials (RCTs) that addresses the question: Does aquatic exercise relieve pain in adults with neurologic or musculoskeletal disease? This question has 2 elements: (1) Is aquatic exercise an effective treatment for pain (ie, better than no treatment)? and (2) How does pain relief in aquatic exercise compare with other interventions?
A search strategy was developed by iterative exploration of 18 databases using a variety of search terms that emphasized sensitivity (high recall) over specificity (precision).15 In the final search strategy, studies were sought from 14 databases, including Medline, AMED, EMBASE, SportDiscus, PEDro, CINAHL, ASSIA, and the Cochrane Library. Over the period January 1980 to June 2006, we used a range of search terms based around the concepts of aquatic exercise (hydrother$, balneo$, aquarobic$, aquatic rehab$, aqua$ and exercise, water and gymnast$, water aerobic$) and pain using the International Association for the Study of Pain terminology.16 Reference and bibliographic lists of retrieved articles and relevant reviews were also examined. The search was limited to English-language works and those that studied adults (people aged ≥18y).
A 3-stage process was used to select studies for inclusion in the review. In the first stage, the title of each identified article was checked against predetermined criteria by 2 reviewers using a standard form and coding sheet. Abstracts and full articles were similarly reviewed in the next 2 stages. In general, a policy of inclusion was adopted—that is, in the absence of any information to the contrary, each article was forwarded to the next stage of the screening process. Criteria for inclusion were titles, abstracts, and/or articles that suggested some aspect of aquatic exercise as defined in adults with neurologic or musculoskeletal pathology and with pain as an outcome measure. In addition, included studies were limited to full reports of RCTs that examined the effectiveness of aquatic exercise compared with no treatment or other interventions, such as land-based exercise or immersion, in adult patients (≥18y) with any neurologic or musculoskeletal pathology and in which at least 1 outcome measure of “subjective pain experience captured by ratings of pain intensity, sensation, and unpleasantness” was reported.17 Studies that considered the prevention of pain in healthy conditions (eg, pregnancy) or which incorporated additional interventions (eg, education) were excluded from the review.
Selected studies were subject to unmasked quality assessment18 by 2 reviewers using the criteria for RCTs recommended by the Scottish Intercollegiate Guidelines Network (SIGN 50).19 An overall assessment of the 9 questions provides a bias rating of low (++), moderate (+), or high (−). A low bias rating indicates a high-quality study in which all or most of the 9 criteria have been fulfilled and where they have not been fulfilled the conclusions of the study are thought unlikely to alter. One study was authored by a reviewer and was therefore assessed by another independent reviewer.
Data Extraction and Analysis
Data extraction was completed using a pilot-tested form to capture information on a range of details including study design, participants, interventions, and outcome measures. All of the primary pain outcomes were continuous and, where it was possible to pool data, meta-analysis was conducted on the results of studies of high to moderate quality20 using the Cochrane Collaboration's review manager software.a Tables of comparisons were set up comparing aquatic exercise with no treatment and with interventions of dry-land exercise and immersion. A random effects model, weighted by sample size, was used to analyze end scores based on posttreatment differences between aquatic exercise and these comparison groups.21 Changes in these scores (effect sizes = [mean1 − mean2]/pooled standard deviation [SD]) and 95% confidence intervals (CIs) were measured in units of SD (standardized mean difference [SMD]) and illustrated graphically using forest plots. Variation in the measured effect was explored using a statistical test for heterogeneity; nonsignificance indicates that the results of the different studies are similar.22 When possible, sensitivity was examined by assessing the effect of removing studies with small sample sizes (low weighting) from the analysis. In addition, between-group differences for all studies, irrespective of quality, were examined for consistency and, in the absence of suitable data, study texts and significance tests were scrutinized. Conclusions were based on both quantitative and qualitative assessment of studies with low to moderate risk of bias.
Seven hundred ninety-three publications were identified and screened against the inclusion and exclusion criteria. Of these publications, 729 were rejected at the title and abstract stage (fig 1). Sixty-nine studies proceeded to the paper screening stage, and 19 of these were accepted for review.23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41 Of the 19, 5 studies were of sufficient quality and had adequate raw data to be entered into meta-analyses under one of 2 comparisons: aquatic exercise versus no-treatment controls and aquatic versus dry land exercise. The study selection and validity assessment process was undertaken by 2 independent reviewers. Where anomalies existed, discussion between the reviewers enabled consensus to be achieved.
Nine studies compared aquatic exercise with no-treatment or waiting-list control groups,26, 28, 30, 32, 33, 35, 38, 39, 41 10 compared it with land exercise,23, 24, 25, 27, 29, 31, 34, 35, 37, 40 and 2 with immersion27, 36 (table 1). Two studies incorporated more than 2 treatment arms.27, 35
||“Primary” Pain Outcomes and Timing||Results of “Primary Pain Outcomes” According to Study||Calculated Effect Size (95% CI)|
|Minor et al23 (USA)||
||Pain subscale of AIMS at 0, 12, 24, and 52wk||Significant improvement of all groups at different time points (P<.05). No significant differences between interventions.||0.28 (–0.21 to 0.76)|
||VAS at 0 and 7wk||Significant improvement in both groups (P<.02). No significant differences between interventions.||DNA|
|Green et al25 (UK)||
||VAS at 0, 3, 6, 9, 12, and 18wk||Significant improvement in both groups. No significant differences between interventions.||DNA|
|Ahern et al26 (Australia)||
||VAS for pain at 1, 2, 4, 6wk||At study end patients who continued with aquatic exercise after phase 1 maintained improvement in pain relief, whereas those who were assigned to the no-treatment control group did not.||DNA|
|Hall et al27 (UK)||
||McGill Pain Questionnaire at 0, 4, and 12wk||No significant differences between interventions.||0.10 (–0.37 to 0.57)|
|Rintala et al28 (Finland)||
||VAS at 0 and 12wk||Aquatic exercise relieved pain significantly compared with control (P=.044).||–0.87 (–1.58 to –0.17)|
|Sjogren et al29 (Australia)||
||VAS at baseline and 6wk||Significant improvement of both groups (P=.001). No significant differences between interventions.||–0.02 (–0.54 to 0.51)|
|McIlveen and Robertson30 (Australia)||
||McGill Pain Questionnaire at 0 and 4wk||No significant differences between interventions.||DNA|
|Jentoft et al31 (Norway)||
||VAS at 0, 24, and 46wk||Significant improvement in aquatic exercise group (P=.006). No significant differences between interventions.||0.14 (–0.45 to 0.74)|
|Patrick et al32 (USA)||
||Pain subscale of HAQ at 0 and 20wk||No significant differences between interventions.||–0.12 (–0.37 to 0.13)|
|Sutherland et al33 (Australia)||
||Pain subscale of MSQOL-54 at 0 and 8wk||Aquatic exercise relieved pain significantly compared with control (P=.01).||0.21 (–0.60 to 1.04)|
|Wyatt et al34 (Sweden)||
||VAS at 0 and 6wk||Aquatic exercise relieved pain significantly compared with control (P≤.05).||–0.86 (–1.49 to –0.22)|
|Foley et al35 (Australia)||
||Pain subscale of WOMAC at 0 and 6wk||Significant improvement in aquatic exercise group (P=.045). No significant differences between interventions.||DNA|
|Altan et al36 (Turkey)||
||VAS at 0, 12, and 24wk||Significant improvement of both groups (P<.05). No significant differences between interventions.||0.08 (–0.48 to 0.63)|
|Yozbatiran et al37 (Turkey)||
||VAS for pain at 0 and 4wk||Significant improvement of both groups (P=.02). No significant differences between interventions.||–0.36 (–1.08 to 0.36)|
|Bilberg et al38 (Sweden)||
||Pain subscale of SF-36 at 1 and 12wk (and for aquatic exercise group 24wk)||Significant improvement of both groups (P<.05). No significant differences between interventions.||0.00 (–0.60 to 0.59)|
|Cochrane et al39 (UK)||
||Pain subscale of WOMAC at 0, 12, and 18mo||Aquatic exercise relieved pain significantly compared with control (P=.031). Improvements had been lost by follow-up.||–0.24 (–0.47 to –0.02)|
|Assis et al40 (Brazil)||
||VAS for pain at 0, 8, and 15wk||Significant improvement of both groups (P<.001, pain reduced by 36%). No significant differences between interventions.||DNA|
|Gusi et al41 (Finland)||
||VAS for pain at 0, 12, and 24wk||Aquatic exercise relieved pain significantly compared with control (P<.05). Improvements had been lost by follow-up.||DNA|
Abbreviations: AIMS, Arthritis Impact Measurement Scale; CLBP, chronic low-back pain; DNA, data not available for effect size calculation; HAQ, Health Assessment Questionnaire; MSQOL-54, Multiple Sclerosis Quality of Life−54; NR, not recorded; OA, osteoarthritis; RA, rheumatoid arthritis; ROM, range of motion; SF-36, 36-Item Short Form Health Survey; VAS, visual analog scale; WOMAC, Western Ontario and McMaster Universities Osteoarthritis Index.
Within the 19 studies, 717 patients participated in hydrotherapy, with an average age of 56.0±11.3 years (range, 25–81y) and, based on available data, an overall men-to-women ratio of 1:3 (not all studies reported this). The number of subjects randomized to the aquatic exercise arm ranged from 7 to 153, with 1 study omitting to report how many patients were randomized to the different treatment arms.34 Most patients presented with rheumatology conditions, and only Sutherland et al33 involved patients with neurologic problems. Of the rheumatology articles, 4 considered fibromyalgia,31, 36, 40, 41 3 chronic low back pain,29, 30, 37 and 11 osteoarthritis or rheumatoid arthritis.23, 24, 25, 26, 27, 28, 32, 35, 38, 39 The duration of symptoms varied from 2.75 to 24.00 years with an average of 9.98±5.50 years.
Drop-out rates for aquatic exercise patients ranged from 0% to 27.4%. Of the 3 studies with drop-out rates exceeding 20% (SIGN criterion), the time from baseline to posttest varied from 6 to 52 weeks. Extraneous causes such as comorbidity, time issues, or travel difficulties were responsible rather than the deleterious side effects of treatment. Three authors reported that a few patients (n=5) experienced an exacerbation of symptoms, but from the information provided, the treatment group affiliation of these patients is difficult to determine.25, 29, 35
Thirteen articles reported the aims of the aquatic exercise intervention and details, however brief, of the specific activities were mentioned in all. By categorizing the activities, it was possible to identify 3 types of exercise program: a general exercise program that included elements of muscle strengthening, increasing range of movement, and functional activities (n=9)24, 25, 26, 27, 29, 30, 32, 34, 36; aerobic exercise (n=9)23, 28, 31, 33, 37, 38, 39, 40, 41; and strengthening exercise (n=1).35 Of the 9 articles in which aerobic exercise was reported, 5 based the exercise intensity on heart rate.23, 28, 31, 40, 41
The intervention setting in 12 studies indicated that hospital or clinic pools were used. Three used community or public swimming pools, and 4 omitted to report the treatment setting. The overall duration of aquatic exercise treatment ranged from 6 weeks or less27, 30 to more than 12 weeks,28, 31, 32, 39, 40 with an average of 33.25±19.20 sessions. Treatments took place on an outpatient basis 2 to 3 times a week for a minimum of 30 and a maximum of 75 minutes (mean, 50.7±12.2min); 3 studies did not report session duration.25, 34, 37 Total treatment time ranged from 4 to 84 hours (mean, 25.5±20.54; median, 22.5). Where reported, treatment was performed in groups of 4 to 24 patients supervised by physiotherapists (n=8) or trained instructors (n=5) and using programs designed by a physiotherapist or fitness professional (n=5). In 11 studies, the average temperature of the water was 32.4°±2.7°C (range, 28°–36°C). Only 4 studies reported the depth of the water,23, 25, 34, 41 which was between waist and chest height.
Pain outcomes were measured before and after intervention in each reviewed study; follow-up data were reported in 8 studies.23, 25, 27, 31, 36, 38, 39, 41 A variety of instruments were used to measure sensory pain, with the 10-cm visual analog scale (VAS) being the most common.24, 25, 26, 28, 29, 31, 34, 36, 37, 40, 41 Other instruments included the McGill Pain Questionnaire27, 30 and pain subscales from a variety of self-report questionnaires (Arthritis Impact Measurement Scale,23 Health Assessment Questionnaire,32 health-related quality of life,33 Western Ontario and McMaster Universities Osteoarthritis Index [WOMAC],35, 39 36-Item Short-Form Health Survey38). Only 3 studies included pain as a primary outcome measure,39, 40, 41 and 2 of these had used this pain outcome in a power calculation for sample size.
Five studies were judged to be at low risk of bias using the SIGN criteria and are therefore judged as high quality.27, 32, 35, 39, 40 Three studies were of moderate26, 31, 38 and 11 studies23, 24, 25, 28, 29, 30, 33, 34, 36, 37, 41 of low quality (table 2). Although all studies were described as randomized, most (91%) of those that had a high level of bias failed to meet this SIGN criterion. Other criteria that were frequently inadequate included randomization concealment and intention-to-treat analysis (both n=14 [73.6%]). Although a priori power calculations are not included within the SIGN checklist we noted that only 6 studies27, 30, 32, 35, 39, 40 included any sample size power calculations and that 3 of these35, 39, 40 were based on pain outcomes. Five studies23, 27, 30, 32, 39 were independently powered to detect either a clinically meaningful or a conventionally large effect size for pain.42, 43, 44
|Study||Clear Question||Acceptable Randomization Method||Adequate Concealment Method||Blinding of Assessors||Groups Similar at Baseline||Only Difference Between Groups Is Aquatic Exercise Intervention||Outcomes Measurement: Standard, Valid, and Reliable?||Percentage of Drop-Outs Before Posttest||ITT||Overall Bias Rating|
|Ahern et al26||WC||WC||NA||AA||PA||NR||AA||NA||NA||+|
|Altan et al36||WC||PA||NA||WC||WC||PA||WC||
|Assis et al40||WC||WC||WC||WC||WC||NA||WC||
|Bilberg et al38||AA||AA||AA||AA||WC||NR||AA||
|Cochrane et al39||WC||WC||WC||WC||WC||WC||WC||
|Foley et al35||WC||WC||WC||AA||WC||AA||WC||
|Green et al25||AA||PA||NA||PA||PA||PA||PA||Overall=25||NA||–|
|Gusi et al41||AA||NR||NA||NA||AA||NA||AA||
|Hall et al27||WC||WC||AA||AA||WC||AA||WC||Overall=6||NA||++|
|Jentoft et al31||WC||AA||NA||WC||WC||WC||WC||
|McIlveen and Robertson30||AA||PA||NA||AA||PA||AA||WC||
|Minor et al23||AA||NR||NA||NA||WC||WC||AA||
|Patrick et al32||WC||WC||NA||NApp⁎||WC||WC||WC||
|Rintala et al28||WC||AA||NA||NA||AA||AA||AA||
|Sjogren et al29||AA||PA||NA||AA||AA||AA||AA||
|Sutherland et al33||AA||NR||NA||PA||AA||AA||AA||NA||WC||–|
|Wyatt et al34||AA||NR||NA||AA||NA||AA||PA||Overall=8.7||NA||–|
|Yozbatiran et al37||WC||NA||NA||NA||AA||PA||AA||NA||NA||–|
Abbreviations: AA, adequately addressed; Gp, group; ITT, intention-to-treat analysis (all subjects are analyzed in the groups to which they were randomly allocated); NA, not addressed; NApp, not applicable; NR, not reported; PA, poorly addressed; WC, well covered.
Legends: +, some of the criteria have been fulfilled. Where they have not been fulfilled the conclusions of the study are thought unlikely to alter; ++, all or most of the criteria have been fulfilled. Where they have not been fulfilled the conclusions of the study are thought very unlikely to alter; −, few or no criteria fulfilled. The conclusions of the study are thought likely or very likely to alter.
Aquatic Exercise Versus No-Treatment Controls
Of the 9 studies in this comparison,26, 28, 30, 32, 33, 35, 38, 39, 41 5 were of moderate to high quality,26, 32, 35, 38, 39 and of those, 3 had data suitable for incorporation into a meta-analysis32, 38, 39 (fig 2A). This showed a small posttreatment effect in favor of aquatic exercise (P=.04; SMD=−.17; 95% CI, −.33 to −.01) (see fig 2A). Removing the smallest study38 (n=22) from the analysis altered the SMD slightly from −.17 to −.19 (95% CI, −.35 to −.02) and the level of significance to P equal to .03. Statistical tests for heterogeneity were not significant. Qualitative analysis, based on review of results reported by researchers showed a lack of consistency between study results, whether of high or low quality. In summary, the evidence shows a small beneficial treatment effect in favor of aquatic exercise compared with no treatment. Only 1 study in the meta-analysis39 independently showed an effect size in favor of pain relief (for WOMAC pain) that is clinically meaningful.42 However, the small number of good-quality studies that could be included into the meta-analysis and the inconsistency of results across all the studies suggest that, currently, we have insufficient evidence to categorically state that aquatic exercise is an effective modality for relieving pain.
Aquatic Exercise Versus Land Exercise
Ten studies were identified in this comparison23, 24, 25, 27, 29, 31, 34, 35, 37, 40; one of these was assessed as moderate quality31 and 3 as high quality.27, 35, 40 Meta-analysis of the moderate- to high-quality studies with available data27, 31 showed no differences between aquatic exercise and land-based exercise (P=.56; SMD=.11; 95% CI, −.27 to .50) (see fig 2B). Data in the remaining 2 high-quality studies, reported as median and interquartile ranges for the WOMAC35 and pain VAS,40 are consistent with these findings. Of the poor-quality studies, only one34 contradicts the overall consistency of results. In summary, the evidence—based on meta-analysis and overall consistency of results—suggests that aquatic exercise and land-based exercise have comparable pain-relieving effects.
Aquatic Exercise Versus Immersion
Two studies were identified in this comparison.27, 36 One of these was assessed as low quality36; meta-analysis of the data of these studies was therefore not performed. Neither study found any postintervention differences in pain outcomes between aquatic exercise and immersion groups. At present, the small number of good-quality studies hampers firm conclusions about the benefits of exercise in water versus immersion.
Is Aquatic Exercise an Effective Treatment for Pain?
When compared with no-treatment controls, meta-analysis shows a small but significant posttreatment effect in favor of hydrotherapy. The 95% CI for this effect comes close to, but does not cross, the line of no treatment effect. In 2 of the studies in the meta-analysis, exercise was performed in water ranging from 29.5° to 33.5°C32, 39; temperature was described as “temperate” in one.38(p503) This encompasses a wide range of water temperature from cool to near thermoneutral (usually described as 34.5°–35°C). There is an assumption, by therapists and patients alike, that warmer water is more conducive to pain relief, and the mechanisms whereby heat alters pain perception are well known. Neuromuscular, hemodynamic, and metabolic responses to skin heating have been described45, 46; however, skin heating through whole-body immersion and core body temperature changes during exercise in water await investigation. The contribution of the warmth of the water to pain relief during hydrotherapy therefore remains speculative.
Several researchers have also reported a relationship between water temperature and adherence, which suggests that thermal comfort is an important environmental factor in patients with a similar profile to those in the studies included in this review.39, 47 Our review found evidence that aquatic exercise has a small effect in relieving pain even at cooler temperatures below thermoneutral. Potentially, this has important implications in terms of water heating costs. In addition, it suggests that effective aquatic exercise can be practiced in community settings that traditionally maintain lower temperatures than hospital-based pools.
Forty-two percent of studies in this review failed to cite water temperature and, because it is considered a critical variable, this should be reported in future aquatic exercise studies. Also, some studies fail to explicitly state the aim of the water-based exercise program and/or to provide an adequate description of its type and intensity (n=6 [31.5%]). The effect of exercise-induced analgesia on pain suggests that pain intensity is reduced after exercise.48 However, in humans, consistent results have been shown only for high-intensity land exercise (ie, ≥70% of maximal aerobic capacity). Exercise prescription is commonly based on predicted maximal heart rate because of the linear relationship between aerobic capacity and heart rate.49 However, Tanaka et al50 have recently questioned the accuracy of exercise prescription based on predicted maximal heart rate, particularly in older people. Furthermore, the variable effect of water temperature on heart rate makes it uncertain that the exercise intensity in the studies reviewed was above the analgesic threshold.51
The interaction between exercise intensity and water temperature, as well as having practical considerations,52 may affect pain differentially. In addition, water depth alters the nature of exercise through buoyancy and hydrostatic pressure. This in turn may influence the physiologic mechanisms underlying pain relief through the relationship between cardiovascular and pain regulatory pathways.53
The duration of the aquatic exercise program is another variable that may play a significant role in pain relief. In his review Janal54 highlights the uncertainty for the optimal duration of exercise-induced analgesia but suggests that the interaction between intensity and duration affects exercise-induced hypoalgesia. We noted that duration of treatment varied from twice a week for 4 weeks27 to 4 times a week for 53 weeks.39 The 3 studies32, 38, 39 comparing aquatic exercise and no-treatment controls are homogenous in terms of the type of exercise performed within the pool environment, but they have different durations of intervention, ranging from 12 to 52 weeks on a twice-weekly basis. It is interesting that those studies that do not report significant differences between groups are those of shorter duration. The duration of aquatic exercise programs for maximum pain relief is both clinically and economically important and warrants further investigation by good-quality longitudinal follow-up studies.
How Does Aquatic Exercise Compare With Other Interventions?
There is a general assumption that hydrotherapy will provide better pain relief than either land-based exercise or immersion alone. However, we found no significant differences between hydrotherapy and immersion in the 2 studies available.27, 36 In addition, no significant between-group differences were noted between aquatic and land-based exercise, which suggests that for those who find exercise on land challenging or tedious, exercise in water provides a similar effect. This lack of difference has been reported in previous studies,23, 31, 40 and currently, given the paucity of evidence, it is difficult to speculate which of the many variables (ie, temperature, exercise intensity and duration, treatment setting) could, either independently or in combination, be critical in pain relief. In contrast to our findings, the popularity of aquatic exercise as a modality for pain relief suggests that any additional benefits compared with land exercise have not yet been captured by the research.
Features of This Review
The impetus for this review was our perception of a disparity between anecdotal reports of significant pain relief from aquatic exercisers and our informal reading of the literature. Thus we chose to focus on the outcome, pain, rather than a specific population. In focusing on pain, we made an a priori selection of the primary pain outcome measure based on the availability of raw data and the most frequently occurring measure across all studies when multiple outcomes for this variable were cited. The lack of consistency in pain outcome measurements across the studies validated our decision to limit our definition to pain intensity; future reviews might be able to incorporate evaluations of pain behavior and cognitive coping strategies as prospective studies include such measures. We selected the RCT checklist produced by SIGN, one of many quality assessment tools, which allows overall assessment of individual components and is included in best practice systems reported by the Agency for Healthcare Research and Quality. However, as Katrak et al55 point out, there is no criterion standard for quality assessment tools in allied health research, and so our results must be considered in the light of the instrument we used.
Key Recommendations for Future Research in Hydrotherapy
We noted a number of substantial research design issues with 57.8% of the studies having important methodologic flaws. Most of these flaws related to aspects of RCT design such as randomization, allocation concealment, and blinding to outcome measurement. In addition, inadequate reporting of the intervention—in terms of setting, water temperature, depth, aim, type, and intensity of exercise—meant that some studies may have been downgraded as a result of poor reporting. Jüni et al56 defend this “guilty until proven innocent” argument with the justification that faulty reporting generally reflects poor methodology. Lack of resources meant that we were unable to contact researchers for further information. We acknowledge the impossibility of patient blinding to aquatic exercise; however, other approaches such as blinding of patients to the research hypothesis and evaluation of the expectations of patients and practitioners are possible and may be particularly important for pain and other self-reported outcome.57, 58 Many of the studies included in this review were general studies of aquatic exercise effectiveness that lacked a primary outcome measure, appropriate power, and adequate follow-up periods. Future studies should address these deficits by including specific details of the intervention, careful and creative consideration of both RCT design and of the literature on minimum clinically important differences for the primary outcome of interest (eg, for pain42, 43, 59), and comprehensive reporting based on current recommendations.60 We noted, as did Geytenbeek,14 that aquatic exercise research concentrates almost exclusively on chronic musculoskeletal conditions. However, the increasing use of aquatic exercise for patients with neurologic disorders suggests that this area is ripe for research.61, 62 Finally, given the importance of predicting patient outcome, data analysis on the basis of improvement versus deterioration and adherence versus nonadherence is also recommended.
In contrast to anecdotal reports of superior pain relief from aquatic exercise, our review shows that water and land-based exercise are similar, although when compared with no treatment, exercise in water provides limited analgesia. Inconsistent results in studies with no-treatment comparison groups combined with the design flaws and reporting omissions throughout the studies reviewed suggests that large, pragmatic studies are required to establish the optimal combinations of exercise type, duration, water temperature and depth, and service delivery for diverse patient populations. Furthermore, discrepancy between the perceived value of hydrotherapy in clinical practice and our findings justifies future research endeavors.