Aquatic exercise for people with intellectual disabilities: findings from a systematic review

Abstract

Background: There is a need for promoting exercise practice among people with intellectual disabilities (ID). Aquatic exercise (AE) could be a viable option in this regard.

Objective: To identify and critically analyze the best available evidence concerning the effects of AE in the physical and mental health of people with ID.

Methods: A systematic review of randomized controlled trials (RCTs) and comparative studies was performed after searching within several databases up to March 2021.

Results: A total of 4 RCTs and 3 comparative studies were found. Their methodological quality ranged from high to low. Aquatic interventions had positive effects on cardiorespiratory, muscular fitness and balance. Mixed effects were seen on parameters related to obesity and cognition. Scant information regarding feasibility and intervention fidelity was provided. In general, interventions details were not discussed in deep.

Conclusion: Preliminary evidence shows that aquatic exercise can be prescribed to people with ID in order to improve their physical health. The impact of aquatic exercise on motor skills, cognitive function and mental health remains unclear. Further research on this topic should provide a detailed description of the interventions carried out, including information on the feasibility and intervention fidelity.

Introduction

People with intellectual disabilities (ID) lead a sedentary lifestyle (Krahn and Fox 2014). Indeed, existing research has highlighted that very few individuals in this cohort seem to meet the minimum physical activity (PA) recommendations (Dairo et al. 2016). This is a matter of concern, as physical inactivity is linked to many conditions and diseases experienced by this population (Durstine et al. 2013). Therefore, the adoption of PA is a key strategy that should be developed among those with ID for health promotion (Pitchford et al. 2018). For children with ID, regular participation in PA would allow them to develop and improve their motor skills, which are, in turn, crucial to remain physically active and fit throughout the life span (Wouters et al. 2019). Comparably, the performance of PA is an important health strategy for adults with ID, as they experience an excess premature mortality rate significantly higher than those without ID (Glover et al. 2017). This is due, in part, to the prevalence of cardiovascular diseases in individuals with ID (O'Leary et al. 2018), often resulting from an inactive lifestyle (Tyrer et al. 2019).

For the first time, The World Health Organization has included persons with ID in the updated guidelines on PA and sedentary behavior, with the subsequent recommendation of the implementation of regular PA among this vulnerable group (Bull et al. 2020). However, encouraging individuals with ID to incorporate more PA into their lives is not an easy task, since most of the available PA programs have been designed for the general population, and do not suit the unique needs of this cohort (St. John et al. 2020). And on top of that, both children and adults with ID face several exercise barriers such as financial limitations, lack of awareness of options, and motivation that discourage them from taking part in PA programs (Bodde and Seo 2009; McGarty and Melville 2018). To solve this situation, identifying the type of PA that could generate greater acceptance and higher levels of adherence in this population has been suggested as a plausible solution (Martínez-Aldao et al. 2019). Likewise, it should bear in mind that PA facilitators including enjoyment, positive experience, social contact and performing PA together with family and friends, are commonly reported for people aging with ID (Caton et al. 2012, van Schijndel-Speet et al. 2014).

In this regard, aquatic exercise (AE) seems to be an interesting option, due to several reasons. Firstly, it is an enjoyable and recreational activity, performed in a safe environment whose properties (i.e. buoyancy and viscosity) facilitate mobility. Indeed, the reduced weight-bearing due to water's buoyancy allows to explore and try out movement patterns without fear of falling or weakness (Driver et al. 2004). Thus, AE can help overcome some of the PA barriers most often reported by individuals with ID, particularly lack of motivation, enjoyment or finding the exercise boring and too difficult (Temple et al. 2017). Secondly, its practice has beneficial effects on a number of health conditions that are prevalent in people with ID (i.e. diabetes, hypertension, or obesity) (Cugusi et al. 2015). In this respect, it has been argued that AE could assist in blood glucose management (Rees et al. 2017), as well as it can reduce arterial stiffness (Park et al. 2019). In addition, the large number of young and adult persons with ID and overweight/obesity (Amo-Setién et al. 2020, Ranjan et al. 2018) may benefit from water's buoyancy and viscosity, as it diminishes stress on their joints (Torres-Ronda et al. 2014) and, thus, AE could be experienced as less unpleasant and more tolerable and safer than land-based exercise. Thirdly, AE can increase physical fitness, a marker of health that is generally low among those with ID (Oppewal and Hilgenkamp 2019). Fitness improvements are due to two properties of water, the resistive forces of buoyancy and viscous drag, that permit a variety of aerobic and muscle-strengthening activities, which subsequently can boost cardiorespiratory function and muscular performance (Driver et al. 2004, Veldema and Jansen 2021). Finally, exercising in water improves swimming ability, an important fact to consider since it diminishes the risk of drowning present in this population (Kerr et al. 2018).

Under these circumstances, it seems necessary to make available the best scientific information related to the feasibility and potential health benefits of AE in people with ID. This goal can be achieved by conducting systematic reviews that synthesize and summarize the scientific evidence about this matter. A number of reviews have been conducted regarding the effects of exercise on those with ID (Bouzas et al. 2019, St. John et al. 2020, Kapsal et al. 2019), but none of them were specifically targeted at AE. Similarly, although a few reviews on the topic of AE and ID have been published (Aleksandrovic et al. 2015, Martin and Dillenburger 2019), all of them exclusively included individuals with autism, and were focused on specific research themes such as fitness, aquatic skills, or social behavior.

In the light of all factors considered above, and given the existent gap in the scientific literature pertaining to the topic, this review aimed at identifying and critically analyze the best available evidence concerning the physical and mental health benefits of AE for people with ID.

Methods

A systematic review was performed using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) methodology (Moher et al. 2009). This was done to ensure that all pertinent literature was sourced, synthetized and critically summarized (Paul et al. 2019), regardless of whether or not a meta-analysis could be executed.

Search strategy

Four electronic databases (MEDLINE/PubMed, PEDro, SPORTDiscus and Scopus) were searched systematically from their inception until March 2021. The following search terms, Boolean operators, and combinations were used: “Aquatic Exercise” OR “Aquatic Intervention” OR “Aquatic Therapy” OR “Aquatic Training” OR “Water Activities” OR “Water-based” OR “Water Exercise” OR “Swimming” AND “Developmental Disabilities” OR “Intellectual Disability” OR “Mental Retardation”.

Eligibility criteria

Randomized controlled trials (RCTs) and comparative studies that provided information respecting the effects of AE on the physical and mental health of people with ID were considered eligible. Investigations were excluded if: (a) AE was performed in combination with other therapies; (b) the study included samples made up of individuals with and without ID and separate data was not provided; (c) the intervention was based on the performance of a single session; (d) the research was not written in English, Portuguese or Spanish language.

Study selection

Two authors screened the titles and abstracts of the identified studies for eligibility. After independently reviewing the selected studies for inclusion, these were compared by both authors to reach an agreement. Once the agreement had been reached, a full-text copy of every potentially relevant study was obtained. If it was unclear whether the study met the selection criteria, advice was sought from a third author and a consensus of opinion was reached. Additionally, the full texts of the investigations that met the inclusion criteria were manually screened for any additional relevant references. We assessed the degree to which independent reviewers agreed upon inclusion criteria using Cohen’s kappa (κ) (Landis and Koch 1977).

Data extraction

Information on study design, participants’ characteristics, AE intervention details, main outcomes and adherence/dropouts, were extracted from the original reports by one researcher and checked by a second investigator. In articles where the scale was not reported, the Diagnostic and Statistical Manual of Mental Disorders – Fourth Edition, Text Revision (DSM-IV-TR) (American Psychiatric Association 1994) was implemented to categorize the severity of ID.

For the purpose of this review, and following a qualitative review on AE to improve cardiovascular and metabolic health (Meredith-Jones et al. 2011), selected studies were classified according to the type of AE intervention carried out. Hence, we distinguished two AE types. On the one hand, swimming, the most popular form of water-based exercise; on the other hand, upright water-based exercises, where the head is out of the water (Meredith-Jones et al. 2011).

Quality appraisal

Methodological quality ratings of the included RCTs were extracted from the Physiotherapy Evidence Database (PEDro) (de Morton 2009). The PEDro scale has 11 items corresponding to an equal number of quality criteria. Each item awards one point when it is satisfied. The first item is not included to calculate the PEDro score; hence, scores range from 0 to 10, with higher scores indicating a better methodological quality of the RCTs. The suggested cut-points to categorize studies by quality were as follows: excellent (9–10), good (6–8), fair (4–5), and poor (<3) (Silverman et al. 2012). Studies not included in PEDro were rated by one author. In the case of doubt, advice was sought from a second author.

The Methodological Index for Non-Randomized Studies (MINORS) (Slim et al. 2003) was used to determine the methodological quality of comparative studies. The MINORS instrument includes 12 items corresponding to an equal number of quality criteria for comparative studies. The items are scored 0 (not reported), 1 (reported but inadequate), and 2 (reported and adequate). The global ideal score is 24 points for comparative studies. The methodological quality of comparative studies was assessed by one author and then checked by a second author. In case of disagreement, advice from a third author was sought. High quality was considered if the total MINORS score was 17 or more, whereas a total score less than 17 was deemed as low quality (Malgie et al. 2020).

Results

Design and samples

From the 658 references initially retrieved and after removal of duplicates, 219 were found eligible for title/abstract screening. The independent reviewers agreed to keep 6 (for full-text review) and to discard 213 of these records (97.3%). Inter-rater agreement between the two independent reviewers for inclusion was substantial (κ = 0.77) (Landis and Koch 1977). Out of the 50 studies whose full-text was analysed, both authors agreed on excluding 44 of them. Advice from a third author was not needed. After revising the references of the selected investigations, one more study meeting the inclusion criteria was identified. Therefore, a total of three RCTs (Boer 2020, Nissim et al. 2019, Suarez-Villadat et al. 2020) and four comparative studies (Boer and de Beer 2019, Lee et al. 2014, Top 2015, Wright and Cowden 1986) were finally included (Figure 1).

Figure 1.

Flow chart of review process and study selection.

The samples ranged between 15 participants (Lee et al. 2014) and 50 participants (Wright and Cowden 1986), with an age range between 12 years (Wright and Cowden 1986) and 59 years (Nissim et al. 2019). A total of four articles included children and/or adolescents (0-17 years) (Lee et al. 2014, Top 2015, Wright and Cowden 1986, Suarez-Villadat et al. 2020), while three papers included adults (30-59 years) (Boer and de Beer 2019, Boer 2020, Nissim et al. 2019).

A total of four investigations reported ID etiology, which included Down Syndrome (DS) (n = 89) (Boer and de Beer 2019, Boer 2020, Lee et al. 2014, Suarez-Villadat et al. 2020) and autism spectrum disorders (ASD) (n = 15) (Lee et al. 2014). Four studies informed about ID level, being mild (n = 59) (Top 2015, Suarez-Villadat et al. 2020), mild to moderate (n = 91) (Nissim et al. 2019, Wright and Cowden 1986), and moderate (n = 16) (Suarez-Villadat et al. 2020). Intelligence quotient (IQ) was reported in four articles (Nissim et al. 2019, Top 2015, Wright and Cowden 1986, Suarez-Villadat et al. 2020) and ranged from 35 to 70. One paper reported that the participants ranged from second to third degree of mental disability (Lee et al. 2014). Ethnicity was not described in any of the reviewed investigations.

Methodological quality

Two RCTs were directly retrieved from the PEDro database, showing a fair methodological quality (Nissim et al. 2019, Suarez-Villadat et al. 2020). The research by Boer (2020) was judged to have a good quality after being independently reviewed by two authors (Table 1).

Table 1.

Methodological quality of the included RCTs

Study (Year)	PEDro items										Score	Quality
	1	2	3	4	5	6	7	8	9	10
Boer (2020)	+	+	+	−	−	−	+	+	+	+	7/10	Good
Nissim et al. (2019)	+	−	+	−	−	+	−	−	+	+	5/10	Fair
Suarez-Villadat et al. (2020)	+	−	+	−	−	−	+	−	+	+	5/10	Fair

Items: 1 = random allocation; 2 = concealed allocation; 3 = baseline comparability; 4 = blind participants; 5 = blind therapists; 6 = blind assessors; 7 = adequate follow-up; 8 = intention-to-treat analysis; 9 = between-group comparisons; 10 = point estimates and variability.

A disagreement in one of the three comparative studies when administering the MINORS scale regarding item 8 (prospective data analysis) was detected. Nevertheless, it did not affect the awarded methodological quality. The mean score obtained by the comparative studies was 14.5. The study by Boer and de Beer (2019) was rated as presenting high quality, while the remaining three investigations (Lee et al. 2014, Top 2015, Wright and Cowden 1986) reached a score classified as low quality (Table 2).

Table 2.

Methodological quality of the included comparative studies

Study	MINORS items												Score
	1	2	3	4	5	6	7	8	9	10	11	12
Boer and de Beer (2019)	2	2	2	2	0	1	1	0	1	2	2	2	17/24
Top (2015)	2	0	2	2	0	2	0	0	1	2	2	1	14/24
Lee et al. (2014)	2	0	2	2	0	2	0	0	1	2	2	1	14/24
Wright and Cowden (1986)	2	2	0	1	0	2	0	0	1	2	2	1	13/24

Items: 1 = a clearly stated aim; 2 = inclusion of consecutive patients; 3 = prospective collection of data; 4 = endpoints appropriate to the aim of the study; 5 = unbiased assessment of the study endpoint; 6 = follow-up period appropriate to the aim of the study; 7 = loss to follow up less than 5%; 8 = prospective calculation of the study size; 9 = an adequate control group; 10 = contemporary groups; 11 = baseline equivalence of groups; 12 = adequate statistical analyses. The items are scored 0 (not reported), 1 (reported but inadequate) or 2 (reported and adequate).

Intervention characteristics

Table 3 shows the intervention characteristics of the included studies. Sessions were mainly based on the performance of swimming (Top 2015, Wright and Cowden 1986, Suarez-Villadat et al. 2020) and upright water-based exercises (Boer and de Beer 2019) either performed alone, or combined (Boer 2020, Lee et al. 2014). Musical support was provided in two studies (Boer and de Beer 2019, Boer 2020).

Table 3.

Characteristics and individual results of included studies.

Authors (Year), Design & Sample	Intervention	AE Session Content & Resources	Outcomes (Measurement Tool)	Findings
Boer (2020) RCT Sample size (n pre/post; sex): 26/26; 0 women Distribution; age (mean ± SD): IG: n = 13; 34.2 ± 5.0 CG: n = 13; 30.3 ± 7.2 BMI (kg/m2; mean ± SD): IG: 33.0 ± 10.1 CG: 35.5 ± 8.3 IQ: NR ID level: NR ID cause (cause; n): DS (all)	Length: 8 weeks IG Activity: Swimming and upright exercises in shallow water for warm-up and preparation to main session Volume: 30 min during the first 4 weeks and 40 min during the last 4 weeks. Warm-up (4 min), preparation to main session (6-7 min), main session (20 min) Frequency: 3 sessions/week Intensity: The instructors continually encouraged participants to swim their lengths at the highest possible intensity and to perform the pre-swimming activities according to the appropriate intensity CG Activity: Normal everyday activities which did not included structured physical activity	Exercises: Walking in circular motion inside the pool, marching in place whilst swinging the arms, few simple stretches (single-arm crossover, chest stretch, hamstring, calf and quad stretch), high intensity running on the spot, lunge jumps, squat jumps, flutter kicks whilst holding onto the side of the pool, repetitive freestyle swim training, swimming lengths whilst holding onto kicking board Ratio instructor:participant: 1:4 Pool temperature: NR Pool dimensions: 1.4 m x 12 m (depth x length) Musical accompaniment: Yes	Anthropometry: Body mass (Seca scale) BMI (kg/m2) Cardiorespiratory fitness: 16-m progressive aerobic cardiovascular endurance run (PACER) 6-minute walk distance test (6MWD) Functionality: 8-ft get-up-and-go test Balance: Static balance (standing on one leg for as long as they could to a maximum period of 10 s whilst looking straight ahead and with their hands placed firmly on their hips) Dynamic balance (walking on a 3.05 m balance beam that is 10.2 cm wide) Muscular strength: Lower-body strength (sit-to-stand test) Upper-body strength (isometric push-up) Abdominal strength (modified curl-up) Flexibility: Upper-body flexibility (shoulder stretch)	Adverse events: No Adherence; dropouts: IG: 67 − 100%, NR CG: 100%, NR Significant results: Intra-group differences: Body mass, BMI, cardiorespiratory fitness, dynamic balance and lower, upper and abdominal strength were improved in IG after intervention Inter-group differences: Body mass, BMI, cardiorespiratory fitness, dynamic balance and lower, upper and abdominal strength were improved in IG compared to CG after intervention
Nissim et al. (2019) RCT Sample size (n pre/post; sex): 64/41; 23 women Distribution; age (mean ± SD): IG: n = 19; 58.95 ± 3.86 CG: n = 22; 58.2 ± 6.09 BMI: NR IQ (range): 35-70 ID level (scale): Mild to moderate; DSM-IV ID cause: NR	Length: 14 weeks IG1: Activity: Upright exercises in shallow water (Ai Chi method) Volume: 20-min sessions with 3 repetitions of each movement Frequency: 2 sessions/week Intensity: NR IG2: Activity: On-land motor intervention (Tai Chi method) Volume/Frequency/Intensity: Same as IG1	Exercises: The first five movements from the Ai Chi method. The protocol included the following movements: contemplating, floating, uplifting, enclosing and folding. Ratio instructor:participant: 1:3-4 Pool temperature: 34 °C Pool dimensions: NR Musical accompaniment: NR	Balance: Dynamic and static balance (Tinetti balance assessment tool) Gait: Tinetti balance assessment tool Fall risk: Tinetti balance assessment tool Verbal working memory: Digital span forward test (DSF)	Adverse events: NR Adherence; dropouts: IG1: 63.2%; 6 IG2: 68.2%; 3 Significant results: Intra-group differences: Balance and gait were improved in IG1 after intervention Inter-group differences: Balance was improved in IG1 compared to IG2 in mid and post intervention
Suarez-Villadat et al. (2020) RCT Sample size (n pre/post; sex): 45/40; 20 women Distribution; age (mean ± SD): IG: n = 15; 13.93 ± 1.25 CG: n = 30; 13.71 ± 1.24 BMI (kg/m2; mean ± SD): IG: 34.18 ± 4.26 CG: 24.77 ± 4.35 IQ (range): >35 ID level (n; scale): Mild (n = 29), moderate (n = 16), severe (n = 0); DSM-IV ID cause (cause; n): DS (all)	Length: 36 weeks IG: Activity: Swimming Volume: 50 min Frequency: 3 sessions/week Intensity: 140-160 beats/min in exercises with technical support elements, 160-180 beats/min in front crawl and 110-130 beats/min in breaststroke technique CG: Activity: Recreational swimming program Volume: 300 m and 400 m Frequency: 2 sessions/week Intensity: NR	Exercises: Front crawl style and breaststroke style, exercises using technical support elements and swimming in front crawl and breaststroke, soft swim and muscle relaxation Ratio instructor:participant: 1:4 Pool temperature: NR Pool dimensions: NR Musical accompaniment: NR	Anthropometry: Body mass (electronic scale) BMI (kg/m2) Waist circumference (non-elastic tape) WtHR (waist circumference/height) % body fat (skinfold thickness and Gonzalez-Aguero’s equation)	Adverse events: No Adherence; dropouts: IG: 66.7%; 0 CG: 100%; 0 Significant results: Intra-group differences: All body composition variables decreased in the IG after intervention Inter-group differences: BMI, waist circumference, WtHR, and percent body fat were decreased in the IG compared to CG after intervention
Boer and de Beer (2019) Comparative Sample size (n pre/post; sex): 25/23; 10 women Distribution; age (mean ± SD): IG: n = 13; 31.2 ± 6.9 CG: n = 10; 31.7 ± 8.4 BMI (kg/m2; mean ± SD): IG: 29.6 ± 6.5 CG: 33.1 ± 7.0 IQ: NR ID level: NR ID cause (cause; n): DS (all)	Length: 6 weeks IG: Activity: Upright exercises in shallow water and normal everyday activities Volume: 35 min during the first 3 weeks and 45 min during the last 3 weeks. Warm-up (5 min), core session (35 min) and cool-down (2 min) Frequency: 3 sessions/week Intensity: The instructors motivated, monitored and ensured optimal training intensity CG: Activity: Normal everyday activities	Exercises: Arm circles, side twists, walk in place, run in place, water scoops, side left lift, flutter kick on back, flutter kick on stomach, jumping jacks, knee twists, side shuffle, squat jumps, lunge jumps and a longer jog in place Ratio instructor:participant: 1:2 Pool temperature: NR Pool dimensions: 1.4 m (depth) Musical accompaniment: Yes	Anthropometry: Body mass (Seca scale) BMI (kg/m2) Cardiorespiratory fitness: 16-m progressive aerobic cardiovascular endurance run (PACER) 6-minute walk distance test (6MWD) Functionality: 8-foot get-up-and-go test Balance: Static balance (standing on a leg for as long as they could to a maximum of 10 s while look straight ahead and with their hands on their hips) Dynamic balance (walking on a 3.05-m balance beam that is 10.16 cm wide) Muscular strength: Lower body strength (sit-to-stand test) Upper body strength (isometric push-up) Abdominal strength (modified curl-up)	Adverse events: No Adherence; dropouts: IG: 94.5%; 1 CG: 94.5%; 1 Significant results: Inter-group differences: Cardiorespiratory fitness, lower and abdominal strength improved in IG compared to CG after intervention
Top (2015) Comparative Sample size (n pre/post; sex): 30/30; 11 women Distribution; age (mean ± SD): IG: n = 14; 17.43 ± 1.55 CG: n = 16; 17.06 ± 1.34 BMI: NR IQ (range): 50-70 ID level (n; scale): Mild (all); Weschler Intelligence Scale test ID cause: NR	Length: 10 weeks IG: Activity: Swimming Volume: 60 min Frequency: 3 sessions/week Intensity: NR CG: Activity: Regular school schedule	Exercises: Activities were prepared based on the Special Olympics Swimming Guide and other resources about swimming Ratio instructor:participant: 1-2:1 Pool temperature: NR Pool dimensions: NR Musical accompaniment: NR	Anthropometry: Body mass (body composition analyzer) Balance: Dynamic balance (BOT-2 Test) Muscular strength (BOT-2 Test) Motor skills: Fine motor precision (BOT-2 Test) Fine motor integration (BOT-2 Test) Manual dexterity (BOT-2 Test) Speed and agility (BOT-2 Test) Coordination: Bilateral coordination (BOT-2 Test) Upper limb (BOT-2 Test)	Adverse events: NR Adherence; dropouts: IG: NR CG: NR Significant results: Intra-group differences: Fine motor precision, fine motor integration parameters, balance and strength were improved in IG after intervention Inter-group differences: Fine motor precision and fine motor integration parameters were different in IG compared to CG after intervention Bilateral coordination was improved in IG compared to CG at baseline after intervention
Lee et al. (2014) Comparative Sample size (n pre/post; sex): 15/15; NR Distribution; age (mean ± SD): IG1: n = 5; 15.60 ± 2.19 IG2: n = 5; 15.80 ± 3.03 CG: n = 5; 16.00 ± 1.87 BMI: NR IQ: NR ID level: Second to third degree ID cause (cause; n): ASD (all)	Length: 12 weeks IG1: Activity: Swimming and upright water-based exercises Volume: Warm up (10 min), main exercise (30 min) and cool down (10 min) Frequency: 3 sessions/week Intensity: Individually tailored IG2: Activity: Swimming and upright water-based exercises + CES treatment Volume: Warm up (10 min), main exercise (30 min) and cool down (10 min) + CES measures (25-30 min) Frequency: NR Intensity: 100μA (weeks 1-4), 200μA (weeks 5-8) and 300μA (weeks 9-12) CG: NR	Exercises: Leg posture, hand posture, respiration, freestyle, and backstroke. Stretching in water, front, side, and back walking in water, kicking, knees to chest, inside and outside movement of feet, running, side step, jumping, free swim Ratio instructor:participant: NR Pool temperature: NR Pool dimensions: NR Musical accompaniment: NR	Cognitive function: Korean Western Aphasia Battery (K-WAB) BDNF (molecular device using the ELISA method) VEGF (molecular device using the ELISA method) IGF-1 (Diasorin using a LIAISON IGF-1 kit and analyzed by CLIA method)	Adverse events: NR Adherence; dropouts: IG1: NR IG2: NR CG: NR Significant results: Intra-group differences: K-WAB, BDNF and VEGF were improved in IG1 after intervention Inter-group differences: K-WAB, BDNF and VEGF were improved in IG1 compared to GC after intervention K-WAB was improved in IG2 compared to IG1 after intervention
Wright and Cowden (1986) Comparative Sample size (n pre/post; sex): 50/50; 11 women Distribution; age (mean ± SD): IG: n = 25; 12 to 18 CG: n = 25; 12 to 17 BMI: NR IQ (mean): IG: 67.72 CG: 66.20 ID level (scale): Mild to moderate; DSM-IV ID cause: NR	Length: 10 weeks IG: Activity: Swimming Volume: 60 min Frequency: 2 sessions/week Intensity: NR CG: Activity: Regular physical education program without swimming program	Exercises: Activities were prepared based on the Special Olympics Swimming Guide Ratio instructor:participant: 1:3-4 Pool temperature: NR Pool dimensions: NR Musical accompaniment: NR	Cardiorespiratory fitness: 9-minute run/walk test Self-concept: The Piers-Harris Children’s Self-Concept Scale or “The Way I Feel About Myself”	Adverse events: NR Adherence; dropouts: IG: NR CG: NR Significant results: Intra-group differences: Cardiorespiratory fitness and self-concept were improved in IG after intervention Inter-group differences: Cardiorespiratory fitness and self-concept were improved in IG compared to CG after intervention

Abbreviations: AE = aquatic exercise; ASD = autism spectrum disorders; BDNF = brain-derived neuropathic factor; BMI = body mass index; BOT-2 = Bruininks-Oseretsky Test of Motor Proficiency-Second Edition; CES = cranial electrotherapy stimulation; CG = control group; DS = Down syndrome; DSF = digital span forward; DSM = Diagnostic and Statistical Manual of Mental Disorders; HRmax = maximum heart rate; ID = intellectual disability; IG = intervention group; IGF-1 = insulin-like growth factor; IQ = intelligence quotient; NR = not reported; RCT = randomized controlled trials; SD = standard deviation; VEGF = vascular endothelial growth factor; WtHR = waist-to-height-ratio.

Interventions lasted between 6 and 36 weeks, with 20-to-60-minute sessions conducted two or three days per week. A number of studies did not report data on exercise intensity (n = 4), water temperature (n = 6), pool depth (n = 5), pool length (n = 6) or support elements (n = 6); while six out of the seven analyzed investigations gave information on the participant to instructor ratio.

Data on adherence to the AE interventions were gathered in four investigations (Boer and de Beer 2019, Boer 2020, Nissim et al. 2019, Suarez-Villadat et al. 2020), and reported compliance rates ranged from 63% to 100%. Dropouts that were not related to the AE interventions were notified in two studies (Boer and de Beer 2019, Nissim et al. 2019). Adverse events were not present in the three investigations that provided information in this matter (Boer and de Beer 2019, Boer 2020, Suarez-Villadat et al. 2020).

Main outcomes

Health-related fitness

A total of five investigations assessed the effects of AE interventions on several health-related fitness dimensions (Boer and de Beer 2019, Boer 2020, Top 2015, Wright and Cowden 1986, Suarez-Villadat et al. 2020). In particular, four studies analyzed the influence of AE interventions on anthropometry measurements. Boer (2020) detected significant, positive differences for the swimming group in body mass (effect size [ES] = −0.10) and body mass index (BMI) (ES = −0.06). Comparably, a standardized swimming program was more effective than a recreational swimming program to significantly enhance BMI, body fat percentage, waist circumference and waist to height ratio (Suarez-Villadat et al. 2020). Besides, when comparing baseline and end-study measurements, the standardized swimming group independently demonstrated significant improvements in all variables of body composition except in subscapular and thigh skinfold. Nevertheless, null results were identified for body mass (Boer and de Beer 2019, Top 2015) and BMI (Boer and de Beer 2019) after the AE interventions. Muscular strength and cardiorespiratory fitness improved in all studies exploring these health-related aspects of fitness after the AE program, with reported ES ranging from 0.34 to 0.72 for strength and from 0.30 to 2.12 for cardiorespiratory fitness (Boer 2020, Boer and de Beer 2019, Top 2015, Wright and Cowden 1986). Instead, null results were found for shoulder flexibility and for 6-min walk test distance and 24-m swim time (Boer 2020).

Balance

Three out of the four investigations that evaluated balance reported significant effects after the AE intervention compared with the baseline. Namely, Boer (2020) noticed that dynamic balance (i.e. walking on balance beam) significantly improved (ES = 0.29) following freestyle swim training. Likewise, another swimming program led to a significant improvement in dynamic balance (i.e. walking forward on a line and standing on one leg on balance beam with eyes open) (Top 2015). Also, Nissim et al. (2019) found that dynamic and static balance, measured with the Tinetti balance assessment tool, exhibited a significant increase following aquatic motor intervention (i.e. Ai-Chi). Specifically, Ai-Chi seemed to be more effective than Tai-Chi in this regard (Nissim et al. 2019). Conversely, Boer and de Beer (2019) reported a lack of significant improvement with and upright water-based exercises intervention in dynamic (i.e. walking on balance beam, ES = 0.43) and especially static balance (i.e. standing on one leg, ES = 0.25).

Motor skills

One investigation (Top 2015) analyzed the effect of a swimming program on motor skills. Statistically significant differences were noted favoring the swimming group over the control group at the end of the intervention for fine motor precision and fine motor integration, whereas null differences in terms of manual dexterity and speed and agility were observed both between groups over time.

Cognitive function

Two articles informed about the impact of AE on cognition. Lee et al. (2014) identified a significant difference between intervention and control groups at post-test favoring the impact of swimming and upright water-based exercises for brain-derived neuropathic factor and vascular endothelial growth factor, neurotransmitters linked to cognitive function, as well as for the cognitive test (Korean Western Aphasia Battery). Greater improvements were detected when this form of AE was combined with cranial electrotherapy stimulation. By contrast, Nissim et al. (2019) did not find significant differences in verbal working memory after the Ai-Chi intervention. In a subsequent analysis, the authors revealed a trend towards improvement in the aquatic motor intervention group from pre-intervention to post-intervention, while the on-land motor intervention group (i.e. Tai-Chi) had no change.

Self-concept

One investigation included self-concept as a study variable by means of the Scheffé Test, which was significantly ameliorated by a swim training program (Wright and Cowden 1986).

Discussion

This systematic review aimed at describing the characteristics, methodological quality and main outcome effects reported in the existent RCTs and comparative studies that have used AE as an intervention strategy for improving the physical and mental health of people with ID. Previous systematic reviews on the topic of exercise for this population have provided scant information on the health benefits conferred by AE. In particular, Kapsal et al. (2019) focused on children and adolescents with ID and did not give further detail about the available research on AE. Correspondingly, St. John et al. (2020) conducted a comprehensive analysis on the effects of exercise in those with ID, but no AE articles were included for analysis in their review. Besides, Bouzas et al. (2019) extensively reviewed the literature on the effects of exercise on the physical fitness of adults with ID; yet uncontrolled studies were searched for inclusion for this review, only three investigations with AE interventions and outcome data were retrieved. Given the lack of research on this subject, the information shown here can offer enough background detail to allow non-specialists working with this population to gain both knowledge and perspective of health-related aspects of AE interventions, as well as having practical implications for health and rehabilitation professionals working with persons with ID of all ages.

We only focused on investigations that included a comparative group. As a consequence, a considerable number of uncontrolled studies were rejected, and seven articles were finally included that presented a high to low methodological quality. These results imply that scientific information on the health-related effects of AE interventions for young and adult individuals with ID is still inconsistent and further research is needed. In spite of this, there are several findings that deserve further discussion, since they can offer some practical help and insight for those caring for individuals with ID or those interested in prescribing AE to this population.

For instance, we noticed that only half of the investigations that analyzed the impact of AE on body composition exhibited significant positive results (Boer 2020, Suarez-Villadat et al. 2020). This is a remarkable finding, as obesity is a prevalent condition that leads to a notable contribution to the reduced life expectancy and increased health needs observed among people with ID (Melville et al. 2007). Instead, the lack of benefits (Boer and de Beer 2019, Top 2015) has been previously noted in other exercise modalities and confirms the idea that exercise often does not promote significant morphological changes in individuals with ID (Bouzas et al. 2019). Indeed, it has been indicated that in order to promote positive body composition changes in overweight/obese adults with ID, practitioners can implement a well-controlled and designed intervention combining diet, PA and behavior change techniques (Ptomey et al. 2018).

Conversely, AE appeared to have a positive influence on important health-related fitness dimensions including cardiorespiratory function and muscular strength, both in children and adults with ID (Boer 2020, Boer and de Beer 2019, Top 2015, Wright and Cowden 1986). These results are in line with earlier observations concerning the physical fitness enhancing effects of AE on two different groups with disabilities. Driver et al. (2004) outlined that AE could increase aerobic fitness of individuals with a brain injury, possibly through improved vital capacity and breath control. Moreover, de Mattos et al. (2016) demonstrated that AE leads to improved muscle function of people with osteoarthritis, mainly due to the work required to overcome the generation of drag forces imposed when the individual moves through the water. Those with ID show a low cardiorespiratory fitness, with further decline incrementing with age, a situation that puts them at higher risk for cardiovascular diseases and all-cause mortality (Oppewal et al. 2013). Similarly, an increased prevalence of muscular strength problems that can lead to reduced physical functionality and lower quality of life in this population has been mentioned (Merchán-Baeza et al. 2020). Therefore, this preliminary evidence suggests that AE interventions seem to be a valuable exercise option that should be advocated among persons with ID.

Another finding worth of mention is that AE appeared to be effective for improving balance levels among adolescents, adults and older people with ID, with three out of four investigations reporting positive effects after intervention periods. Consistent with the literature, the preliminary positive impact of AE on balance (Boer 2020, Nissim et al. 2019, Top 2015) can be attributed to the fact that hydrostatic pressure and viscosity might yield individuals with multisensory stimulation (i.e. proprioceptive and sensory feedback) and postural support to further enhance postural and balance control (Methajarunon et al. 2016). These results confirm the idea that balance is potentially trainable in individuals with ID (Enkelaar et al. 2012), and imply that AE might be a useful rehabilitation strategy for preventing balance-related problems, such as fall risk. In addition, we add some valuable information to the current body of knowledge on the question of the impact of exercise on the balance levels of young persons with ID, considering the scientific evidence in this regard has been qualified as inconclusive (Maïano et al. 2019, Maïano et al. 2019).

Similarly, it has been suggested that scientific information respecting the impact of motor skill interventions in young individuals with ID remains unclear (Maïano et al. 2019). According to our review, AE may be an interesting option for ameliorating motor skills in this population. However, we only found one investigation on the topic of AE and motor skills (Top 2015), indicating that further research on this subject is needed. This limited evidence on the effects of AE on motor skills has also been underlined in other persons with disabilities (Roostaei et al. 2017).

On the contrary, AE showed mixed results on cognitive function. Previous studies have proposed that exercising is an important strategy for maintaining cognition in adults with ID (Ptomey et al. 2018). Our findings do not follow this idea, due to the improvements in cognitive function were seen in children (Lee et al. 2014) but not in adults (Nissim et al. 2019). The intensity and nature of the AE being performed might explain this variable response, considering the cognitive performance-enhancing effects of moderate-intensity swimming (Shoemaker et al. 2019). In this sense, whilst Lee et al. (2014) conducted a program based on swimming and upright water-based exercises at individually tailored intensity, Nissim et al. (2019) opted for Ai Chi, which comprised continuous, slow movements, as part of their intervention. Another possible reason could be based on the severity of ID, taking into account that the participants in the study by Nissim et al. (2019) presented very low IQ. Eventually, AE appeared to be effective for enhancing self-concept, which is generally poor in children and adolescents with ID (Nader-Grosbois 2014).

According to our findings, there are important gaps in the existing scientific literature addressing the issue of the physical and mental health impact of AE for people with ID, which further studies should properly undertake. Firstly, none of the reviewed investigations provided information on the degree to which interventions were implemented as originally planned. In this vein, identifying intervention fidelity is central to the interpretation of the results, and whether the interventions produced the intended outcomes (Ibrahim and Sidani 2016). Secondly, it could not be determined if AE helped participants to overcome the PA barriers salient for those with ID, since data on the participant’s views or their parents/caregivers’ opinions about this matter were not collected. This is a key issue to be addressed in order to increase PA participation among individuals with ID, given that Temple (2007) has heightened the need for making sure that the effort required to engage in PA is manageable and perceived as achievable. Accordingly, one effective way of gathering useful information on the participant’s experience, challenges and acceptance in relation to the proposed activities would be using questionnaires (Weterings et al. 2019). Moreover, although financial limitations are a common PA barrier among those with ID (Bodde and Seo 2009), the analyzed papers lacked to measure the AE programs’ cost-effectiveness. Besides, it is well-known that this population is prone to experiencing motivational, attentional and motor control issues, as well as using medication that might predispose to inactivity (Weterings et al. 2020); and this combination of factors jeopardizes the feasibility of an exercise intervention. However, scant information on recruitment rates, program retention, adverse effects, attendance, group size feasibility and non-compliance were reported in the reviewed studies. Thus, the feasibility of AE interventions could not be further discussed. Notably, it has been stated that ensuring a suitable and safe pool environment is a priority when implementing AE programs (Kelly and Darrah 2005). Still, though in the included works the the participant to instructor ratio was generally noted as an important factor in relation to help measures, only a few of them gave detail on pool characteristics, or supporting material. This is a matter of concern, since fear (i.e. risk of being injured or having a fall, feeling unsafe) is often mentioned as a PA barrier by persons with ID (Bossink et al. 2017). Taken all together, these facts strongly limit the transparency and replicability of the proposed interventions.

We should also add that the included articles varied in terms of participants' age and severity of intellectual disability (with the latter not always specified explicitly). This fact prevents the generalization of the reported findings to all people with ID. This heterogeneity also did not make possible to perform a meta-analysis. Finally, no study analyzed the impact of AE on different mental disorders that are prevalent in this population (i.e. challenging behavior, anxiety or depression). In addition, there are limitations related to the language restrictions, to the fact that we did not review the grey literature, and to the publication bias, which may have conditioned the results of this review.

Conclusion

Few randomized controlled and comparative studies addressing the physical and mental health benefits conferred by AE in individuals with ID have been carried out so far. Preliminary evidence shows that this exercise modality can lead to improvements in cardiorespiratory and muscular fitness, as well as in balance. The impact of AE on motor skills, cognitive function and mental health remains unclear, while the feasibility and cost-effectiveness of AE programs have not been adequately researched. In this regard, further research on both children and adults with ID is needed.

Conflict of interest

The authors declare that there is no conflict of interest.