Leonardo Da Vinci, one of the most prolific artists/scientists of the Renaissance period once stated, “Water is the driving force of all nature.” Today, the influence of water on our daily lives has not changed. Considering 71% of the earth’s surface and 60% to 70% of the human body is water, this element surrounds us and is considered vital for survival. Furthermore, the U.S. population continues to embrace this medium with its commitment to 10.4 million residential and 309,000 public swimming pools available nationwide (1).
Aquatic exercise (AE) and swimming are currently enjoyed by millions of participants, providing unique health and wellness benefits across the life span. This article dives into the health and training benefits of both immersion and exercising in water, specifically using nonswimming, vertical, and “head out” activities. It is intended to better equip health/fitness professionals with basic knowledge to advocate AE to clients and to provide insights and skills so AE may be integrated into their personal health plan. For the purpose of this article, AE is defined as an adaptation of land-based physical activity (i.e., walking, jogging, calisthenics, and locomotor/resistive movements) to a water medium, often performed in an upright stance (2). AE provides evidence-based aquatic programs coached or recommended by health/fitness professionals to clients previously cleared for exercise by a health care provider.
Although the aquatics industry currently campaigns to promote the widespread benefits of AE, it is still generally underutilized and often overlooked by populations who could benefit the most. For example, aquatic high-intensity interval training (AHIIT) can help fill a gap for clients looking to adopt a novel and challenging form of aerobic activity to their existing fitness routine.
In 2018, the American College of Sports Medicine formed a Presidential Task Force to better promote the importance and benefits of swimming and AE. Although the aquatics industry currently campaigns to promote the widespread benefits of AE, it is still generally underutilized and often overlooked by populations who could benefit the most. For example, aquatic high-intensity interval training (A-HIIT) can help fill a gap for clients looking to adopt a novel and challenging form of aerobic activity to their existing fitness routine. The proper prescription of HIIT for an aquatic environment is considered safe for healthy and most clinical populations and has gained popularity for its reported positive benefits of cardiorespiratory and metabolic outcomes (3). The following sections will explore the characteristics and benefits of exercise in an aquatic environment, how interval training is applied to shallow water for cardiometabolic adaptations, and provide an evidence-based method of A-HIIT training and progressions.
CHARACTERISTICS OF THE WATER ENVIRONMENT WHILE IMMERSED
The physical properties of water are uniquely different from a land-based environment and are largely responsible for the underlying mechanisms that promote physiological adaptations during exercise. The following properties will influence a client when immersed in a static position (without movement) (4):
- Defined as mass per unit volume, the density of water varies with its state (gas, liquid, or solid) and is quite stable through the narrow temperature ranges used for aquatic activity.
- Water is approximately 800 times as dense as air at sea level.
- Water is essentially incompressible in its liquid state and exerts approximately 1 mmHg of pressure per 0.5-inch water depth. Consequently, a person standing upright in water at clavicle depth will experience water pressure at the ankle well in excess of venous and lymphatic pressure and close to systolic blood pressure.
- Buoyancy is produced by the differential between the specific gravity of water (defined as 1.0 g/cm3) compared with that of the human body, which averages 0.974 g/cm3. Lean body mass is approximately 1.1 g/cm3, whereas fat mass is 0.9 g/cm3. Consequently, the upward force of buoyancy offloads immersed joints (40% offloaded when immersed to pelvis, 50% offloaded when immersed to the umbilicus, and 60% or more depending on arm positions when immersed to the xiphoid). When immersed to the neck, only the weight of the head loads the spine.
- Water is an efficient conductor of heat, easily transferring heat from or to the body, depending on water and body temperature. The thermal conductivity of water is 25× greater than air. Water that is cooler than one’s body temperature will reduce the risk of overheating. Warm water can efficiently heat joints and injured tissues, decreasing pain perception and enhancing blood flow to an injured area.
- For moderate to vigorous exercise, general recommendations range from 78°F to 83°F (25°C to 28°C) (2).
- For postrehab or special needs clients, temperature guidelines suggest the following (5):
- ▪ 82°F–88°F (28°C to 31°C) for more active clients and patients with multiple sclerosis
- ▪ 88°F–92°F (32°C to 33°C) for less active clients such as those with arthritis or women
- ▪ 92°F–96°F (32°C to 35.5°C) for less active clients with hypertonicity (chronic muscle contraction) or spasticity (uncontrollable muscles that are tight or stiff)
- • Viscosity refers to the magnitude of internal friction within a fluid with movement and is a time-dependent property driven by movement speed. Highly viscous fluids require more energy to move through a fluid volume. The resisting force is dependent on the frontal surface of the moving object and the rearward shape. Frontal resistance and rear drag will both oppose movement.
- • With greater movement speeds, the equation governing the energy required becomes highly complex and logarithmic in magnitude.
BENEFITS OF IMMERSION
The benefits of immersion alone are numerous, particularly when clients are submerged in warm water. The combined effects of all water properties promote the aquatic environment as a highly effective medium to foster health, recovery, rehabilitation, health/fitness, and recreational outcomes. These benefits include the following:
Cardiac function. During immersion, hydrostatic pressure pushes blood from superficial to deeper blood vessels. As the depth of immersion increases, blood is compressed upward, first into the large capacity vessels of the pelvis and abdomen then, with greater depths, above the diaphragm into the chest. Although immersed to the neck, nearly 3/4 of a liter of blood is displaced, with 2/3 of that blood into large pulmonary vessels, and one-third into the heart. This increase in end-diastolic ventricular volume results in up to a 30% increase in stroke volume and cardiac output above resting values (6). Because immersion also affects decreasing peripheral vascular arterial tone, the combined effects of decreased circulatory resistance to blood flow and improved efficiency of heart contraction add to the efficiency of cardiac function. Clients or clinical patients such as those with mild to moderate congestive heart failure can improve V̇O2peak, in addition to quality of living, and other health/fitness-related outcomes from this therapeutic environment (7,8).
Clients or clinical patients such as those with hypertension and mild to moderate congestive heart failure can improve quality of living, cardiovascular, and other health fitness-related outcomes from this therapeutic environment. Deep water exercise or exercise in shallow water with the chest submerged may be useful for strengthening muscles of respiration, with implications for improved activities of daily living or rehabilitation of those who suffer respiratory weakness or other diseases of the lungs.
Blood pressure. The body compensates for the increase in cardiac output by decreasing arterial blood vessel tone, causing arteries to relax without an increase in blood pressure. Normotensive and, in many cases, hypertensive individuals will experience a lower resting blood pressure while immersed. The magnitude of this drop can be related to water temperature with warmer water yielding the greatest effects. It also should be noted that initial exposure to warm or cold water may cause a temporary rise in blood pressure. In past years, it was considered that individuals with hypertension should avoid hot tubs or aquatic therapy when, in fact, such patients often benefit from warm water immersion (9).
Muscle blood flow. Increased blood volume promotes blood flow into deeper tissues. The increase in flow rates improves endothelial function within muscle and helps to meet metabolic demands and improved oxygen delivery to the working muscle. These effects may be beneficial for tissue healing or muscular recovery from exercise (7).
Respiratory function. The hydrostatic effects of immersion also compress the chest wall while resisting expansion during inspiration. This, in combination with increased intrathoracic blood volume, results in a dramatic increase in the work of breathing during chest-deep immersion, measured at a 60% increase at rest (10). By working the respiratory muscles against these resistive loading forces, an improvement in respiratory efficiency and endurance can occur through the strengthening of respiratory musculature. During vigorous exercise, the added effects of viscosity and hydrostatic pressure promote an even greater respiratory workload, while requiring increased breathing rates and respiratory volume. In this circumstance, as the body responds with increased blood flow to respiratory muscles, a metabolic reflex shunts blood from “non-mission critical” oxygen consumptive structures. As a result, deep water exercise or exercise in shallow water with the chest submerged may be useful for strengthening muscles of respiration, with implications for improved activities of daily living (ADLs) or rehabilitation of those who suffer respiratory weakness or other diseases of the lungs (7).
Brain function. Immersion in warm water has been found to produce a profound impact on the autonomic nervous system. Warm and neutral water temperature immersion causes a dramatic downregulation of the sympathetic system, allowing the sympathetic and parasympathetic components to come into balance. At the same time, cerebral blood flow increases, promoting oxygenation and cerebrovascular function. Such responses may lead to improved cognitive and executive abilities and may have implications for populations who suffer communication disabilities or those with dementia (11,12).
BENEFITS OF AE
Because of the buoyancy effects of water, AE is the ideal modality for clients with musculoskeletal or cardiometabolic disorders. Water offers less joint impact allowing for exercise to be performed both at higher intensities and longer duration compared with land. Specifically, by using buoyancy to reduce impact during full body resistance work, speeds may be adjusted to allow for intensity self-regulation, including increasing the work and volume without increasing the risk of injury (13). For training clients, one of the best examples of this format uses the time-efficient and effective HIIT approach. Aquatic HIIT is ideal for those with limited mobility, discomfort on land, neurological conditions (muscular dystrophy and Parkinson’s disease), coordination deficits, neuropathy, and cardiometabolic conditions (prediabetes, diabetes, and metabolic syndrome), for those overweight/obese, or for those who seek long-time adherence to a “friendly and fun” alternative to land-based activities (13).
Interventions examining physiological and other health outcomes have shown similar cardiorespiratory, metabolic, musculoskeletal, physical function, ADLs, mood/pain, and quality-of-life benefits compared with traditional land-based forms of training (14–17). In addition, evidence of physiological responses from training for clients who would benefit from adding AE to their training program is summarized in Table 1.
Notably, recent evidence has shown more favorable effects with acute AE exercise and AE training programs compared with other land-based forms in the following areas:
- acute endothelial and blood pressure responses
- greater augmented cerebral blood flow
- greater improvements in pulmonary function
- reduced pain and depression
These results are considered provocative, warrant further investigation, and provide additional support regarding the unique advantages of exercise in an aquatic medium (24,25).
Contraindications to immersion and AE (4,5,24)
- contagious disease or fever
- diarrhea or bowel incontinence
- uncontrolled medical conditions (i.e., HTN or hypotension, DM, seizures)
- skin conditions, open wounds, or chemical allergies
- immunocompromise (weakened immune system by drugs or illness, i.e., uncontrolled asthma)
- behavioral or cognitive status that would compromise safety of client or those in the pool
- myocarditis <6 months or myocardial infarction <6 weeks (4)
- “does not participate in regular exercise, demonstrates signs or symptoms suggestive of CV, metabolic, or renal disease” and has not received medical clearance in accordance with the American College of Sports Medicine preparticipation screening algorithm
- oxygen saturation < 88% to 90%
- within 48 hours of intravenous chemotherapy, oral dependency upon chemotherapy agent
- inability for client to safely access the pool (i.e., weight, ADL skills, cognitive, or behavioral concerns)
Aquatic-based exercise programs provide a wide variety of training options for clients or participants on the health continuum, ranging from rehabilitation to fitness, and athletic performance. By contrast, aquatic physical therapy is defined by the Academy of Aquatic Physical Therapy as “the evidence-based and skilled practice of physical therapy in an aquatic environment by a physical therapist, or a physical therapist assistant under the supervision of a physical therapist” (5). The focus of this article is AE for health and fitness, although through an understanding of a therapist’s role, health/fitness professionals can help facilitate an appropriate intervention for a client.
WATER ENVIRONMENT WITH EXERCISE
During exercise, water’s static properties act dynamically against the body. Responses will vary with temperature, depth, individual body differences, equipment applied, surface area presented, speed, and propulsion generated in the direction of the current’s flows or the force of buoyancy’s upward lift. When the static properties are altered by exercises that are designed to engage water’s properties, the resulting dynamic responses are responsible for the intensity-driven stimuli, which will provoke a training effect.
Altered static and dynamic properties
Altered physiological and perceptual responses
To achieve an optimal safe and effective training effect, an AE prescription must consider water’s properties within the context of training principles and safety. Maximizing the use of drag and buoyancy and refining program content to increase potential benefits is a key component of AE prescription (15). Well-coordinated, full body movements, paced appropriately, performed by moving in the right direction, and in the right depth, allow clients to “take charge” of their activities to meet their health objectives. A water-specific exercise prescription can effectively offer clients a “gateway” to adopting activity for those who are reluctant to exercise on land and/or expand comfortable training options as their health conditions change over time. Table 2 examines considerations for designing an AE prescription, with specific tips for application (13).
Well-coordinated, full body movements, paced appropriately, performed by moving in the right direction, and in the right depth, allow clients to “take charge” of their activities to meet their health objectives. A water-specific exercise prescription can effectively offer clients a “gateway” to adopting activity for those who are reluctant to exercise on land and/or expand comfortable training options as their health conditions change over time
“CATCH A WAVE” INTERVAL COACHING
The dynamic properties provide an excellent environment for interval training (Figure 1). The following is an interval coaching progression to guide clients through variations in movements and speed (2):
- 1. Start. Cue a coordinated move and/or variation(s) to gear up.
- 2. Transition. Cue a quick “down-up” interval to shift gears between variations, or in response to rate of perceived exertion (RPE).
- Slow speed, smaller move, and stabilize (jog and scull)
- 3. Repeat sequence.
Check perceived exertion by using RPE, which measures the “intensity, strain, or discomfort, and/or fatigue felt during exercise” (25). Adaptation to water’s resistance may take some training, so progress gradually. Using the perceived exertion scale, clients can apply numerical rating of the overall body, arms, legs, or chest/breathing. A “full body pace check” helps determine an appropriate speed, based on the duration that they “feel” they could continue the exercise (while maintaining the same ROM, quality, and speed). Have clients try different speeds that match with RPE using the OMNI Aquatic Exercise Scale of Perceived Exertion (Figure 2) (2,25):
- Easy/somewhat easy: RPE 2–4 (3 minutes or longer)
- Somewhat hard: RPE 4–6 (2 minutes)
- Hard: RPE 7–8 (30 seconds–1 minute)
- Extremely hard: RPE 9+ (15–30 seconds)
BRIDGING CLIENTS TO SHALLOW WATER INTERVAL TRAINING
Even without a pool in your facility, assign AE “homework” so your clients can train independently in a community or private pool, adopting a “surf and turf” approach to a well-rounded program. Quick Splash! HIIT Rx Client Handout, available online (link below), provides a shallow water HIIT workout with video clips demonstrating the exercise protocols and coaching methods used in the study by Nagle et al. (2).
QUICK SPLASH! HIIT Rx CASE STUDY
Dave, 45 years old, was assessed as overweight with controlled hypertension. He′s doing well in his land-based exercise program, but he’d like to add some lower impact, vigorous training to help improve his jump shots in his one-on-one basketball games. He enjoys occasional swims in his apartment’s shallow water pool.
Quick Splash! HIIT Rx Client Handout: http://links.lww.com/FIT/A125.
Sample Interval Protocols Table:http://links.lww.com/FIT/A124.
Clips demonstrate coaching methods, exercises, variations, and intervals from the protocols used by study participants in Nagle et al. (2,13).
Jog: see Supplemental Digital Content 1, Video 1, http://links.lww.com/FIT/A120
Jump: see Supplemental Digital Content 2, Video 2, http://links.lww.com/FIT/A121
Hover jump and jog combination: see Supplemental Digital Content 3, Video 3, http://links.lww.com/FIT/A122, and Supplemental Digital Content 4, Video 4, http://links.lww.com/FIT/A123
BRIDGING THE GAP
Immersion and AE result in profound physiological and psychosocial responses for improved health and wellness. By understanding the science of water’s properties and how they are engaged during exercise, trainers are better equipped to provide safe and effective aquatic-based exercise prescriptions for clients.
Interval training in water offers clients an opportunity to use a HIIT approach in a more comfortable and supportive environment.
The authors thank the following: Cathy Maloney-Hills, PT, DPT, TPS, Courage Kenny Rehabilitation Institute, part of Allina Health, Minneapolis, MN, for editorial contributions; Robert J. Robertson, University of Pittsburgh; Brian Templien, director of Sports Video Services, Pitt Athletics, University of Pittsburgh, PA; and Nora L. Constantino, Ph.D., associate professor Community Health Science, University of Nevada, Reno for video exercise performance.
Photos: Tracy Frankel or Mary E. Sanders, unless noted. Photos are courtesy of WaterFit.
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