The unique properties of water (buoyancy and resistance) provide a safe and effective modality for both relaxation and vigorous exercise, yet the health benefits of water workouts are not widely known. Current public health trends—especially rising rates of obesity, coupled with an aging population, associated chronic conditions and rising healthcare costs—are a call to new action for both treatment and prevention.

Physical activity is positively related to physical function and health, yet some individuals may have chronic or age-related conditions that limit their ability to perform physical activity on land. Barriers to exercise include fear of falling, pain, discomfort and lack of fitness. Water exercise has shown promise as an effective, yet comfortable and safe, physical activity option for people who are unable to exercise on land, but the health benefit lessons are still being learned.

Established Research

Water programs in deep and shallow water, where exercise intensities are similar to those on land, successfully meet the American College of Sports Medicine’s recommendations (3–6 METs, or units of metabolic demand) with regard to cardiorespiratory training and kilocalorie expenditure for weight loss. In addition, depending on movement speed, equipment can provide as much as 35.9 pounds (lb) (16.3 kilograms [kg]) overload on a working muscle (Pöyhönen et al. 2000, 2002).

Water immersion usually reduces blood pressure and stress hormones. In addition, immersion strengthens respiratory muscles, enhances cardiovascular efficiency, improves kidney function and increases muscle blood circulation. As a person steps into water and becomes submerged, the blood is being pushed upward, increasing the volume in the large pulmonary vessels and heart. Stroke volume and cardiac output increase at a rate similar to that which occurs on land. (For further analysis of past research, see Sanders 2001 and Sanders 2003.)

Now let’s examine new topics that have emerged in the literature, and look at ways to apply these findings at the pool.

Fibromyalgia and Depression

Fibromyalgia is a condition characterized by widespread musculoskeletal pain and fatigue, which stress and other factors can
exacerbate. People diagnosed with fibromyalgia have a low tolerance for land exercise, so water may be an ideal environment for them (Sanders 2000). Assis and colleagues (2006) randomly assigned 60 sedentary women aged 18–60 years to either land or deep-water training for 15 weeks. Four women dropped out of each program. Comparing outcomes for the 52 remaining subjects, the researchers found that both groups had similar gains in cardiovascular endurance and reported decreased pain levels, feeling “much better” at the end of the program. Compared with the land group, the deep-water group showed decreased levels of depression sooner (at 8 weeks, compared with 15), a trend that continued until the end of the study. This research may have positive implications for fibromyalgia sufferers.

Brass and Federoff (2007) measured depression and pain in women aged 34–58 years. Participants attended two shallow-water exercise sessions for 10 weeks and performed stretching, light aerobics and some strength training, progressing from 20 to 50 minutes’ duration. Subjects reported decreased pain and depression immediately following the program and for 3 months afterward.


Nagle and colleagues (2007) explored weight loss during a 16-week study that compared two groups of women who dieted and exercised. All participants received behavioral guidance on weight loss and were taught how to reduce dietary intake.

The 44 obese, sedentary women with a body mass index (BMI) of 34.9 (mean age, 40), were randomly assigned to an aquatic-
exercise-plus-land-walking group or a land-walking-only group.

Aerobic conditioning time progressed from 20 to 40 minutes per day by the ninth week. Intensity was monitored using rating of perceived exertion (RPE) and heart rate monitors, with a moderate level of intensity set as a goal. As participants progressed, exercises were modified to meet intensity goals. Aquatic exercise was performed in shallow and deep water using rhythmic, whole-body movements that kept participants warm. Subjects exercised at their own pace, achieving an RPE of 7–8 during cardio segments (on a scale of 1–10, with 10 being the highest intensity). Program design included circuit interval stations, continuous cardio conditioning and calisthenics. Equipment such as noodles and foam dumbbells were used
occasionally. Outdoor supervised land walking sessions included inclines. The Lifestyle, Exercise, Attitudes, Relationships, and Nutrition (LEARN) weight loss program was implemented, with individual dietary and caloric goals assigned and tracked.

Both groups lost weight and increased their cardiorespiratory fitness, strength and health-related quality of life. Between the water-plus-land group and the land-only groups, slightly greater losses in body weight were reported for the former (6.8 kg or 14.96 lb vs. 5.5 kg or 12.32 lb); the difference was not significant. The water-plus-land and land-only groups both decreased waist circumferences, by 6.63 centimeters (cm) and 7.23 cm respectively. Flexibility improved significantly for both groups, with the water-plus-land group improving somewhat more than the land-only group. Exercise enjoyment was significantly higher for the water-plus-land group, while both groups improved in health-related quality-of-life areas. Both groups significantly decreased caloric and fat intakes.

Some other interesting research looked at the use of underwater treadmills. Greene et al. (2009) assigned 57 participants, aged 24–66 years (mean age, 44), to exercise 3 days per week for 12 weeks on a treadmill, either on land or submerged in water. Participants were physically inactive and overweight or obese, with an average BMI of 30.5 kg/m2. Water depth was standardized at the forth intercostals (approximately nipple depth), where the average water-equivalent weight was about 25% of that on land. Flow jets were directed at the umbilicus for each water participant. Exercise intensity and duration were controlled so that all participants expended an equivalent number of kilocalories per session (land and water).

Both land and water groups significantly improved VO2max (+3.95 ml/kg/min on land and +3.26 ml/kg/min in water) and reduced percent body fat (-1.3%). Trunk, hip and waist girths, as well as BMI, were all significantly lower in both training conditions. Lean mass increased under both conditions, but the increase was greater for water exercisers. Both land and water treadmill users experienced similar weight loss and cardiovascular responses as long as frequency, intensity and duration (i.e., caloric expenditure) of exercise were matched between training modes.

Of note is the fact that bariatric surgery is increasing as a treatment for obesity. McCullough et al. (2006) reported that there were no medical complications for bariatric patients with a presurgical baseline BMI < 45 kg/m2 or a peak VO2 ≥ 15.8 ml/kg/min. Because of both the offloading and training effects, water training may provide a valuable pre- or post-surgical treatment for bariatric patients of any age.

Hip and Knee Osteoarthritis

In 2008, the Osteoarthritis Research Society International issued recommendations for evidence-based management of osteo-arthritis of the hip and knee. Goals included decreasing joint pain and stiffness, stabilizing and increasing joint mobility, reducing physical limitations and disability, improving health-related quality of life, limiting progression of joint damage and providing patient education (Zhang et al. 2008). Recommended treatments included water-based exercise.

A systematic research review by Bartels et al. (2007) provides clues about the effectiveness of aquatic exercise for knee and hip osteoarthritis treatment. Their review of six trials involving 800 participants revealed small to moderate improvements in function and quality-of-life measures. A minor effect was reported for pain reduction in programs that included treatment for both hip and knee. During one study, in which pain was measured for participants with hip osteoarthritis only, no effect was noted for pain reduction, function or quality of life. When knee
osteoarthritis was examined alone, the aquatic exercise had a large effect on reducing pain.

Wang and colleagues (2007) recruited 38 men and women (mean age, 66 years) with osteoarthritis of the hip or knee and randomly assigned them to either a 12-week aquatic exercise program or a nonexercise control group. The exercise consisted of 50-minute shallow-water sessions, 3 days per week, in a pool where the temperature ranged from 30 to 32 degrees Celsius (86–90 degrees Fahrenheit). At the end of the study, participants had significantly improved knee extension (45.5%), hip extension (11.5%) and hip abduction (14.3%). Muscle strength improved for knee extension (18.5%) and flexion (12%), and for hip flexion (11.9%), abduction (24.8%) and adduction (13.5%). No injuries or worsening of the joint condition were reported.

Another study, by Hinman, Heywood and Day (2007), randomized 71 subjects (mean age, 62.4 years) with osteoarthritis of the hip or knee into 6 weeks of aquatic or no aquatic exercise. Participants exercised for 45–60 minutes in shallow water (water temperature, 34°C, or 93°F). Intensity was progressed over time as appropriate for each individual. Exercises were functionally targeted and included squats, lunges, knee flexion and extension, stepping up and down and long walking segments. Compared with the control group, participants in the 6-week aquatic program reported less pain (72%) and joint stiffness, with greater physical function (75%). The authors suggested that the warm water might have helped with muscle relaxation. Hip strength also increased. Attendance during the intervention was high, and only 49% reported mild joint discomfort. Benefits were maintained for 6 weeks after completion of supervised training, with 84% of participants continuing the exercises independently.

Muscle Activation

Electromyography (EMG) analysis in water is a new research field that comes with a number of technical and safety challenges stemming from the use of electrical components in the water
environment. Insights into how muscles are activated in water compared with land are important in helping trainers design appropriate programs for clients. Those who need to exercise during recovery from injury may benefit from less muscle activation during the healing process, while others aiming to strengthen weak muscles can choose to meet their training objectives in the comfort of water.

Masumoto & Mercer (2008) reviewed a number of EMG studies on muscle activity during water exercise. Masumoto et al. (2004, 2005, 2007a, 2007b, 2008), Masumoto, Delion & Mercer (2009) and Kaneda et al. (2008), measured EMG responses for participants walking on an underwater treadmill or running in deep water. To set the pace, treadmills were submerged with a 0% incline, and speed could be regulated. With regard to muscle
activity in shallow water, Masumoto and Mercer (2008) reported on EMG studies in which participants walked on submerged treadmills at xiphoid process depth, where weight bearing is approximately 80% compared with on land. In most cases, shallow-water subjects walked with water currents streaming against the body.

The reviewed studies showed that under certain conditions, immersion might limit neuromuscular function as a result of decreased impact, depth and/or hydrostatic pressure. Investigators theorized that one factor involved might be reduced stimulation of proprioception, which could potentially affect balance mechanisms.

The findings, outlined in the sections below and summarized in the sidebar “Muscle Activation Chart in Water Under Different Conditions,” suggest some exciting possibilities for
water programming.

Shallow-Water Walking

Under different conditions, muscle activation responses differ.

Land vs. Water: Cardio Intensity. When exercise intensity was matched to cardiorespiratory and RPE responses, muscle activity was approximately 70% lower during both forward and backward walking in water, compared with land walking, in younger and older participants. Despite this difference, cardiovascular intensity was similar in both environments. It is thought that in water, the arms moving through the water’s resistance may contribute to higher cardio overload. Slower walking speeds (approximately 57% lower)—caused by water’s buoyancy and resistance—could also decrease muscle activation in the pool. (Masumoto et al. 2004, 2008)

Land vs. Water: Speed. When land and water walking speeds were matched, cardiorespiratory and RPE responses were higher in water. Walkers had lower peak muscle activity in water than on dry land, although water walkers had higher average muscle activity for the quadriceps (rectus femoris and vastus lateralis), hamstrings (biceps femoris) and gastrocnemius. Investigators suspected that in order to keep pace by pushing through water, the water walkers had to engage propulsive forces, using more muscle activity over a longer period of time. (Masumoto et al. 2008)

When participants walked at self-selected speeds, they chose slower speeds in water, and only the hamstrings produced greater muscle
activation than on land. This could be related to the need to increase propulsive force to overcome drag in water. (Miyoshi et al. 2004)

Depth and Speed. Investigators reported that the gastrocnemius was used more as speed increased, while the soleus was
engaged more as lower-extremity load increased. Changing
water depth can change load. The researchers thought that greater load and shallower water increased the somatosensory input that stimulated soleus motor neurons. (Miyoshi et al. 2006)

Forward vs. Backward Walking: Muscle Activation. Compared with walking forward against the current flow, walking backward against the current was found to result in higher cardiorespiratory responses. With backward walking, there was higher activation of the paraspinal muscles or erector spinae (61%), the quadriceps (83%) and the tibialis anterior (47%). This was thought to be related to propulsion, locomotion and body
mechanics. (Masumoto et al. 2005, 2007a)

Young Versus Old: Forward Walking. Muscle activity was different for younger and older participants at three different speeds (1.8, 2.4 and 3.0 kilometers per hour). Older participants took smaller steps, resulting in greater stride frequency over time, and used more of their quadriceps and hamstrings and less of their gastrocnemius compared with younger water walkers. Investigators attributed these differences to age-related changes, with greater hip power compensating for lower gastrocnemius use during walking. (Masumoto et al. 2007b)

Land vs. Deep Water: Walking and Running. When researchers examined muscle activation during land walking and deep-
water running performed at three self-selected speeds (slow, moderate and fast), they discovered that deep-water runners had lower muscle activation for the soleus and medial gastrocnemius and higher activation of the quadriceps and hamstrings compared with land walkers matched for speed. This occurred at all three paces. The investigators concluded that at sufficient speed, deep-water running could provide effective conditioning for the quadriceps and hamstrings. (Kaneda et al. 2008)

Land vs. Deep Water: Running. Muscle activation was measured during treadmill running and deep-water running at matched RPE levels of 11 (fairly light), 13 (somewhat hard) and 15 (hard). Results indicated lower muscle activation for deep-
water running at each RPE by the tibialis anterior and gastrocnemius, which would indicate a need for participants to exercise at a higher RPE in water to achieve the same level of muscle activity. Mercer (one of the study authors) further stated, “In looking at the deep-water training studies that have been published, it seems the ones that included high-intensity intervals during deep-water running have produced the best training response, which agrees with our findings.” (Masumoto, Delion & Mercer 2009)

Water Buoyancy and Bone Health

In water, forces acting on the body include buoyancy and drag. Buoyancy force works against gravity, and drag force works to oppose movement. Drag force working against the body can aid or impede muscle activation.

Water’s support provides a safe environment for participants diagnosed with osteopenia or osteoporosis, who may also be at risk for falling. The osteogenic effects of water exercise are not well-known. In a 1997 study on the effects of water exercise on bone health, Bravo et al. found that, after a year, participants (women aged 50–70 with osteopenia) had a 1% decrease in lumbar-spine bone mineral density and no change in hip bone density. However, subjects showed improvements in coordination, agility, muscular strength, flexibility and cardiovascular endurance. Functional skills also improved, and the water exercises might have helped increase the amount of activity performed on land, which in turn could have helped minimize bone loss over the 12-month period.

More recently, Ay and Yurtkuran (2005) learned that overweight, sedentary, postmenopausal women (mean age, 54 years) increased calcaneal bone mass after 6 months of shallow-water training 3 days per week, even though water’s buoyancy reduces impact. Aquatic exercises included jumping, walking and swaying for up to 40 minutes. The study found that calcaneal broadband ultrasound attenuation (BUA) increased by 3.1% in the aquatic-exercise group and 4.2% in the weight-bearing exercise group (by contrast, it decreased by 1.3% in nonexercising controls). Although this study was small and short-term, there appeared to be a positive affect on calcaneal bone density.

Final Splash

There is much to be learned, and as more studies are published, more programs can be developed to meet the activity needs of an aging population. According to Bruce Becker, MD, clinical professor in the department of rehabilitation medicine at the University of Washington School of Medicine in Seattle, water exercise for health could become the next important trend in health care. Becker has stated that our nation “could save huge amounts in healthcare expenses if the public were educated about the value of aquatic activity, if political powers directed public expenditures toward pool construction to improve public access and [if] the medical establishment understood the potential value of aquatic activity across a wide range of clinical problems” (Becker 2007).

Trainers who understand effective water exercise methods and know how to integrate water moves into land-based programs can adopt a mix-and-match approach to activity plans for clients. Fill your own toolkit with emerging practices, and prepare yourself for times when water is the best, and perhaps only, option.

Poolside Applications

With the latest research in mind, here are some training tips.

Fybromyalgia or Depression

  • Recommend shallow- or deep-water exercise to clients with either of these conditions.
  • Teach and remind clients to work at their own pace and modify moves as needed.
  • Guide clients to be mindful about emotional states so they become aware of positive changes, which may lead to long-term adherence.
  • Given the broad range of fitness levels among people with fibromyalgia, organize classes based on fitness level as well as medical condition.
  • Include water exercise as an effective confidence-building “gateway” for students wishing to move on to land-based activity.

Overweight and Obesity

  • Include shallow- and deep-water activity along with land walking programs for weight loss, weight maintenance and enjoyment.
  • Allow participants to work at a sufficient speed and intensity needed to achieve a moderate-intensity exertion rating (7–8 on the RPE scale) during cardio conditioning.
  • Ensure privacy—have a locker room poolside for comfort.
  • To quantify speed, recommend that clients with lower-extremity limitations walk and jog on a variable-speed underwater treadmill instead of (or in addition to) using a land treadmill.
  • Coach participants to walk forcefully in water that is between navel and nipple depth, and travel through water to create currents and exercise at a pace that mimics land intensity.
  • Include water in exercise recommendations for obese, metabolic syndrome and bariatric surgery clients who need to reduce weight and risk of surgery.

Hip and Knee Osteoarthritis

  • Conduct small-group, skills-training sessions in warm water to help promote pain-free and effective exercise that reduces pain and increases function. (Individualized and progressive exercise programs, when mastered, can lead to long-term, independent exercise adherence and result in better living on land.)
  • Check pain levels and modify exercises or reduce intensity as needed.
  • Encourage early and long-term participation, and guide students to add new types of exercise that cross over to more land activity.

Water Walking

  • Include water walking for clients who need to decrease exercise intensity below what is tolerated on land (i.e., rehab or pain management clients).
  • Change speed, current flow, direction of movement and/or impact (water depth) to affect muscle activation.
  • Alternate forward and backward walking for a work-recovery cycle for the quadriceps, hamstrings and gastrocnemius and for balance.
  • Cue participants to walk backward in shallow water to increase cardiovascular intensity, beyond what they may be able to achieve while walking forward. To increase intensity further, encourage them to walk forward and backward against a current.
  • To train for improved gait on land, teach good posture and land-based walking mechanics as you motivate older adults to enlarge their stride in water, working against the properties of water that impede movement.
  • To provide sufficient overload to improve land walking, motivate participants to apply “maximal effort.”
  • Teach clients to push forcefully through the foot to maximize use of the gastrocnemius.
  • As muscle activation for the tibialis anterior during forward walking is lower for older participants, cue participants to lead with “toes up” to maximize activation in water. Include land-based training for further activation.
  • In deep-water running programs, include interval training conducted at a higher RPE level than you would use on land, to maximize muscle activation.

Buoyancy and Bone Health

  • Include moderate-intensity impact exercises, such as jumping and walking, in shallow-water depths. An increase in time spent in shallow water at waist depth or less, where weight bearing may be about 28% of body weight, can help optimize bone health.
Muscle Activation in Water Under Different Conditions

* Higher levels of intensity (RPE 15) during deep-water running elicited similar muscle activity levels as lower intensities (RPE 11) on the land treadmill.


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Mary E. Sanders, PhD

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