Metabolic and Cardiovascular Response to Shallow Water Exercise in Young and Older Women : Medicine & Science in Sports & Exercise

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APPLIED SCIENCES: Physical Fitness and Performance

Metabolic and Cardiovascular Response to Shallow Water Exercise in Young and Older Women


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Medicine & Science in Sports & Exercise: April 2003 - Volume 35 - Issue 4 - p 675-681
doi: 10.1249/01.MSS.0000058359.87713.99
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The acute and chronic physiological responses of aquatic activities such as swimming and deep-water running have received considerable attention in the literature (9,10,14,17,18,26–28). Over recent years, shallow water exercise has gained popularity in both private and collegiate fitness programs (4,6–8,15,17,20), and received attention in scientific (4,6) and lay (3,21–23) publications. Shallow water exercise programs emphasize modified land-based exercises performed primarily in a vertical orientation in shallow water, approximately waist to shoulder level (23). This form of aquatic activity is ideal for older adults, especially for those with orthopedic challenges and balance impairment, which makes impact exercise on land difficult (17). Furthermore, shallow water exercise is becoming increasingly popular at universities and recreational facilities where college-age women, varsity sport teams, and group fitness enthusiasts participate in aquacise classes (11).

Eckerson and Anderson (6) found that in healthy collegiate females shallow water exercise elicited a heart rate and oxygen uptake response corresponding to 82% and 48%, respectively, of maximal values achieved on a treadmill. The researchers concluded that shallow water activity may provide an alternative mode of exercise for improving cardiovascular fitness. It should be noted that the female participants performed jogging in place, and a variety of upper- and lower-body movements that resulted in little translatory motion. Enhanced aerobic capacity and reduced percent adipose tissue have also been reported in females (19–40 yr) after shallow water exercise training programs spanning 7–10 wk (6). Few studies have been conducted investigating the physiological load of shallow water exercise in older adults (4). Recently, D’Acquisto et al. (4) investigated the metabolic and cardiovascular demands of shallow water activity performed by older females ranging in age from 60 to 80 yr. Investigators found that during a 40-min shallow water exercise session, participants were able to maintain an effort that elicited a cardiovascular response (∼66–78% of age predicted HRmax) that meets intensity guidelines set by the American College of Sports Medicine to realize health-related benefits (1).

To our knowledge, no study has compared the physiological responses of young and older adult women during submaximal and maximal shallow water activity. Such information would be useful to physical therapists, exercise physiologists, fitness instructors, and others who prescribe shallow water exercise. Therefore, the purpose of this study was to compare the metabolic and cardiovascular responses between older adult women (63–72 yr) and college-age women (19–24 yr) performing shallow water exercise.



Eleven older women, with a mean age of 66.7 ± 0.9 yr, were recruited from university and local community shallow water exercise classes. In addition, 11 younger women, mean age of 21.3 ± 0.4 yr, were recruited from the university’s physical education activity program. All participants had experience with water exercise. Written informed consent was obtained from all subjects before the experiment. The university’s Human Subject Review Committee approved the investigation.

All volunteers were prescreened with a Physical Activity Readiness Questionnaire. Only individuals not taking cardiac medication participated. The older group underwent a modified Bruce treadmill stress test. A 12-lead electrocardiogram strip was obtained and reviewed by a medical doctor. Only individuals who passed the treadmill stress test participated in the study.

Table 1 presents physical characteristics for the younger and older groups.

Physical characteristics of participants.

Resting Metabolic Measurements

Participants were provided with standard written and oral instructions before testing. They were asked to abstain from vigorous exercise the day before testing, encouraged to rest well the evening prior, and to arrive to the lab between 05:30 and 08:30 after a 12-h fast. Participants were also encouraged to arrive to the lab in a hydrated state.

Upon arrival to the lab, participants rested in a seated position for 5-min and subsequently were prefitted with a breathing apparatus (Hans Rudolph, Inc., Kansas City, MO) plus headgear and nose clip and then allowed to breathe quietly for several minutes. The valve apparatus was then removed. Participants then rested in a supine position in a dimmed, quiet room. Investigators periodically checked to make sure participants were relaxed but not sleeping. Room temperature was approximately 22°C.

After 40 min of rest, heart rate (2 × 60-s count) and blood pressure (two to three measurements) were obtained. Subsequently, the breathing valve apparatus was fitted, and two samples of expired air were collected into meteorological balloons (8-min collection periods). Expired air was analyzed for oxygen and carbon dioxide using a calibrated Ergo-oxyscreen (Yaeger, Hoechberg, Germany) while volume was determined with a dry gas meter. Oxygen uptake values from the two collection periods were averaged and represented the subject’s resting metabolic rate (RMR). After the collection periods, body composition analysis was assessed by skinfold measurements. A seven-site equation by Jackson and Pollock (13) was applied to the college age females for estimation of body density in combination with the equation for percent adipose tissue (25). A four-site formula was employed for the estimation of percent adipose tissue (29) for the older group. Height was measured, and body weight was obtained with a standard clinical balance (Detecto-Medics, Brooklyn, NY).

Submaximal Shallow Water Exercise Bouts

Subjects participated in an individual shallow water exercise session that occurred within 2 wk after the resting measurements. The intent of this testing session was to have participants perform five submaximal shallow water exercise bouts ranging from low to moderate effort so that individual regression equations among selected physiological parameters could be established. Prewater exercise instructions (see below) were delivered with the intention that participants would self-select work efforts resulting in an incremental increase in physiological load. A previous study (4) has shown that the same preexercise instructions delivered to a group of healthy older female adults elicited cardiovascular responses typical of a shallow water exercise workout. Minor variations in preexercise instructions were made for the younger group in the present study (described below) to assure that an incremental increase in physiological load would be achieved.

Participants were instructed to report to the pool during morning hours, in a well-rested, and hydrated state. Participants were requested not to eat three hours before testing and to consume no caffeine on the morning of testing. Upon arrival to the pool, subjects were weighed, fitted with a Polar heart rate monitor (Kempele, Finland), and subsequently allowed to rest quietly for 10 min in a seated position. During a shallow water warm-up (5–10 min), participants were familiarized with the breathing valve setup. Participants performed five shallow water exercise bouts, approximately 8 min in length, ranging from light to moderate effort as well as a maximal exercise bout. All shallow water exercise was performed in a 25-m pool with the water level ranging from the xiphoid process to the axillary region for all participants. Water temperature varied from 27.5 to 28.0°C. The instructions were read to the participant just before each exercise bout.

Bout 1

Do not use your arms; let arms rest (float) on top of the water. Be relaxed, and remember use no arms. Your legs are walking at a normal pace. You should be able to carry on a conversation and not be out of breath. Maintain an even pace.

Bout 2

Bring arms down to sides and swing naturally through the water. Legs are walking at a normal pace. You should be able to carry on a conversation and not be out of breath. Maintain an even pace.

Bout 3

Use slightly bigger steps (longer strides) with your legs and a more forceful arm swing. You should feel like you are walking with a purpose (older adults: like going to answer the phone, going after your golf ball, or going after your grandchild) (younger adults: like walking across campus to get to class on time). You should still be able to carry on a conversation and not be out of breath. Remember to maintain an even pace.

Bout 4

Your legs are now in a jog with arms pumping underwater at your sides. You should feel like you are “folk dancing” (or some type of rhythmical dance) at a fairly good pace. No kicking or high knees, though. Maintain an even pace.

Bout 5

Jog with arms performing breaststroke-like movements under and toward the surface of the water. You should feel like you are in an aquacise class working on a particular exercise routine. Maintain an even pace.

The younger group received the same set of instructions with several variations, again, with the intent of eliciting shallow water exercise efforts ranging from light to moderate. They started with bout 2, outlined above. Bout 5 outlined above was bout 4 for the younger group. The fifth submaximal effort involved the same instructions (breaststroke-like movements . . .); however, the younger group wore webbed gloves for added resistance.

Participants were stopped approximately 3 min into each exercise bout to be fitted with a two-way breathing valve (15–30 s stop). The breathing valve was connected to a collection apparatus (5) that one of the investigators held as the participant resumed exercise across the shallow end of the pool. During each exercise bout, two meteorological bags of expired air were collected (∼75–90 s samples) between minutes 5 and 8, and analyzed for percent oxygen and carbon dioxide with a calibrated Ergo-oxyscreen (Yaeger). The volume of air in the bag was determined with a calibrated dry gas meter. Metabolic values obtained from the two samples were averaged and represented the metabolic response for the exercise bout. Heart rate was monitored continuously throughout all exercise bouts (Polar). Heart rate values measured during the expired air collection period were averaged and used to represent the heart rate response for each respective bout of exercise. Immediately after each exercise bout, the participants were asked to rate their perceived effort based on Borg’s 6–20 rating of perceived exertion scale (2). Participants were allowed to rest 4–5 min between each exercise bout.

Maximal Shallow Water Exercise Bout

Participants rested for 10–15 min after the last submaximal effort before performing a maximal 300-m shallow water exercise bout. During the maximal effort, webbed gloves were worn by both groups to help increase propelling force in the water. Participants were asked to perform the first 100 m of exercise jogging with breaststroke arms, the same as submaximal bout 5. For the next 100 m, they performed a side-hopping motion of the lower extremities with simultaneous alternating adduction/abduction movements of the upper extremities. While traveling, water was pushed out with the palms of the hands, so as to increase resistance. Just before the last 100 m, participants were stopped (15–30 s) and fitted with the two-way breathing valve apparatus. During the fitting time, participants were reminded to move as quickly as possible during the final 100 m using breaststroke-like arm motion while jogging. Participants were encouraged to reach the 250-m mark feeling like they only could go an additional 50 m, no more, no less. Two meteorological bags of expired air were collected (∼30–40 s each) during the last 50 m of exercise and analyzed for percent oxygen and carbon dioxide and volume. The highest recorded 1-min HR value represented HRpeak. As soon as the participant finished, RPE was recorded.

Shallow Water Exercise Class

Participants took part in a 40-min shallow water exercise class within 2 wk of completing the submaximal and maximal tests. Participants were asked to arrive at the pool between 06:30 and 07:00 in groups of four to five. The class was led by an experienced aquacise instructor and consisted of a 10-min warm-up, body of the workout (25 min; broken up into body I and II), and a 5-min cool-down. Heart rate was monitored throughout the exercise session with a Polar heart rate monitor. RPE was obtained at the end of the warm-up, after body I and II of the workout, and at the conclusion of the cool-down. The selected exercises for each workout stage represented common movements performed during a typical shallow water exercise class. The warm-up consisted of walking with shoulder rolls, arm movements performed through multiple planes, and dynamic stretches. The main body of the workout consisted of whole-body traveling while performing a variety of upper- and lower-extremity movements. Body I of the main workout lasted 12 min and emphasized lateral traveling with long-lever pendulum-like movements of the extremities. Participants also performed vigorous forward, backward, and circular jogging, and side crab-like jogs with arms pushing, pulling, pressing, and mimicking breaststroke. The second part of the main workout, body II (13 min), emphasized more forceful combinations of movements (cross-country ski, leaps, kicks, leg crossovers, and vigorous hopping movements) focusing on traveling in multiple directions, and bounding off the bottom of the pool when possible. The cool-down consisted of slowed walking and dynamic stretches.

Calculations and Statistical Analyses

Submaximal shallow water exercise bouts.

The exercise metabolic equivalent (MET) for each self-selected submaximal bout was calculated as the exercise oxygen consumption divided by the resting oxygen consumption. Percent V̇O2peak and percent HRpeak for each bout was calculated by dividing the submaximal exercise oxygen consumption and HR response by V̇O2peak and HRpeak, respectively, and multiplying these quotients by 100. Rate of caloric expenditure (kcal·min−1) for each exercise bout was determined from oxygen uptake and the RER. Simple linear regression was employed to develop individual equations for MET versus %V̇O2peak, RPE versus %V̇O2peak, %HRpeak versus %V̇O2peak, MET versus HR, and kcal per minute versus %V̇O2peak. Conventional statistics were employed to determine means and standard errors.

Comparison of younger with older adults.

The younger and older groups were compared on selected physiological parameters at given relative submaximal shallow water exercise intensities. The estimated MET, RPE, %HRpeak, and kcal per minute responses at standard relative intensities (40, 50, 60, 70, and 80% V̇O2peak) were determined from individual regression equations (described above). The measured HR for each phase of the 40-min shallow water exercise class was entered as the independent variable in the MET on HR regression equation to estimate MET level.

Two-way ANOVA with repeated measures were employed to analyze data between younger and older groups across the levels of relative intensity (% V̇O2peak) and different stages of the aquacise class (warm-up, body I, body II, and cool-down). When a significant interaction was found, a simple main effects analysis (12) was employed to examine any differences between the young and older groups at each level of relative intensity (% V̇O2peak) and at each phase of the 40-min workout session. As part of this analysis, changes over the different levels of relative intensity (%V̇O2peak), and the SWE stages, for each level of age, were examined. Tukey post hoc analysis was employed for this latter analysis when a significant F-value was found. Level of significance was set at 0.05.


Resting oxygen uptake, blood pressure, and heart rate are presented in Table 2. No differences were found between groups for heart rate; however, relative oxygen uptake was lower for the older group. In addition, systolic blood pressure was lower for the younger participants.

Resting oxygen uptake, blood pressure, and heart rate.

Maximal exercise data are presented in Table 3. V̇O2 values between the samples of expired air collected over the final 50 m were not different for each group (P < 0.05); therefore, values were averaged to obtain the final V̇O2peak. The younger group was found to have a greater peak MET and V̇O2 (P < 0.05), whereas RPE for both groups was approximately 17. RER values > 1.1, HR > 90% of land-based predicted maximum, RPE indicating “very hard” exertion, in addition to no difference in oxygen uptake between the two collection periods over the final 50 m, suggests that both the younger and older groups may have reached their highest oxygen uptake during the maximal shallow water exercise effort.

Selected responses during the maximal shallow water exercise bout.

There was no difference (P > 0.05) in oxygen uptake between collection periods (bag 1 vs bag 2) for both the young and older groups for each self-selected submaximal bout. Consequently, oxygen uptake values for bags 1 and 2 for each bout were averaged, and the resulting V̇O2 was used to calculate the exercise MET level. Descriptive data for MET, HR, and RPE responses to self-selected submaximal exercise bouts 1–5 are displayed in Table 4. On average, MET response varied from ∼3.5 to 8.6 for the younger group and ∼3.2 to 6.3 for the older group for bouts 1–5. The younger group, on average, had a HR response which varied from ∼95 bpm (bout 1) to 144 bpm (bout 5). For the older group, HR varied from ∼92 bpm to 124 bpm for bouts 1–5. For bouts 1–5, RPE ranged from 7.6 to 13.6, and 9.2 to 14.5 for the younger and older groups, respectively.

Submaximal MET, HR, and RPE responses to self-selected shallow water exercise bouts 1–5 for the younger (Y) and older (O) groups.

Selected responses at given relative intensities (%V̇O2peak) for the young and older groups are displayed in Table 5. MET, RPE, %HRpeak, and kcal per minute are estimated values based on individual linear regression analysis. For the older adults, correlation coefficients for %V̇O2peak versus MET, RPE, %HRpeak, and kcal per minute were 1.0 (P = 0.0001 ± 0.0000), 0.909 ± 0.028 (P = 0.055 ± 0.021), 0.972 ± 0.006 (P = 0.008 ± 0.004), and 0.999 ± 0.000 (P = 0.0001 ± 0.0000), respectively. Correlation coefficients for the same set of relationships in the younger group were 1.0 (P = 0.0001 ± 0.0000), 0.958 ± 0.012 (P = 0.018 ± 0.006), 0.980 ± 0.008 (P = 0.006 ± 0.003), and 0.999 ± 0.000 (P = 0.0003 ± 0.0001), respectively. MET and kcal per minute responses were greater for the younger group at each relative intensity (P < 0.05). %HRpeak values were consistently higher for the younger group at each relative intensity level, with differences occurring at 50 through 80%V̇O2peak (P < 0.05). RPE responses were similar (P > 0.05) between groups at each relative intensity. All pair wise comparisons within each group (young and older) across relative intensity levels were significant (P < 0.05) for all dependent measures (MET, RPE, %HRpeak, and kcal·min−1).

MET, RPE, %HRpeak, and kcal·min−1 at given relative intensity levels (%V̇O2peak) of shallow water exercise.

Table 6 displays measured HR, RPE, and estimated MET response during the shallow water exercise class. MET levels were estimated based on individual regression equations between MET and HR responses obtained during SWE bouts 1–5. Correlation coefficient values between MET and HR for the older and younger adults were 0.972 ± 0.006 (P = 0.008 ± 0.004) and 0.977 ± 0.006 (P = 0.009 ± 0.005), respectively.

HR, MET, and RPE responses of young and older females performing in a 40-min shallow water exercise class.

The younger group maintained a greater HR for body I (120 vs 112 bpm) with the difference increasing during body II (129 vs 116 bpm). However, these differences were not significant (P > 0.05). Heart rates were similar between the groups during the warm-up and cool-down. In addition, the younger group worked at a greater estimated MET level during body I (6.4 vs 5.4) with the difference increasing during body II (7.3 vs 5.8), whereas little difference was found during the warm-up and cool-down. Overall, the older group worked at a greater relative effort (% METpeak), 61.5 ± 2.5 vs 48.8 ± 2.2 (P < 0.05). In addition, the older group had an overall greater RPE, 11.8 vs 10.4 (P < 0.05).


The primary intent of this investigation was to compare the metabolic and cardiovascular responses of younger and older female adults performing shallow water exercise. In addition, this investigation classified shallow water exercise intensity in terms of metabolic and cardiovascular load and perceived exertion at given relative intensities. Shallow water exercise offers a unique challenge when attempting to control exercise intensity for the purpose of physiological testing. The exercise protocol employed for the submaximal and maximal test efforts was found to incrementally increase physiological load. Findings from the submaximal exercise efforts were utilized to determine individual relationships between metabolic response (MET) and heart rate, then to use the relationship to predict metabolic load from measured heart rate responses during a 40-min shallow water exercise class. In addition, test results were used to study the relationship between MET, %HRpeak, and RPE versus a relative intensity measure (% V̇O2peak). Subsequently, the former variables were estimated at standard relative intensities ranging from 40 to 80% V̇O2peak in order to compare between the younger and older populations.

An important aspect of this study was that the metabolic requirements of shallow water exercise were described as a multiple of the measured resting V̇O2. The assumption of a RMR of 3.5 mL O2·min−1·kg−1 is typically made when estimating the metabolic demand of exercise (1). The younger group did have a RMR (∼3.3 mL O2·min−1·kg−1) that was similar to the standard reported one MET value of 3.5; however, the older population had a considerably lower RMR (∼2.8 mL O2·min−1·kg−1). The lower value for the older adults suggests that attempting to describe the metabolic requirements of shallow water exercise based on the standard one MET value would have resulted in an underestimation of MET requirements. For example, if an older adult in the present study consumed 16 mL O2·min−1·kg−1 during shallow water exercise, the estimated MET level would be approximately 4.6 when using the standard one MET value (3.5) versus 5.7 METs when using the one MET value of 2.8 measured in this investigation. These findings clearly illustrate that variations in RMR from the typical average textbook value of 3.5 need to be considered when quantitating the metabolic requirements of exercise.

This study directly measured V̇O2peak and HR while participants performed increasingly more demanding movements incorporating large muscle groups. To our knowledge, this is the first study that has measured V̇O2peak and HR in younger and older females performing whole-body forward movements across the shallow end of a pool. Such measurements allowed for a more accurate calculation of relative intensity (% V̇O2peak, % METpeak, and % HRpeak) versus the use of age predicted HRmax equations such as the conventional 220 − age (1) or 210 − age, as has been suggested for shallow water exercise (16).

RPE values for both groups were ∼9 at 40% V̇O2 and ∼14 at 80% V̇O2peak, suggesting that on average shallow water exercise elicited the same perceptual response regardless of age at a given relative intensity. Our findings support those of Sidney and Shephard (24), who reported that at any given relative intensity during land-based activity, RPE values were similar between younger and older populations. Pollock et al. (19) report RPE values of 12–13 and 14–16 for “moderate” and “hard” exercise intensity levels, respectively. Percent HRpeak values associated with an RPE range of ∼9–12 and 12–14 in this study corresponded to a measured physiological load (∼55–69 and 70–89% HRpeak, respectively) typically classified as “moderate” and “hard” (1). This finding is interesting and suggests that shallow water exercise performed at an exertional perception corresponding to “somewhat hard” (∼12–14) is associated with a relative physiological load range (70–89% HRpeak) that will improve or maintain maximal aerobic power in individuals exercising in adult fitness settings (1).

The older group was found to self-select a greater relative exercise effort compared with their younger counterparts during the shallow water class. This finding is interesting especially because both groups performed the same movement patterns, and received coaching instructions and encouragement from the same instructor. Results presented in Table 6 indicate that the older and younger groups worked at ∼74 and 59% METpeak, respectively, during the main phase of the workout. During the warm-up (W) and cool-down (C) the older group also worked at a greater relative intensity (W = ∼58, C = ∼53% METpeak) compared with the younger group (W and C ∼40% METpeak). Furthermore, the older adults, on average, consistently self-selected a greater relative physiological intensity for the submaximal exercise efforts (bouts 1–5 protocol) in which both groups received identical preexercise instructions from the same investigator. It is possible that the older adults were more motivated and focused, and consequently ended up working at a greater relative effort especially because they recognized they were being compared with a younger population.

Another explanation may be related to differences in body composition. The younger and older groups were similar in lean body mass; however, the older group had a significantly greater percent adipose tissue, 21.0 versus 31.4%. The estimated average adipose tissue weight for the older and younger group was 21.8 and 12.4 kg, respectively. The greater amount of adipose tissue in the older group perhaps resulted in a greater buoyancy effect compared with the younger group while exercising in the water. The older adults, therefore, needed to apply more propelling force with the upper and lower extremities to execute the required movements to counteract the effects of the added buoyancy. The net result was perhaps a recruitment of more muscle mass, increased metabolic load, and consequently a relative exercise intensity that exceeded that of the younger participants.

The warm-up and cool-down portions of the shallow water session corresponded to ∼57–58% (RPE = ∼8) and ∼63–65% (RPE = ∼10) HRpeak for the younger and older groups, respectively (Table 6). The warm-up and cool-down would be classified as “moderate” intensity, according to ACSM’s classification of physical activity intensity based on activity lasting up to 60 min (1). Body I and body II of the workout were performed at ∼66 and 71% HRpeak (RPE = 11.6 and 13.5, respectively) for the younger group, giving a classification on the upper end of “moderate” and “hard.” The older adults worked at ∼72 and 74% HRpeak (RPE = 12.8 and 14.2) for body I and II of the main workout. This intensity level corresponded to “hard” according to ACSM (1). These findings clearly illustrate that both the younger and older participants self-selected efforts that meet ACSM’s intensity guidelines to realize enhanced cardiovascular and muscular fitness.

In summary, the information presented in this investigation provides a more complete image of the exercise stimulus provided by shallow water exercise. On a practical note, aquatic fitness instructors should recognize that older female adults may self-select a greater relative intensity when provided with the same shallow water exercise cues that would be delivered to a younger group. In addition, exercise instructors should be aware that buoyancy in shallow water may be directly impacted by a participant’s body composition and thereby influence the level of relative intensity self-selected by the exerciser.

The researchers would like to express their gratitude to Dr. Terry Devietti, Department of Psychology, Central Washington University, for his assistance in the statistical analysis of the data.


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