Part I. The Training Effects of Aquatic Exercise
Water exercise is rapidly growing in popularity. Exercise enthusiasts, athletes, elderly, and the physically challenged are discovering aquatic exercise programs that suit their fitness desires. An advantage of aquatic exercise is that it can involve the upper and lower extremities through optimal ranges of motion while minimizing joint stress.

Despite the numerous attributes of aquatic exercise, few randomized, controlled studies have been completed substantiating the benefits of exercise in this medium. This review summarizes the published research, from articles and abstracts, and will be presented in three sections: 1) the training effects of aquatic exercise, 2) shallow and deep water exercise responses, and 3) selected topics of aquatic exercise.

The Aquatic Medium—800 Times as Dense as Air
Weight bearing, land-based exercise presents a challenge to the joints and soft tissues of the body. The repetitional strain imposed on the tissues by ground striking can lead to injury. The buoyant force of water results in up to a 90% reduction in body weight in the water. Because of the cushioning effect of water, individuals potentially at risk to bodily stress from weight-bearing exercise, such as the elderly, obese, individuals with a soft tissue injury, or those with an orthopedic disorder, may find water to be the most desirable environment for exercise. Yet, at the same time, water is capable of providing a full-body resistance. The density of water is approximately 800 times that of air, which is an important contribution to the energy cost of aquatic exercise (Di Prampero, 1986) . Thus, the water environment allows for high levels of energy expenditure with relatively little strain to the body.

Maximal Oxygen Consumption—Deep Water Running Training Studies
The majority of research completed on deep water running (DWR) has evaluated the ability to sustain (Bushman et al., 1997; Eyestone, Fellingham, George, & Fisher, 1993; Quinn, Sedory, & Fisher, 1994; Wilber, Moffatt, Scott, Lee, & Cucuzzo, 1996) or improve (Michaud, Brennan, Wilder, & Sherman, 1995) aerobic capacity following DWR. The cardiovascular and metabolic effects of chronic DWR versus land-based running is of particular interest to competitive runners suffering from musculoskeletal injury, or those wanting to reduce the stress associated with rigorous training. Table 1 summarizes the current research investigating the chronic effects of DWR training on maximal treadmill responses. Presently only a limited number of studies exist describing the training effects of DWR exercise programs. Due to variations in training programs, testing procedures and study lengths (4-10 weeks), conclusions are tentative.

Those studies conducted utilizing endurance trained subjects have found DWR training to be successful in the maintenance of aerobic performance (Bushman et al., 1997; Hertler, Provost-Craig, Sestili, Hove, & Fees, 1992; Wilber et al., 1996) . Wilber et al. recruited 16 trained male runners aged 20-40 years old to examine the effects of a 6-week DWR program on maximal oxygen consumption (VO2max), lactate threshold and running economy. The DWR training maintained maximal oxygen consumption (pre = 58.7 ± 4.7, post = 59.6 ± 5.4 ml/kg/min) in the highly fit subjects. These results are consistent with Bushman et al. who found similar maximal (pre = 63.4 ± 1.3, post = 62.2 ± 1.3 ml/kg/min) and submaximal VO2 responses (pre = 44.8 ± 1.2, post = 45.3 ± 1.5 ml/kg/min) after only 4 weeks of DWR training in 11 competitive runners (males N = 10; females N = 1; mean = 32.5 yr).

In the study conducted by Wilber et al. (1996), ventilatory threshold was sustained at approximately 80% of VO2max (Day 0 = 46.5 ± 6.14 ml/ kg/min, 79% VO2max, Day 42 = 47.4 ± 6.7 ml/kg/min, 80% VO2max), while VEmax (Day 0 = 120.1 ± 13.3 l/min, Day 42 = 131.1 ± 17.6 l/min) and treadmill run time to exhaustion (Day 0 = 16.6 ± 5.4 min, Day 42 = 17.3 ± 5.7 min) were not significantly altered through DWR training. Although DWR uses different proportions of upper to lower body muscle mass compared to land-based running, Wilber et al. and Bushman et al. (1997) both concluded that chronic DWR training created the physiological stimuli necessary to facilitate the maintenance of running economy.

Small inconsistent changes in maximal blood lactate responses have been reported after chronic DWR training (Bushman et al., 1997; Wilber et al., 1996) . Wilber et al. observed slight increases in maximal blood lactate values as a result of DWR training (7.8 ± 2.2 mM to 8.3 ± 2.0 mM) while decreases (pre = 9.3 ± 0.6 mM; post = 8.3 ± 0.5 mM) were noted by Bushman and colleagues. The heightened blood lactate response in the Bushman et al. study is most likely a reflection of greater VO2max values of these subjects.

Hertler et al. (1992) compared treadmill exercise to DWR training in 13 young runners (aged 18-26 yr). Subjects trained on land 3 days per week, for 4 weeks, and then half of the subjects began a DWR program while the rest continued to run on land. To equalize the training, groups were matched for total exercise time and RPE. Post-training maximal treadmill tests indicated no changes occurred in VO2max between the treadmill and DWR exercise training groups. This finding implies that DWR training can be effective in maintaining VO2max in aerobically trained subjects.

In a practical assessment of endurance performance, Bushman et al. (1997) had subjects complete a simulated 5-kilometer (5-k) run at a race pace on the treadmill to assess the crossover effects of DWR training to treadmill exercise. Analysis revealed that pre-training 5-k run times (1142.7 ± 39.5 s) were not significantly altered following 4 weeks of DWR training (1149.8 ± 36.9 s). Similarly, in an investigation by Eyestone et al. (1993) , 32 recreational trained runners (VO2max = 56.29 ± 1.49 ml/kg/min) achieved small but non-significant improvements (1.21%) in two-mile run performance (indoor track) after 6 weeks of DWR. However, post-tests revealed VO2max declined in these subjects by 4.9% (post-test VO2max = 53.52 ± 1.61 ml/kg/min). This may just represent daily variations in VO2. Another possible reason for these decrements in VO2max by Eyestone’s subjects is that the exercise prescription used minimal standards accepted by the American College of Sports Medicine (ACSM) for maintenance of cardiovascular fitness during the entire study. The training regimen established by Eyestone and colleagues consisted of subjects initially training 3 days per week and increasing to 5 days per week during weeks 3-6. Duration of exercise was established at 20 minutes in week 1 progressing to 30 minutes during the final month. Exercise intensity began at 70% of heart rate maximum (HRmax) and increased to 80% HRmax over the last 4 weeks of the study. The hydrostatic effects of the water may have caused Eyestone et al. to underestimate the training heart rate during DWR. Along with daily variations in VO2 this underestimation of training intensity also could have been a major contributing factor leading to the decline in VO2max in these fit participants. In studies where VO2max has been maintained, exercise training has closely resembled land-based training.

Wilber et al. (1996) exercised aerobically trained subjects 5 days a week, alternating high intensity shorter workouts (90-100% VO2max for 30 minutes) with moderately intense longer sessions (70-75% VO2max for 60 minutes). Similarly, Bushman et al. (1997) employed a training regimen consisting of DWR 5-6 days a week integrating two long and short interval days, one long run and an easy recovery run. These training schedules not only reflect actual training routines of these competitive athletes but more importantly insure adequate exercise intensity for the maintenance VO2max.
Only one published training study investigated the effects of DWR with older adults (mean age of controls 57.5 ± 2.3 yr, mean age of experimental group = 63.1 ± 1.6 yr). In this investigation Long et al. (1996) reported significant VO2max improvements in a group of 35 sedentary older women after a 10-week DWR program.

Quinn and colleagues (1994) found that untrained females were unable to sustain VO2max though DWR. In their study, 7 young untrained females (mean = 21.7 yr) performed 6 weeks of land-based training (LBT) followed by 4 weeks of DWR. Evaluation of VO2max occurred on three separate occasions: before and after the land-based running training and at the conclusion of the DWR program. Participants trained 4 days a week for a duration of 30 minutes per day. Untrained subjects improved VO2max after 6 weeks of outdoor running (post-LBT = 42.9 ± 3.2 ml/kg/min) only to have these gains return to pre-training baseline values after 4 weeks of DWR (pre-training = 39.9 ± 3.6, post-DWR 40.0 ± 1.8 ml/kg/min). Similar to Eyestone et al. (1993), exercise training intensity during the DWR training protocol may have also effected the outcome of this study. Unlike the land-based training protocol which varied exercise intensity from 60-80% heart rate reserve (HRR), the DWR program employed only steady state exercise at one intensity (80% of HHR within 10 bpm). Since acute heart rate responses are decreased in water due to hydrostatic pressure, this steady rate intensity may not have been adequate to maintain VO2max. Upon completion of the project, the authors indicated the importance of adding interval, varying tempo and fartlek workouts to the DWR training routine. This suggests that there may be a critical intensity or threshold which must be achieved if VO2max is to be maintained or improved through DWR. The frequency and duration spent training at this critical threshold is yet to be elucidated.

Morrow, Jensen & Peace (1996) divided 11 subjects into either DWR (female = 3, males = 3) or land-based (female = 2, male = 3) exercise groups. Subjects trained three days a week for 35 minutes a session at 80% of HRmax as determined by mode specific VO2max tests. Additionally, subjects performed a timed 2.4-k run. Both training groups significantly improved in VO2max (p < 0.01). DWR training also decreased run time (p = 0.06). No mode specific differences between the two training methods (land vs. water) were observed indicating that DWR can improve VO2max in a similar fashion as land-based exercise.

Michaud and colleagues (1995) had 10 inactive volunteers (female = 8; males = 2; mean = 32 yr) complete maximal treadmill and DWR tests prior to and following an 8-week aerobic interval DWR program. Improvements in VO2max of 10.7% and 20.1% for treadmill and DWR, respectively were observed after DWR training. Recruits exercised 3 times per week with workouts ranging from 25-45 minutes a session. Interval length varied from 30 seconds to 7 minutes in duration, with exercise heart rates averaging 63% to 83% of maximal treadmill heart rate. Michaud and associates propose the large increases resulted from a combination of the high intensity workouts, unfit subjects, and the specificity of training and testing involved in the study. By measuring pre-and post-training VO2max while DWR a specificity of testing and training was clearly established. Furthermore, this research also supports a significant crossover effect of DWR to land-based training in untrained volunteers. The results of these training studies support the use of DWR as an alternative form of exercise to land-based training for maintenance of aerobic capacity in trained athletes as well as possible VO2max improvements for unfit participants.

Table 1. Effects of Deep Water Running on VO2max
1st Author of Study # Subjects Length of DWR Training Status Pre—Post VO2max
(ml/kg/min) %Change in VO2max
Bushman
1997 11 subjects
(10M, 1F) 4 wks Trained 63.4±1.3—62.2±1.3 Decrease 1.8% NS

Eyestone
1993 32M 6 wks Trained 56.3±1.5—53.5±1.6 Decrease 4.9% SIG

Michaud
1995 10 subjects
(2M, 8F) 8 wks Untrained 29.3±.8.2—32.8±8.0 Increase 10.6% SIG

Quinn
1994 7F 4 wks Untrained 42.9±3.2—40.0±1.8* Decrease 6.75% SIG

Wilber
1996 16M 6 wks Trained Treadmill
58.4±2.3—60.1±3.6
DWR
58.7±4.7—59.6±5.4 Treadmill
Increase 3% NS
DWR
Increase 1.5% NS

NS=not significant
SIG=significant
*Represents VO2max after land-based training but prior to DWR
Note: Table includes studies reporting pre- and post-training VO2max data

Maximal Oxygen Consumption—Shallow Water Aerobics

Research investigating the chronic effects of shallow water aerobic exercise on VO2max improvements has been favorable. Various types of aquatic exercise have been tested including aqua aerobics, aqua step, shallow water running, and deep water aerobic training. However, regardless of training mode, relative improvements have ranged from 5.6% to 18.9% with only a single study reporting a small, non-significant decrease (0.82%) in aerobic capacity (see Table 2). These positive changes in maximal oxygen consumption following aqua training match the improvements attained in chronic land-based exercise. One project conducted by Hoeger et al. (1992) directly examined the training effects of an identical aerobics program performed on land (low-impact) and in the water. Forty-nine untrained female subjects (water n = 20; land n= 15; control n = 14) participated in the 8-week study with the experimental groups exercising 3 times per week. The aerobic portion of the training session was 20 minutes in duration with exercise intensity maintained between 70-85% of HRR. Both the land-based (low-impact) and shallow water aerobics groups made similar gains in aerobic fitness, with a 14.8% relative improvement in estimated VO2max using a Bruce protocol (pre = 31 ± 6.8, post = 35.6 ± 7.0 ml/kg/min) observed in the shallow water aerobics group. Total treadmill time was also significantly increased (by one minute) following shallow water training. In agreement with Hoeger et al., a smaller yet significant 5.6% increase in VO2max (34.8 ± 4.1 to 36.7 ± 5.2 ml/kg/min) and an improved run time to exhaustion (pre = 15.8 ± 3.7 min, post = 19.4 ± 5.0 min) was also observed by Abraham (1994) following eleven weeks of shallow water aerobics.

As with other aerobic exercise modes, cardiovascular benefits of aqua training are not restricted by age. Twenty female (40 ± 13.99 yr) volunteers were divided into young (28 ± 6.5 yr) and older (52 ± 8.3 yr) adults to assess the effects of age on improvements in cardiovascular fitness following an shallow water aerobics program (Sanders, 1993) . Training included 8 weeks of shallow and deep water workouts using aquatic exercise equipment. Heart rates were maintained between 74-84% of predicted heart rate maximum during exercise. Post-training submaximal treadmill tests (Astrand-Rhyming) revealed increases in aerobic capacity of 13.7% and 8.8% for both the young and old, respectively. The authors suggest that an initial lack of muscular strength may have been responsible for the differences observed in younger and older subjects. Nevertheless, these findings support the use of shallow water exercise for cardiovascular improvements in the aging population.

Simpson and Lemon (1995) examined the effects of a chronic deep water aerobic training program. Eighteen adult females (22-39 years old) were buoyed in the water either by using foam waist belts or ankle cuffs. All subjects trained a minimum of 3 days per week for 50 minutes per session using various aqua aerobics movements. The exercise program in deep water significantly improved estimated VO2max (pre = 29.5 ± 1.8, post = 35.1 ± 1.9 ml/kg/min) in both groups as assessed from an Astrand-Rhyming submaximal treadmill test.

One new area under investigation is aqua stepping which adapts bench stepping on land to the water. The resistive properties of water makes aqua stepping very challenging in both phases of the step movement compared to land-based stepping where the down phase is aided by gravity. Currently, only two studies have been conducted determining the effects of aqua step on gains in VO2max. Gaspard et al. (1995) found that a 7-week aqua step program produced a 7% relative improvement in VO2max. In contrast, Seefeldt and Abraham (1996) observed a small non-significant decrease (0.82%) after 11 weeks of aqua step training with 24 inactive college females. Both training programs were similar with the most important difference being the intensity of exercise. Seefeldt and Abraham reported average ratings of perceived exertion (RPE) during training of 11. 8 (fairly light). This was reflected by average training heart rates of 69.5% of HRmax, which were very low in comparison with the heart rates reported by Gaspard et al. (80% HRmax). Additionally, these higher heart rates may partially be attributed to the music tempo employed in each study. Music tempos of 125-147 bpm and 80-120 bpm were used during workout sessions by Gaspard et al. and Seefeldt et al., respectively. The higher exercise intensity conceivably provided the training stimulus necessary for VO2 improvements achieved by Gaspard’s subjects.

One unique project conducted by Hamer and Morton (1990) assessed the chronic adaptations incurred from shallow water running in 1 meter deep water. Untrained subjects used a high knee running technique immersing the forearms and hands in the water. A 9% increase in VO2max (pre = 49.32 ± 5.42, post = 53.98 ± 4.83 ml/kg/min) resulted after 8 weeks of water running.

Table 2. Effects of Shallow Water Aerobics Exercise on VO2max

1st Author of Study # Subjects Length of DWR Training Status & Type of Training Pre—Post VO2max
(ml/kg/min) %Change in VO2max
Abraham
1994 14F 11 wks Untrained
Water Aerobics 34.8±4.1—36.7±5.2 Increase 5.6% SIG

Gaspard
1995 21F 7 wks Untrained
Aqua Step 39.9±5.5—42.7±5.8 Increase 7% SIG

Hoeger
1992 49F 8 wks Untrained
Water Aerobics 31.0±6.8—35.6±7.0 Increase 14.8% SIG

Sanders
1993 20F 8 wks Untrained
Shallow & DW Aerobics Young participants
32.3±2.7—36.7±2.5
Older participants
25.0±2.8—27.2±2.5 Young
Increase 13.7% SIG
Older
Increase 8.8% SIG

Seefelt
1996 22F 11 wks Untrained
Aqua Step 35.4±6.6—35.2±9.2 Decrease .82% NS

Simpson
1995 18F Untrained
Aqua Aerobics in DW 29.5±1.8—35.1±1.9 Increase 18.9% SIG

NS=not significant
SIG=significant

Oxygen Consumption Summary
Of the five articles testing the effects of DWR training on land-based (treadmill) VO2max improvements, three found decreases in VO2max after DWR training with two of these studies recording significant improvements (Table 1). However, it should be noted that these decreases possibly were the result of improper manipulation of training variables (frequency, intensity, and duration). Both Eyestone et al. (1993) and Quinn et al. (1994) trained subjects using the minimum ACSM guidelines which might explain the small but significant decreases in VO2max. The changes in VO2max after DWR training range from a 6.75% relative decrease (Quinn et al.) to a 10.6% relative increase in a study by Michaud et al. (1995) . In spite of the current conflicting research, DWR still remains an attractive option compared to the 14-16% decrements in aerobic capacity seen when exercise training ceases due to injury or layoffs.

Table 2 describes the effects of shallow water exercise on maximal oxygen consumption. Results of these studies indicate shallow water exercise not only leads to significant improvements in VO2 but that the cardiovascular benefits are similar to those achieved following chronic land-based exercise. The only reported training study (Seefeldt & Abraham, 1996) which found non-significant decreases in maximal oxygen consumption could be traced to an inadequate physiological training stimulus. The subjects exercised at a training intensity that elicited heart rates averaging only 69.5% of HRmax. This intensity, especially for the population of young college-aged females, incorporated by Seefeldt and Abraham may not have been sufficient to achieve gains in maximal oxygen consumption.
Currently, there is a need for more research testing the chronic physiological responses to water exercise. Better control measures as well as standardized training protocols are recommended.

Heart Rate
The Heart Rate Issue in Aquatic Exercise
It is important to discuss the heart rate response to the aquatic medium. It has been shown that the heart rate response in water depends considerably on water temperature (Avellini, Shapiro, & Pandolf, 1983) . Head-out, underwater exercise at 25&Mac251;C (77°F) has been shown to produce a lower heart rate response than land, at a set oxygen consumption. Increasing the water temperature to 30-35&Mac251;C (86°-95°F) shows little difference from land-based heart rate response (Craig & Dvorak, 1969).

In addition, the hydrostatic effects of water cause a shift of blood volume from the periphery of the body to the thorax (Arborelius, Balldin, Lilja, & Lundgren, 1972) . This increases the central venous pressure, stroke volume and cardiac output, which leads to a decrease in heart rate. This is evidenced in water that is at chest level. The combined influence of water temperature and hydrostatic pressure help to explain why, at a given VO2, heart rate has been shown to be up to 20 bpm lower in water than on land (Mougios & Deligiannis, 1993).

Resting Heart Rate
One physiological adaptation to regular cardiovascular exercise is a reduction in heart rate at rest. However, little research has been conducted to support this decrease following shallow or deep water exercise. Currently, two research projects have observed reductions in resting heart rate after chronic shallow water exercise training (Hoeger et al., 1992; Simpson & Lemon, 1995) . Simpson and Lemon found resting heart rates were reduced by 11 bpm (before = 77.7 ± 2.4 bpm; after = 66.3 ± 1.7 bpm) (p < 0.01) upon completion of an 8-week deep water exercise training program. An 8-week training study by Hoeger et al. compared the heart rate responses following an identical shallow water and land-based (low-impact) aerobics program. Both shallow water and land-based aerobics programs led to similar decreases in resting heart rate (water pre = 77 ± 9.3 bpm, post = 70 ± 7.5 bpm, land = 76 ± 10.8 bpm, post = 70 ± 7.7 bpm). These research projects confirm the idea that sedentary individuals may attain decreases in resting heart rate which average approximately one bpm for each week of training during the initial weeks of exercise training (Wilmore & Costill, 1994) .

Changes in resting heart rate through DWR has yet to become an important training variable of study. This is because the subject population for the majority of the current investigations has been aerobically trained athletes with the focus on performance training outcomes. Additionally, through land-based training, these fit subjects have already achieved very low resting heart rates.

Submaximal Heart Rate
Training intensity is a critical factor essential for the development of cardiovascular fitness in land-based exercise. During aqua exercise, intensity appears to be the most important consideration for improvements in cardiovascular conditioning primarily because of the differing circulatory responses to water immersion compared with land-based training. Many of the studies reporting non-significant decreases in VO2max following aquatic exercise may have occurred as a direct result of underestimating submaximal heart rates during water immersion training (Eyestone et al., 1993; Quinn et al., 1994; Seefeldt & Abraham, 1996) . Conversely, studies achieving submaximal heart rates which reached the upper end of the target zone (70-85% of heart rate maximum) during training produced gains in cardiovascular fitness. This higher training intensity may be necessary to nullify the effects of hydrostatic pressure which dampens heart rates when water levels are at or above the chest level. This also implies that the ACSM guidelines for improvement of cardiovascular fitness may need to be adapted for aquatic training, since current standards prescribe only for land-based exercise.

Chronic exercise training seems to have little effect on the hydrostatic pressures exerted by water on the cardiovascular system. Therefore, it seems logical that chronic changes in heart rate responses would follow a similar trend as acute heart rate responses with submaximal DWR training producing heart rates which are approximately 10-15 beats lower than those attained during treadmill running at matched intensities. As of yet research has not compared post training submaximal DWR heart rates to submaximal heart rates of treadmill exercise using sedentary subjects. For endurance trained individuals, submaximal heart rate response during DWR is notably reduced compared to treadmill responses. Wilber et al. (1996) reported average heart rates that were 14% lower during DWR compared to treadmill exercise in endurance trained runners. Similarly, after 4 weeks of training Hertler et al. (1992) documented significantly lower submaximal heart rates for experienced runners during DWR (123.5 ± 20.1 bpm) compared to the treadmill running (169.5 ± 10.9 bpm), while training at identical RPE’s.

Additionally, there are conflicting reports as to the cross-over effects of water running to treadmill exercise. Bushman et al. (1997) determined that four weeks of DWR had no impact on post-training submaximal treadmill heart rate responses when compared to pre-training submaximal values (pre = 158 ± 5.0 bpm; post = 158 ± 4.4 bpm). In contrast, after training subjects in shallow water, Hamer and Morton (1990) observed lower heart rates on 5 different submaximal treadmill workloads. In shallow water running 1 meter deep (WR) Hamer and Morton found heart rates of sedentary subjects to be 10-12 bpm lower during submaximal water running compared to treadmill running. Interestingly, Hamer and Morton noted that as the intensity of exercise increased towards VO2max, the disparity of heart rate response between the two modes of exercise were diminished to within only a 5 bpm difference (50% VO2max: WR 122 ± 8, TM 134 ± 10 bpm; 90% VO2max: WR 168 ± 11 bpm, TM 173 ± 8 bpm).

Maximal Heart Rate

Chronic land-based exercise training has little if any effect on maximal heart rate (MHR). In aquatic exercise the physiological changes due to hydrostatic pressure and temperature of water warrants comparisons between maximal heart rate responses in water versus land-based exercise. In aerobically trained athletes, no differences in HRmax were observed during maximal DWR (193.9 ± 8.8 bpm) and treadmill (192.1 ± 10.2 bpm) tests before or after 6 weeks of DWR training (Wilber et al., 1996) . Quinn et al. (1994) also found maximal heart rates to be similar for DWR (192 ± 6 bpm) and treadmill (190 ± 6 bpm) exercise in sedentary females after a 4 week exercise regimen. These results for inactive subjects were not supported by Michaud et al. (1995) who found DWR elicited maximal heart rates that were approximately 15 beats lower compared with treadmill prior to and following DWR training (DWR pre = 172 ± 16.7; post = 175 ± 13.9 bpm; treadmill pre = 187 ± 11.9; post = 189 ± 11.2 bpm). These differences in HRmax during DWR result from several factors including altered muscle recruitment patterns, more upper body involvement, hydrostatic pressure, and water temperature.

Hamer and Morton (1990) noticed differences in HRmax only in pre-training measures while running in shallow water compared to the treadmill. Inactive females achieved MHR values significantly lower while running in shallow water as opposed to the treadmill (WR = 194 ± 9, TM 198 ± 6 bpm). This indicates familiarity of DWR may also play a role in an individual’s ability to perform a successful maximal test. Gaspard et al. (1995) observed no change in MHR on pre or post training treadmill tests after 7 weeks of aquatic stepping.

Heart Rate Summary
The heart rate response to water exercise is based primarily on the depth of the water, water temperature and intensity of the workout. The effects of water exercise on resting heart rate have been reported by two investigators finding that heart rate at rest does decrease as a result of chronic water aerobics exercise. Hoeger et al. (1992) and Simpson and Lemon (1995) found reductions in resting heart rate of 7 bpm and 11 bpm, respectively. These values are similar to those reported after land-based training.

A comparison of submaximal heart rate of DWR versus treadmill exercise finds that on average heart rates are 3-15% lower in water compared to the treadmill. For trained individuals, water immersion to the neck lowers heart rates to a greater extent at submaixmal intensities than it does to untrained subjects. Hertler et al. (1992) found a 36% relative decrease in submaximal heart during DWR versus treadmill exercise while at the same intensity.

In general there have been little changes in maximal heart rate on a treadmill after aquatic exercise training. Investigators have found no difference in maximal heart rate (Bushman et al., 1997; Gaspard et al., 1995; Hamer & Morton, 1990; Quinn et al., 1994) , an increase (Wilber et al., 1996) , and a decrease in maximal heart rate (Michaud et al., 1995) after a DWR training protocol.

Body Composition
The positive effects of habitual land-based exercise training on body composition are supported in the literature with the most important benefit being the reduction of percent body fat. Research efforts to substantiate similar body fat changes following shallow and deep water training has provided varying results. One major reason for the discrepancy is that the majority of aquatic training studies are of short duration, ranging in length from 4-11 weeks. Investigators agree that a minimum of 8 weeks are necessary for training effects to occur in most physiological variables. This may be particularly true for adaptations in body composition since dietary intake also plays a major role. No dietary restrictions or considerations were given to subjects involved in any of the training studies reviewed. Additionally, several studies conducted on DWR have utilized endurance trained athletes. Significant changes in body fat are not expected for these highly trained subjects who commonly have very lean physiques. One such study performed by Wilber et al. (1996) measured body fat by hydrostatic weighing prior to and following 6 weeks of DWR training with 16 endurance runners (VO2max = 58.6 ± 3.6 ml/kg/min). DWR training closely replicated on-land training with exercise sessions being conducted 5 days per week. A 3.6% increase in percent body fat was observed after the training regimen in these fit runners.

Quinn and colleagues (1994) had untrained females perform 6 weeks of land-based running prior to 4 weeks of DWR. An initial 6.7% decrease in body fat was noted following the 6 weeks of land-based training. However, after 4 weeks of DWR training, body fat increased by 2.1% (pre-training = 24.6 ± 3.5 %, post-land-based running = 22.9 ± 4.2%, post-DWR = 23.4 ± 4.3%). In contrast to Quinn et al., Michaud and associates (Michaud et al., 1995) found that 8 weeks of DWR training in 10 healthy untrained subjects provided an adequate stimulus for body fat reduction. Subjects exercised 3 days a week for 8 weeks at an intensity between 63-83% of HRmax. Post-testing via skinfold measurements found a 2.6% decrease in body fat.

Simpson and Lemon (1995) used bioelectrical impedance to assess body fat percentage finding a 2.7% relative decrease in percent fat after 8 weeks of deep water aerobic exercise. Gaspard et al. (1995) used hydrostatic weighing before and after 7 weeks of aqua step training to assess body composition in 21 untrained college aged females. Results showed no significant differences within the experimental group or when compared to controls.

Significant decreases in body fat have been observed in several studies conducted on shallow water aerobics. Training 3 days per week for 50 min a session, an 11-week training program completed by sedentary college-aged women (pre = 24.2 ± 3.3 kg, post = 22.8 ± 3.0 kg) produced a 5.6% relative decrease in body fat (Abraham et al., 1994) . In agreement with Abraham, Hoeger et al. (1992) reported decreases in body fat with previously sedentary women exercising 3 days a week, 20 minutes a day at 70-85% of heart rate reserve for 8 weeks. A 7.5% change in percent fat (pre = 26. 4 ± 7.4%, post = 24.4 ± 6.7%) as measured by skinfold thickness were similar to the 5% decreases seen in the land-based low-impact aerobics group (pre-test = 21.8 ± 5.0%, post-test = 20.7 ± 4.5%;). No changes occurred in the control group.
Sanders and Rippee (1994) examined the effects of shallow and deep water exercise on body fat of young (28 ± 6.5 yr) and older (52 ± 8.3 yr) women. All subjects achieved significant reductions in body fat following the 8-week community based program with decreases of 11.9% for the young and 5.8% for the older participants.

Table 3. Effects of Aquatic Exercise on Body Composition
1st Author of Study # Subjects &
Fitness Status Length of DWR Type of Training Pre—Post Body Composition %BF %Change in
Body Comp
Abraham
1994 14F
Untrained 11 wks Aqua Aerobics 24.2±3.3—22.8±3.0 -5.8%

Hoger
1992 49F
Untrained 8 wks Aqua
Aerobics 26.4±7.4—24.4±6.7 -7.5%

Michaud
1995 8F, 2M
Untrained 8 wks DWR 30.0±7.4—29.2±7.2 -2.6%

Quinn
1994 7F
Untrained 4 wks DWR 22.9±4.2—23.4±4.3 +2.1%

Sanders
1994 20F
Untrained 8 wks Shallow & DWR 25.1±1.2—22.1±1.1*
25.7±1.7—24.2±1.5† -11.9%
-5.8%

Seefeldt
1996 24F
Untrained 11 wks Aqua
Step 21.1±6.8—22.1±1.2 +4.7%

Simpson
1995
18F
Untrained 8 wks DWR 35.9±2.0—36.4±1.9 +3.6%

Wilber
1996 16M
Trained
6 wks DWR 13.8±4.5—14.3±4.7 +3.6%

*Young subjects: 28 ± 6.5 yr
†Older subjects: 52 ± 8.3 yr

Body Composition Summary
Similar to land-based research, the effects of chronic aqua training on changes in body composition vary. Research findings range from a 4.7% increase to a 11.9% decrease in body fat in studies lasting 4-11 weeks. A trend of body fat decreases were observed in training programs lasting 8 weeks or greater. Four of the six studies recorded decreases in body fat in training protocols of this duration (Abraham et al., 1994; Hoeger et al., 1992; Michaud et al., 1995; Sanders, 1993) . This supports the opinion that training effects are achieved in studies 8 weeks or greater in length. Furthermore, experiments without dietary control will necessitate lengthier exercise training before notable changes in body fat are attained.

Muscular Strength and Endurance
Several researchers have examined the effects of water exercise on muscular fitness. Shallow water aerobic exercise holds promise as a means of enhancing muscular strength in untrained participants. After 8 weeks, untrained females (26 ± 5.9 yr) who performed shallow water aerobics exercise achieved significantly greater gains in several strength measures such as left knee flexion at 300 degrees/s-1, left shoulder extension at 60, 180, 300 degrees/s-1, left shoulder flexion at 60 degrees/s-1 and right shoulder extension at 60, 180, 300 degrees/s-1 compared with the control group (Hoeger et al., 1992) . In agreement, Sanders and Rippee (1994) found improvements in both strength and endurance in young (28 ± 6.3 yr) and older (52 ± 8.3 yr) women. Subjects participated in an 8-week community based program with exercise sessions being a combination of shallow and deep water aerobic exercise using aquatic equipment. Bench press and curl-ups were evaluated pre- and post-training to assess muscular fitness. Although no specific muscular strength and endurance exercises were incorporated into the aqua aerobics routine, increases in these muscular fitness parameters occurred. In the bench press, an astonishing 136% improvement (pre = 14.2 ± 1.95 kg, post = 33.6 ± 4.91) was observed in the young while the older subjects improved their lift by 180% (pre = 6.8 ± 7.4 kg, post = 19.1 ± 2.9 kg). Similarly, muscular endurance was also enhanced in both groups as determined by the one minute curl-up (Y’s Way to Fitness) test. The younger participants improved curl-up pre-test scores of 19.1 ± 2.9 repetitions to 34.89 ± 2.86 on the post-test, while the older subjects significantly increased the number of successful curl-ups completed from an initial score of 1.75 ± 1.08 to 13.25 ± 4.25 on the post evaluation. Based on these results, both young and old improved muscular fitness through shallow water training. However, it is noted that the dramatic increases in musculoskeletal fitness observed in these studies can be partially credited to the initially low level of fitness of the subjects.

Simpson has also found strength benefits from aerobic exercise in deep water. After 8 weeks of training subjects (n = 18) improved isokinetic quadriceps (pre = 50.5 ± 3.2 Nm, post = 55.2 ± 3.3 Nm) and hamstring (pre = 35.4 ± 2.4 Nm, post = 41.9 ± 3.0 Nm) strength (p < 0.01).
Hamer and Morton (1990) tested changes in musculoskeletal parameters before and following 8 weeks of shallow water running (1 meter in depth). An interval training program was conducted 3 days per week for 20-45 minutes per workout. Testing employed isokinetic resistance (Cybex II dynamometer) equipment to measure peak power, and initial and final peak torque during repeated maximal contractions of knee extensors and flexors. Subjects completed a muscular fatigue test which consisted of 50 maximal contractions at 120 degrees/s-1 in a 2 minute period. Differences occurred between the training group and controls in final mean peak torque (final three contractions) for knee extensors (experimental group = 98 Nm; controls = 85 Nm).

Hertler et al. (1992) found that experienced runners were capable of maintaining leg strength through DWR. Researchers had runners complete a 4-week land-based running program prior to dividing the subjects in half with participants either continuing the land-based training or engaging in a deep water running. Isokinetic testing measured concentric and eccentric contraction of upper and lower leg and found no difference in leg strength between DWR and land-based running after training.

Muscular Strength and Endurance Summary
All of the above projects testing either strength or endurance found improvements in some aspect of muscular fitness. This indicates that the resistive properties of water possibly facilitate the development of muscular strength and endurance in inactive participants while maintaining leg strength of competitive athletes. The findings are especially promising for older adults. More research in this area is necessary before solid conclusions can be drawn concerning this aspect of health-related fitness.

Flexibility
Only four training studies have addressed this often overlooked aspect of health-related fitness. The primary test used by all of the investigators to test flexibility was the sit-and-reach test, which evaluates low back and hamstring flexibility. Hoeger et al. (1992) observed a 10.5% improvement in the modified sit-and-reach measurements (pre = 37.9 ± 7.6 cm; post = 41.9 ± 8.9 cm) following 8 weeks of shallow water aerobics. This was not surprising to the researchers since low back and hamstring exercises were integrated into the stretching and warm-up phase of the program. Seedfeldt and Abraham (1996) also incorporated flexibility exercises into the total conditioning program during 11 weeks of aqua step training. Twenty-two subjects were assessed on the sit-and-reach test with 5.4% improvement being achieved, which approached significance (pre = 18.4 ± 3.4 in, post = 19.4 ± 5.4 in).

Findings do not seem to be limited to shallow water aerobics. Simpson and Lemon (1995) also noted a 7.3% increase, although not significant (p > 0.07), in the sit-and-reach flexibility test (pre = 34.1 ± 2.1 cm, post = 36.6 ± 1.8 cm) after 8 weeks of deep water aerobic training. Sanders and Rippee (1994) combined both shallow and deep water exercise during 8 weeks of water aerobics. As part of the experiment subjects were separated into young (28 ± 6.5 yr) and older (52 ± 8.3 yr) adults. Results revealed small, non-significant, improvements in the sit-and-reach for the younger (pre = 15.9 in, post = 16.0 in) and older (pre = 12.8 in, post = 13.5 in) subjects.

Flexibility Summary
Current studies, although small in number support the improvement of flexibility through shallow and deep water exercise. Exercise participants are able to use the buoyant properties of water to decrease joint stress while gaining flexibility. Additional research needs to be conducted using various flexibility tests as well as training regimens of increased length.

Part II: Shallow and Deep Water Exercise Responses
Treadmill running is considered the ‘gold standard’ exercise modality to which all other modalities are compared. Studies comparing treadmill to other modalities such as cycling, simulated cross-country skiing, rowing, and stepping have shown treadmill running to elicit the highest energy expenditure and oxygen consumption (Thomas, Ziogas, Smith, Zhang, & Londeree, 1995; Zeni, Hoffman, & Clifford, 1996) . It therefore can be assumed that water exercise comparisons to treadmill running will have similar findings. However, the true relationship of water exercise to treadmill running (and other forms of land exercise) can only be determined through experimental research. Knowledge of the acute physiological responses of aquatic exercise programs helps the applied professional make correct decisions on safe and effective programming for participants. Part II of this aquatic review will summarize the responses to shallow and deep water exercise.
Comparisons of Submaximal Land and Water Exercise in Waist-To-Chest Deep Water

A pioneer aquatic investigation examined the oxygen consumption and heart rate responses of walking and jogging in waist deep water and on land with six males (21 - 42 yr) (Blanche, Evans, Cureton, & Purvis, 1978) . Water temperature was 30 degrees C (86°F) to 31 degrees C (88°F). In waist deep water, walking and jogging produced similar heart rate responses to land while oxygen consumption was higher in water. It was concluded that the water resistance in waist deep water while walking and jogging results in high levels of energy expenditure with relatively little strain on the lower extremities.

Hered et al. (1997) compared aquatic exercise using the arms and legs, and legs only, on land and in chest deep water at four different intensity levels with 12 females (mean = 20 yr). Results indicated that heart rates were lower in water than on land while oxygen consumption at 2 of the 4 intensities were significantly higher in water. Subjects incorporating both the arms and legs had the highest heart rates regardless of the environment (land or water). This study substantiates that adding the arms to leg exercise in chest deep water increases the energy expenditure cost of the aquatic activity.

One investigation studied the effect of walking on land and in water (at a matched cadence of 103 bpm), with and without an external elastic resistance belt, in ten male and eight female college-aged participants (Robert, Jones, & Bobo, 1996) . The elastic belt (tubing) allowed for more resistance to be applied to the arms and shoulders during exercise. Water temperature ranged from 22.2 degrees C (72°F) to 25.6 degrees C (78°F). Treadmill walking had significantly higher oxygen consumption and kilocalorie expenditure than matched exercise in chest-deep water. The resistance belt was not of sufficient magnitude to affect the oxygen cost or caloric cost of the exercise on land and in water.
Comparison of Aerobic Exercise on Land to Water

In a comparison of identical aerobic exercise routines on land and in water with ten female subjects (mean = 43 yr), land exercise produced significantly higher oxygen consumption results (Heberlein, Perez, Wygand, & Connor, 1987) . However, the cardiovascular stimulus for the hydroaerobics program was within ACSM guidelines for the improvement of cardiovascular endurance. Having the subjects perform the same exact aerobic exercise routines on land and water may have impaired the participant responses due to the varying effects of water density (800 times greater) compared to land.

Cassady and Nielsen (1992) evaluated heart rate and oxygen consumption of 40 subjects (20 males, 20 females, mean = 25 yr) performing upper extremity and lower extremity exercise on land and in water, at three different cadences. The oxygen consumption responses were greatest during water exercise, whereas heart rate, expressed as a percent of age-predicted heart rate maximum was highest on land, attributable in part to the hydrostatic pressure of water.
Maximal Intensity Land and Water Exercise Comparisons in Chest Deep Water

One investigation compared maximal oxygen consumption (VO2max), maximum heart rate (HRmax) and ratings of perceived exertion (RPE) of treadmill running to aquatic exercise (in chest deep water) with 19 males and 11 females (Hoeger, Hopkins, Barber, & Gibson, 1992) . The aquatic exercise consisted of arm and leg work which was gradually increased by speeding up the movement to attain maximal work output. Maximal treadmill exercise elicited a significantly higher response in VO2max, HRmax and RPE. This is not surprising since treadmill exercise has been shown to produce higher VO2max values when compared to other modalities (Thomas et al., 1995) .
Comparison of Bench Stepping on Land and in Water

Evans and Cureton (1996) compared oxygen consumption, heart rate and perceptual response of bench stepping on land and in chest-deep water. Ten women completed 5-minute trials of aqua bench stepping (29 steps/minutes) at three different bench heights (0, 7 in, 12.5 in) using a traditional stepping pattern and an arms and legs stepping pattern (water only). Water temperature varied between 29 degrees C (84°F) and 32 degrees C (90°F). Heart rates and oxygen consumption were lower in the water, although the perceived exertion response was very similar for stepping in water and on land. The added use of arms to legs increased oxygen consumption demands of the movement to 48%, 58%, and 78% of VO2peak, for the step heights 0, 7 in, and 12.5 in, respectively. Thus, bench stepping with the use of the arms in water meets ACSM guidelines for the improvement of aerobic capacity (50% to 85% VO2max).

Heart Rate and Oxygen Consumption of Shallow Water Exercise
Eckerson and Anderson (1992) explored the energy expenditure of shallow water aquatic exercise. In approximately 1 meter of water, 16 college females (20 yr) performed shallow water exercise routines. Maximal metabolic and cardiovascular data for the subjects was also obtained from land tests on a treadmill. When compared to treadmill effort, shallow water exercise resulted in mean heart rate responses that were 74% of heart rate reserve and 82% of HRmax, while VO2 was 48% of VO2max (minimally meeting ACSM guidelines). Subjects burned an average of 5.7 kilocalories per minute during the aquatic exercises.

Another investigation studied the effects of rhythmic aquatic calisthenics (stretching, jogging in place, modified lap swimming, simulated crawling, and treading water) on heart rate and oxygen consumption, at three different intensities (Vickery, Cureton, & Langstaff, 1983) . The researchers found heart rates of 70% to 77% and oxygen uptakes of 51% to 57% (meeting ACSM guidelines) of maximal values. The caloric expenditure ranged from 5.9 to 6.5 kilocalories per minute for the various programs.

Deep Water Running Studies
Treadmill Walking/Running vs. Deep Water Walking/Running
Coad et al. (1987) studied the energy costs of treadmill walking and running versus matched speeds of deep water walking and running with 14 subjects. Subjects wore wet vests while exercising in the water. Results indicated that deep water walking required significantly greater metabolic costs than treadmill walking. Deep water running and treadmill running were very similar in energy expenditure.
VO2 (L/min) HR (b/min) Kcal Expenditure
Treadmill walking .850 101 4.0
Treadmill running 2.35 163 11.8
Deep water walking 1.8 130 8.78
Deep water running 2.30 162 11.5

DeMaere et al. (1997) compared five-minutes trials of deep water running to treadmill running at 60% and 80% of VO2peak in eight cross-country runners. Deep water running and treadmill walking at similar intensities resulted in similar energy expenditure values.
VO2 (ml/kg/min) HR (b/min) Kcal Expenditure
Water 60% VO2peak 39.6 143 13.5
Treadmill 60% VO2peak 40.7 143 13.8
Water 80% VO2peak 54.9 172 18.9
Treadmill 80% VO2peak 55.4 173 19.2

Svedenhag and Seger (1992) compared running on land to vest-supported deep water running with 10 trained male runners (26 yr). Subjects ran at heart rates of 115, 130, 145, 155-160 bpm and also exercised to maximal exercise intensity. Maximal oxygen uptake (4.03 vs 4.60 l/min) and maximal heat rate (172 vs 188 bpm) was lower during water running. The authors suggest the lower maximal heart rates may be attributable to an increase in heart blood volumes, while the influence of different test procedures in the water vs. land may partially explain the differences in VO2max. RPE values were higher for deep water running as were the blood lactate concentrations at any given VO2. These responses may be due to a decreased blood flow in the legs during deep water running as well as the altered leg muscle activation patterns of deep water running.

An investigation by Glass, Wilson, Blessing and Miller (1995) compared the maximal physiological costs of deep water running to treadmill running using ten male and ten female subjects (26 yr). Treadmill running produced higher VO2 and heart rate values. However, heart rate was measured by palpation, and water temperatures were reported to be 24°C (75°F), which has been shown to be associated with a lowered exercise heart rate response. Treadmill running elicited higher metabolic training intensities than deep water running when equated for the same level of RPE. The authors suggested that due to the density of water, subjects utilized more anaerobic energy because of the increased challenge to the exercising muscles, and thus had lower VO2 and heart rate values. In addition, the use of the arms and legs against the water resistance contributed to higher lactate levels for deep water running as compared to treadmill running.

Frangolias, Rhodes, and Taunton (1996) compared the cardiovascular responses of maximal deep water running to treadmill running utilizing 22 endurance runners (8 female, 14 males, ages 21 to 35 yr) who were divided into experienced and inexperienced deep water running groups and given maximal exertion tests on the treadmill and in the water. Experienced deep water runners were classified as those doing at least 6 deep water running workouts per month for 6 months prior to the study. Results indicated that the more familiar subjects were with deep water running, the smaller the difference in maximal oxygen uptake values between water and land running. Experienced deep water runners had VO2max values on land and in water that were within 3.8 ml/kg/min whereas the difference in the inexperienced deep water runners was 10.3 ml/kg/min. Underwater video analysis revealed that inexperienced deep water runners were unable to maintain upright positions in the water and more likely to cup the water with their hands, propelling themselves slightly forward. Leg patterns of the inexperienced deep water runners adapted to a shorter stride cycle, similar to a swimming kick motion, which increased the contribution of the upper body. Maximal heart rate results indicated no significant differences in maximal heart rate in land vs. water in the experienced deep water runners. The researchers concluded that the more familiar individuals are with deep water running, the more closely matched the physiological responses of the two exercise mediums.

In another study using experienced deep water runners, Frangolias and Rhodes (1995) found higher maximal metabolic values on land compared to deep water running with 13 distance runners (21-35 yr). Experienced deep water runners were defined as those who incorporated at least 6 DWR workouts per month into their training program for six months prior to the study. Maximal VO2 and heart rate values were approximately 8% lower in water as compared to land. Also, lower ventilatory threshold (which is a marker for the body’s production of lactic acid) values were noted for DWR as compared to treadmill running at the same RPE and respiratory exchange ratio (the ratio of carbon dioxide produced and oxygen consumed) levels. However, when ventilatory threshold was expressed as a percentage of the respective DWR or treadmill VO2 values, there was no statistical difference. This suggests that factors dampening the effect of maximal effort also appear to be factors limiting VO2 at the ventilatory threshold. The authors suggest that the differences observed in maximal values in land versus water are most likely related to hydrostatic responses, gravitational effects, and running styles in the different mediums. It is noted that during exercise in water there is a tendency for breathing frequency to be higher and tidal volume lower in submaximal (80% of VO2max) and maximal exercise (Sheldahl et al., 1987) . This suggests that the cost of breathing in DWR increases and a larger portion of oxygen is consumed by the respiratory muscles during water exercise as compared to land. This may function to limit the oxygen available to the leg muscles. Researchers also reported similar blood lactate responses during submaximal, maximal and recovery periods in land and water. This implies that variations in arm and leg actions (DWR technique) as well as the recruitment patterns in the deep water running that may limit oxygen consumption also contribute to the onset of blood lactate.

Michaud et al. (1995) compared the physiological, perceptual and metabolic responses of peak and moderate intensity deep water running to treadmill running with six trained male runners (mean = 25 yr). Peak oxygen consumption and heart rate were 12% and 8% greater for treadmill running than deep water running At similar relative and absolute exercise intensities, blood lactate and respiratory exchange ratio were significantly greater during deep water running. No significant difference was found in submaximal heart rate responses between trials. Subjects in this study were inexperienced DWR and received only three familiarization trials in deep water running. Water temperature was maintained at approximately 29°C (84°F) to 30°C (86°F). Submaximal trials were 75% of treadmill VO2peak on treadmill (TM 75%), 70% of deep water running VO2peak in water (DW 70%), and 75% of treadmill VO2peak in water (TM 75%-W). Oxygen consumption at 75% of deep water running VO2peak was significantly lower than the other trials. No difference in heart rate occurred between trials. For both blood lactate and respiratory exchange levels, the water responses were significantly higher than land (TM 75%-W > DW 70% > TM 75%). At the same absolute exercise intensity, RPE values were higher in deep water running. The authors suggest that the mechanics of DWR are not as similar to land running as has been suggested by others.

Butts, Tucker and Smith (1991) investigated the maximal responses to treadmill and deep water running in 12 high school female cross country runners (mean = 15 yr). Subjects were taught DWR technique prior to testing, but had no previous experience with this form of training. Peak heart rate and oxygen consumption was higher on the treadmill than in water by 9% and 13%, respectively. The authors suggest the lower DWR metabolic responses may be attributable to a number of factors, including the cooling effect of the water temperature (29°C {84°F}), the hydrostatic forces exerted by water, the low body fat of the subjects (mean = 17.6%), and mechanical differences observed in deep water running due to the buoyancy effect of water. It was concluded that DWR provided numerous rehabilitation and training possibilities for athletes.
Maximal Gender Responses to Treadmill and Deep Water Running

Any investigation comparing maximal physiological responses between women and men is complicated by differences in body composition, physical size, and level of training. The larger percentage of body fat observed in women is a chief contributing factor to the lower cardiorespiratory observations (Pate & Kriska, 1984) . These differences in body composition may also facilitate buoyancy, possibly resulting in a reduced metabolic response in women when compared to men (making the water exercise more economical for women) (Brown, Chitwood, Beason, & McLemore, 1997) .

Butts, Tucker and Greening (1991) compared maximal physiological responses to treadmill running and deep water running in 12 trained men (mean = 20.6 yr) and 12 trained women (mean = 21.9 yr). Subjects were familiarized prior to testing with treadmill and deep water running exercise. Water temperature was 29°C (84°F). Men and women had significantly lower maximal VO2 and heart rate responses in water. The DWR VO2max values in water for men and women were 9% and 16% lower, respectively. The DWR maximal heart rate values in water for men and women were 5% lower. Respiratory exchange ratio was similar in both the water and on land. The authors concluded that the magnitude of these differences in water exercise and treadmill running is not different from that comparing treadmill running to other modalities and in no way precludes deep water running as an effective training technique.

Brown et al. (1997) explored the physiological differences to deep water running and treadmill running and differentiated them by gender with 12 untrained men (mean = 21 yr) and 12 untrained women (mean = 20 yr). This investigation matched running cadences at a wider range of intensities to compare the two modalities. Subjects were familiarized to DWR with at least 2 DWR practice sessions prior to testing. Water temperature averaged 29.6°C (85°F). At all submaximal intensities, with running cadences matched in water and on land, deep water running resulted in higher VO2 responses. The authors concluded that at matched cadences in submaximal exercise, subjects were working harder during DWR. Heart rate was not significantly different between genders on land or in water (although heart rate on the treadmill was 6% and 10% higher than DWR for men and women, respectively). Men had significantly higher VO2max responses compared to women and treadmill VO2max values were 13% and 24% higher for men and women than in deep water running. A very interesting finding of this study was that at matched running cadences, submaximal physiological responses for men and women were higher during DWR as compared to treadmill running.

Submaximal Energy Expenditure
An investigation with 8 male competitive runners (18 to 42 yr) running at a submaximal pace for 30 minutes showed deep water running incurred higher oxygen consumption values, respiratory exchange ratios, and RPE levels than normal treadmill running and road running (Richie &, 1991) . Heart rates were similar in the three experimental conditions. When subjects exercised at a self-selected ‘hard’ pace on the treadmill, metabolic values were higher than in deep water running. It was concluded that submaximal exercise can be sufficiently and effectively completed in deep water.
Differences in Deep Water and Treadmill Running Mechanics

It has been suggested that there is greater involvement of the anaerobic energy system during water exercise because of the additional recruitment of smaller muscle groups (Michaud et al., 1995) . Subjects have reported more fatigue in the arms, shoulders, hips, and legs during DWR, with potentially greater use of the upper body and less use of the lower body (Michaud et al., 1995) . The propulsion mechanics of the muscles in the legs when running are different than water, where the body is suspended and not working against gravity. In deep water running there is no weight-bearing and hence no push-off phase against a hard surface. Therefore, although deep water running mimics running on land, several important factors differentiate the two activities.

Table 4. Kilocalorie Expenditure
The following are some kilocalorie expenditure comparisons of different exercise modalities.
Exercise Mode Kilocalorie Expenditure
Aquatic exercise 5.7 - 6.5 kcal.min-1
Aerobic dance 6.2 - 6.6 kcal.min-1
Circuit training 5.1 - 6.1 kcal.min-1
Step aerobics 6.7 - 7.7 kcal.min-1
Running 11 min mile 8.0 kcal.min-1
Running 9 min mile 11.4 kcal.min-1
Walking normal pace 4.7 kcal.min-1
Deep water walking 8.8 kcal.min-1
Deep water running 11.5 kcal.min-1

Table 5. MET Values Table
Met levels are a unit of measurement frequently used to designate the energy costs of exercise programs. One MET equals 3.5 ml/kg/min. This table will provide MET data for various aquatic exercise programs. Due to variation in fitness levels of subjects and gender, these values are best used as approximations for the aquatic activity.

1st Author of Study & Yr Type of Aquatic Exercise Gender MET Levels
Cassady 1992 Upper extremity only Female 2.9-4.1
Cassady 1992 Upper extremity only Male 3.3-5.7
Cassady 1992 Lower extremity only Female 4.0-7.0
Cassady 1992 Lower extremity only Male 4.6-9.2
Echerson 1992 Aqua exercise in 1-meter of water Female 5.25
Vickery 1983 Waist-to-chest deep aqua calisthenics Female 6.7-8.3
Hered 1996 Chest deep aqua exercise with arms and legs Female 4.8-6.8
Evans 1996 Bench stepping, 7 inch step (no arms/with arms) Female 4.2/7.4
Evans 1996 Bench stepping, 12 inch step (no arms/with arms) Female 6.5/9.9
Kirby 1984 Running in chest deep water Female
& Male 7.1
Heberlein 1987 Aqua exercise in chest deep Female 5.4
Michaud 1995 Deep water running at 76% HRmax Male 11.0
Richie 1991 Deep water running at 83% HRmax Male 13.1

Summary Points
From this review of literature on the cardiovascular and energy expenditure responses to aquatic exercise, the following is a summary of findings:
Adding arms to leg exercise in chest deep water significantly increases the energy cost of the workout. This may equal or exceed matched exercise performed on land.
Water jogging and running in waist-deep water results in equal or even greater cardiovascular responses compared to similar exercise on land.
Aqua exercise routines can meet ACSM guidelines for the improvement of cardiorespiratory endurance. However, the ACSM guidelines for improvement of cardiovascular fitness may need to be adapted for aquatic training, since current standards prescribe only for land- based exercise.
Bench stepping exercise in water, using the arms, meets ACSM guidelines for the improvement of cardiorespiratory endurance.
Water exercise using elastic resistance with the upper body does not significantly increase energy expenditure.
Investigations have found the cardiorespiratory responses of deep water running to be less than, similar, and greater than treadmill running on land.
Blood lactate levels in deep water running have been shown to be higher and lower to land exercise which may reflect variations in arm and leg actions and exercise protocols.
Ratings of perceived exertion for DWR appear to be elevated due to higher blood lactate levels and upper extremity muscular fatigue.
The hydrostatic pressure and altered running style (due to different muscle activity patterns of DWR) contribute to a greater involvement of the anaerobic energy system during deep water running.
There is an increase in breathing frequency and cost of breathing during water exercise which leads to the respiratory muscles consuming more oxygen. This may function to limit the oxygen available for the legs.
The more familiar subjects are to DWR, the smaller the difference between VO2max values between land and water.
Exercise heart rate and oxygen consumption comparisons of teenage females in land and water exercise appear to result in similar responses to those seen in adults.

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