Maximal Physiological Responses between Aquatic and Land Exercise in Overweight Women

Abstract

Purpose: 

To investigate the maximal physiological responses between aquatic and land-based graded exercise tests in overweight women.

Methods: 

Twenty healthy, overweight (body mass index (BMI) ≥ 25 kg·m⁻²), Caucasian women (mean ± SD; age 48 ± 7 yr, BMI 30 ± 4 kg·m⁻²) completed a deep water running (DWR) and treadmill walking (TMW) graded exercise test. Maximal responses during the DWR and TMW graded exercise tests were compared using paired t-tests. Comparisons were made in the incidence of achievement of maximal oxygen consumption (V˙O_2max) criteria between DWR and TMW protocols. Criteria were a plateau in V˙O₂ (change < 2.1 mL·kg·min⁻¹), heart rate (HR) equal to or above the age-adjusted maximum, and respiratory exchange ratio (RER) ≥ 1.15.

Results: 

Maximal responses for V˙O_2max (22.5 ± 4.86 vs 27.7 ± 4.73 mL·kg·min⁻¹), HR_max (159 ± 16 vs 170 ± 12 bpm), and RER (1.03 ± 0.06 vs 1.10 ± 0.06) were significantly lower (P < 0.01) for the DWR test compared with the TMW test, respectively. Achievement of various V˙O_2max criteria was demonstrated more consistently during the TMW test than the DWR test.

Conclusion: 

Maximal physiological responses of overweight women to DWR and TMW are significantly different but are comparable with other populations. As the maximal responses for DWR compared with TMW differ, the use of land-based criteria for V˙O_2max is not recommended for a graded DWR exercise test.

Maximal oxygen consumption (V˙O_2max) is considered the principal measure of cardiovascular fitness (^24,26) and is taken as a function of cardiac output and oxygen extraction from blood, as it reflects an individual's capability for uptake, transport, and use of oxygen (^15,22,24,26). Accurate determination of V˙O_2max during a maximal exercise test is considered advantageous to exercise prescription, as exercise training is frequently undertaken at a specified percentage of V˙O_2max (^20,23).

Maximal oxygen consumption is traditionally defined as the oxygen intake attained during maximal exercise that does not increase with an increase in intensity (¹⁵). Early work by Taylor, Buskirk, and Henschel (²⁶), viewed the absence of an increase in oxygen consumption (V˙O₂), despite an increase in workload measured in a second constant-load protocol following a first constant-load protocol, as the main objective criterion for establishing V˙O_2max.

However, this plateau in V˙O₂ is not always reached in subjects (^9,12), and in the absence of a plateau, secondary criteria are used to indicate that V˙O_2max has been achieved. These include attainment of a specific respiratory exchange ratio (RER), a percentage of age-adjusted maximum heart rate (HR_max), and a certain concentration of blood lactate (⁹). Before accepting values as maximal, it is preferable that two of the four proposed criteria be exceeded (⁹). Additionally, subjective criteria such as a rating of perceived exertion (RPE) (²) can be used to assess the quality of a graded exercise test (¹²). Difficulties arise, however, in determining whether an individual has attained their actual V˙O_2max. There is variability in the cutoff points used for each of the secondary criteria (^11,16), and there are no universal standards for the exact criteria that should be used to establish that a true V˙O_2max has been reached (¹⁶).

There is strong support for the hypothesis that there is a peak and subsequent plateau in V˙O₂ during maximal exercise (^1,14). However, this concept has recently been disputed due to the lack of published data reporting a definitive plateau in V˙O₂ during a maximal exercise test (¹⁴). Some subjects may not be able to demonstrate a plateau in V˙O₂, and some types of exercise are more likely to produce a leveling off in V˙O₂ than others (^9,23,26). Nevertheless, lack of a plateau in V˙O₂ does not mean subjects have not achieved their "true" V˙O_2max (¹²). When an individual reaches his or her maximum tolerable work rate during a maximal exercise test but has not demonstrated a plateau in V˙O₂, he or she has attained peak oxygen consumption (V˙O_2peak) (²⁸). Although V˙O_2peak does not satisfy the definition of V˙O_2max, the V˙O_2peak value can be equivalent to the measured V˙O_2max based on criteria in normal subjects (^10,28).

Maximal oxygen consumption is typically measured during a continuous, multistage exercise test of increasing intensity (²⁰). However, the directly measured V˙O_2max varies with the type of exercise (²³). Therefore, obtaining accurate and valid V˙O_2max values is of physiological importance when different modes of exercise are used (¹⁰). Deep water running (DWR) is an effective form of cardiovascular conditioning for individuals who require a low-impact aerobic workout (²¹). The physiological effects associated with water immersion also cause maximal exercise responses to differ when compared with land-based exercise (^6,18,25). Consequently, mode-specific maximal exercise testing is essential for individuals undertaking DWR.

Protocols to elicit maximal DWR effort have included subjective increases in intensity (²⁵), specific increases in cadence (²⁹), and addition of weight to increase resistance (¹⁷). Many DWR studies are undertaken using trained male runners (^6,13,25), and often the criteria for V˙O_2max are vague or not stated. Attainment of V˙O_2max during DWR is based on criteria developed during maximal exercise tests undertaken on land (^4,17,18). Consequently, there is inconsistency in values for DWR criteria.

The purpose of this study was to investigate the maximal physiological responses to aquatic and land-based graded exercise tests in overweight women. Maximal responses during a treadmill walking (TMW) graded exercise test were compared with maximal responses during a DWR graded exercise test. Comparisons were made in the incidence of achievement of the various V˙O_2max criteria between DWR and TMW protocols. Finally, the validity of the DWR protocol was investigated in this population using maximal responses of heart rate (HR) and V˙O₂ between DWR and TMW graded exercise tests.

METHODS

Participants.

Twenty healthy, overweight (body mass index (BMI) ≥ 25 kg·m⁻²) Caucasian women participated in this study. Participants were recruited by advertisements in the local newspaper and flyers placed on local notice boards. Written informed consent was obtained from each individual and a medical screening and exercise history questionnaire completed. Exclusion criteria included BMI < 25 kg·m⁻², a history of heart disease, diabetes, orthopedic limitations, pulmonary diseases any other health problem that may interfere with exercise testing, and regular use of medication known to affect metabolism. Participants were regarded as recreationally active. All had performed DWR in the past and were undertaking this on average 1 d·wk⁻¹ prior to entrance to the study. This study was approved by the University of Otago human ethics committee.

Experimental protocol.

Height and weight were measured without shoes and in a swimsuit using a stadiometer and electronic scales (Digi D1-10, Teraoka Seiko Ltd, Tokyo, Japan). Body mass index was calculated as weight (kg)/height squared (m²).

Each participant completed a DWR graded exercise test followed by a TMW graded exercise test. A minimum of 24 h separated the tests. The DWR test was conducted in the swimming flume at the School of Physical Education, University of Otago, following the protocol of Mercer and Jensen (¹⁷). The flume was 1.80 m deep, and water was maintained at a constant temperature of 29°C. Participants performed DWR wearing a flotation belt, to which a tether was attached and run through a series of pulleys. A bucket was attached to the end of the tether and suspended in front of the participant at the edge of the pool. Participants were instructed to warm up in the pool and familiarize themselves with the breathing apparatus prior to beginning the test. The protocol was continuous and consisted of 1-min stages. To provide a graded response, a weight of 0.57 kg was placed in the bucket at the beginning of each stage. Participants received strong verbal encouragement throughout the entire session. If the participant was unable to meet the intensity at any given stage, they were pulled back and the bucket dropped, indicating the endpoint of the test (Fig. 1).

FIGURE 1: Illustration of the graded maximal deep water run (DWR) set up from Mercer and Jensen (¹⁷). Participants performed DWR wearing a flotation belt while tethered to a series of pulley attaching to a bucket. To provide a graded response, weight was placed in the bucket at the beginning of each 1-min stage. The end point of the test was when the bucket dropped and touched the plank, indicating that the participant was unable to meet the required exercise demand. Figure adapted from Mercer and Jensen (¹⁷), publisher Taylor and Francis, www.informaworld.com.

The TMW test was also continuous and consisted of 1-min stages. It was conducted in an exercise physiology laboratory at the School of Physical Education, University of Otago, on a motorized treadmill (Quinton Instrument Company, Series 90 Q65) following the Harbor Protocol (²⁸). Participants completed 3 min of warm-up of zero-grade walking at a comfortable speed. This also provided familiarization with the treadmill and breathing equipment. Speed during the TMW test was set and maintained at the individualized comfortable walking speed determined during the warm-up, ranging from 4.1 to 6.0 km·h⁻¹. Grade was increased at a constant, preselected amount in all participants of 1% each minute. Participants received strong verbal encouragement throughout the entire test. The test was terminated when the participant could no longer continue and signaled to stop.

Data collection.

Continuous respiratory gas analysis using indirect calorimetry was undertaken using a Sensormedics 2900 metabolic cart (Sensormedics). Heart rate was recorded throughout both tests using a heart rate monitor (Polar). Gas analyzers were calibrated prior to each test with a known concentration of oxygen and carbon dioxide. Volume was also calibrated prior to each test using a standard 3-L syringe. While undertaking both exercise tests, participants breathed through a two-way mouthpiece, which was securely fitted to their mouth via a custom-made headpiece. The highest V˙O₂ and HR achieved in each test were accepted as V˙O_2max and HR_max respectively, regardless of whether V˙O_2max criteria were met. Maximal RER and ventilatory rate (V˙E_max) were taken as values corresponding with V˙O_2max. Participants were asked to identify their RPE at the end point of each test using the Borg 6- to 20-point scale (²). Prior to the first maximal exercise test, all participants were familiarized with the Borg 6-20 RPE scale. The time taken to reach V˙O_2max was recorded.

Specific criteria used in this investigation to determine V˙O_2max were a plateau in V˙O₂ (change in V˙O₂ < 2.1 mL·kg·min⁻¹ between successive increases in intensity), an HR equal to age-adjusted maximal values, and RER ≥ 1.15 (¹²). These criteria have been described and used previously in land-based maximal exercise tests (^9,12) and were objective criteria for termination of both the DWR and TMW protocols.

Statistical analysis.

Data are presented as group means and standard deviations (SD). Maximal physiological responses between DWR and TMW exercise tests were examined using paired t-tests. Achievement of V˙O_2max criteria are expressed as percentages for DWR and TMW exercise tests. Validity of the DWR protocol was tested by calculating the Pearson's product-moment coefficient of correlation for V˙O_2max and HR_max between DWR and TMW for each individual. Statistical Package for the Social Sciences (SPSS) was used for all statistical analyses (SPSS Inc, v. 14.0, Chicago, IL). Significance was accepted at P < 0.05.

RESULTS

Table 1 represents the physical characteristics of female participants. From the 20 participants, 10 women were premenopausal and 10 women were postmenopausal. The maximal physiological values obtained on each protocol are presented in Table 2. These are the highest values obtained for each variable during the DWR and TMW tests, regardless of whether V˙O_2max criteria were met.

TABLE 1: Physical characteristics of participants (N = 20).

TABLE 2: Maximal group responses during deep water running (DWR) and treadmill walking (TMW) graded exercise tests for overweight women (N = 20).

Maximal relative V˙O₂, HR, RER, and V˙E_max were significantly lower (P < 0.001) for the DWR test compared with the TMW test. The maximal relative V˙O₂ during DWR was 19% lower than during TMW. At maximal work, HR was 6% lower during DWR than during TMW. This corresponds to an HR 11 bpm lower in water compared with land. The amount of time taken to reach V˙O_2max was significantly different (P < 0.001) between TMW and DWR protocols. The duration of the DWR test was less than half the duration of the TMW test. There was no difference (P > 0.05) in RPE between protocols.

The results for the achievement of the various V˙O_2max criteria (¹²) appear in Figure 1. Criterion values for HR, RER, and a plateau in V˙O₂ were achieved more consistently during the TMW test (Fig. 2). Of the total number of participants, 45% were able to achieve two or more of the criteria during the DWR protocol, while 95% of participants were able to achieve two or more of the criteria during the TMW protocol.

FIGURE 2: Proportion of subjects (N = 20) achieving V˙O_2max criteria from Duncan et al. (¹³), during deep water running (DWR) and treadmill walk (TMW) protocols. Criteria were a plateau in V˙O₂ (change < 2.1 mL·kg·min⁻¹), heart rate (HR) equal to or above the age-adjusted maximum, and respiratory exchange ratio (RER) ≥ 1.15 (¹³).

Data demonstrating the validity of the DWR protocol in comparison with the TMW protocol are presented in Figures 3 and 4. A significant positive correlation was evident between DWR and TMW for V˙O_2max (r = 0.70, P < 0.01) (Fig. 3) and HR_max (r = 0.65, P < 0.01) (Fig. 4).

FIGURE 3: Relationship between deep water running (DWR) and treadmill walking (TMW) protocols for maximal oxygen consumption (V˙O_2max, in milliliters per kilogram per minute) in overweight women (N = 20).

FIGURE 4: Relationship between deep water running (DWR) and treadmill walking (TMW) protocols for maximal heart rate (HR_max, in beats per minute) in overweight women (N = 20).

DISCUSSION

The purpose of this study was to investigate the maximal physiological responses to aquatic and land-based graded exercise tests in overweight women. In agreement with other studies comparing maximal responses of DWR to TMW (^{3,4,6,13,17,25,27}), the present study demonstrated significantly lower V˙O_2max during DWR compared with TMW. In this study, V˙O_2max was 19% lower during DWR than TMW. When compared with TMW, young males demonstrate an 8-19% reduction in V˙O_2max during DWR (^4,13,17,25), while in young women this reduction is 21-25% (^4,17). In healthy older women, a 29% lower V˙O_2max during DWR compared with TMW has been observed (³). The result of the present study is comparable with that between young men and women, indicating that in this sample of middle-aged, overweight women, cardiovascular responses are similar to other populations during maximal DWR and TMW. Factors such as the exertional force required to overcome viscosity friction of the water medium, differences in DWR technique, and different muscle-activation patterns contribute to the reduction in relative V˙O₂ during DWR (^5,25). The buoyancy factor associated with exercise undertaken in the water may also contribute to a decreased ability to perform maximal exercise in the water compared with on land. Therefore, the lower V˙O_2max during DWR is suggested to be a consequence of the decrease in maximal mechanical work that can possibly be performed in the water (^4,13).

During water immersion, blood volume is redistributed centrally. This increases venous return, possibly leading to a higher stroke volume and a lower HR during exercise (¹³). However, no clear consensus exists to explain the mechanisms responsible for the reduction in maximal HR during exercise in water compared with land (^{6,13,19,21,25,27}). In the current study, a 6% reduction in maximal HR response during DWR compared with TMW was observed and is comparable with values obtained in previous studies using young males (^4,13,17,25), young women (¹⁷), older women (³), and sedentary subjects (¹⁸). Water temperature was maintained at 29°C to prevent alterations in HR with temperature above or below the thermoneutral range during exercise of 26-29°C (^8,21). Despite the use of different maximal testing protocols between studies, HR_max is consistently lower during DWR than TMW (^17,21).

Respiratory exchange ratio responses at maximal effort were lower for DWR compared with TMW in the current study, which is in agreement with previous research (^3,6,13,25,27). The significantly lower V˙E_max demonstrated during DWR, also demonstrated by others (⁶), may be associated with reduced maximal RER (¹⁶). The lower maximal RER may have reflected reduced relative exercise intensity that can be achieved at maximum effort during DWR, as demonstrated by the decreased maximal V˙O₂ and HR responses. The possible differences in intensity and work rate between DWR and TMW could result in the differences in responses seen in the two modalities. However, the aim of this study was not to assess intensity or work rate in water compared with land; it was to assess maximal responses in water versus land. Therefore, further research into differences in intensity and work rate during exercise in an aquatic environment compared with those on land would be beneficial.

Perceived exertion during maximal DWR and TMW was equivalent for overweight women, supporting results from previous research using the 6-20 Borg scale (^{3,4,6,13,17,25,27}). The RPE value of 17 demonstrated in the current study falls within the RPE of 16-18 that is reliably associated with a true maximal effort (⁷).

Obtaining accurate and valid V˙O_2max values is of physiological importance when different modes of exercise are used (¹⁰). Consequently, because of the significant differences identified for maximal physiological responses between DWR and TMW, difficulty arises when using the same V˙O_2max criteria. Comparisons were made in the incidence of achievement of the various V˙O_2max criteria between DWR and TMW protocols. It was consistently more difficult for overweight women to achieve V˙O_2max criteria during DWR than TMW. Due to a significantly lower HR and RER during maximal DWR compared with TMW, participants were less likely to achieve V˙O_2max criteria established for land-based TMW (¹²). This indicates the need for mode-specific V˙O_2max criteria to be developed and used for DWR. Accordingly, secondary V˙O_2max criteria specific to DWR are needed when determining V˙O_2max values from a maximal DWR test.

The primary criterion of a plateau in V˙O₂ was more commonly demonstrated in both DWR and TMW tests for overweight women in this study compared with secondary criteria. The success of attaining a plateau in V˙O₂ indicates that the DWR and TMW protocols used were successful in eliciting a maximum effort from the individuals tested. However, when participants do not satisfy V˙O_2max criteria but do display effort to volitional exhaustion, it may be advisable to use the terminology of V˙O_2peak and heart rate peak (HR_peak) rather than V˙O_2max and HR_max.

A potential limitation in this study was the time taken to reach exhaustion in the DWR protocol. The mean time to V˙O_2max for the DWR test was 4.78 min, with the TMW mean time being 11.18 min. It is recommended that a graded maximal treadmill test protocol elicits V˙O_2max within 8-12 min (⁷). However, the shorter duration to reach V˙O_2max during the present DWR test compares with that of Mercer and Jensen (¹⁷), who had a mean DWR time of 6.8 min using the same protocol, and that of Town and Bradley (²⁷), who found a DWR duration of 4 min. It is suggested that the DWR duration in the current study is acceptable to elicit peak responses, demonstrated by no difference in V˙O_2max for female participants performing a non-weight-bearing cycle maximal protocol between durations of 5, 8, 12, and 16 min (³⁰). Water immersion may have caused greater physiological and psychological strain on the participants, contributing to the reduced time taken to reach maximum V˙O_2max during DWR. Consequently, the short duration of the DWR protocol may have contributed to the failure to reach a true V˙O_2max value (²¹). This indicates that the highest value obtained during the DWR test may be best represented as V˙O_2peak, and, likewise, it may be equivalent to the V˙O_2max value (^10,28).

Finally, the validity of the DWR protocol was investigated through comparison of maximal V˙O₂ and HR responses between DWR and TMW. The positive correlations between maximal variables of DWR and TMW suggest validity of the DWR protocol as a graded exercise test in overweight women. The significant correlations between DWR and TMW protocols in the current study are similar to those found by Mercer and Jensen (17) in a group consisting of young men and women of normal weight for V˙O_2max (r = 0.70, P < 0.01; r = 0.88, P = 0.001) and HR_max (r = 0.65, P < 0.01; r = 0.64, P = 0.001). Results from the current study and past research demonstrate similar relationships for maximal V˙O₂ and HR responses between weight-bearing and non-weight-bearing exercise regardless of age and body weight (¹⁷).

In conclusion, maximal responses of overweight, middle-aged women to DWR and TMW are significantly different; however, they are comparable with those of other populations. Appropriate prescription of aquatic and land-based exercise training may be implemented using valid, mode-specific graded exercise tests, taking into account the attenuation of maximal V˙O₂ and HR in water. Because of differing maximal physiological values for DWR compared with TMW, the use of land-based V˙O_2max criteria is not recommended for a DWR graded exercise test. Therefore, further investigation is required to determine the optimal values for describing DWR V˙O_2max criteria.

The authors would like to thank the research participants for their dedication and enthusiasm. Special thanks to Kim Meredith-Jones for invaluable assistance throughout this investigation.