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Using Heart Rate to Prescribe Physical Exercise During Head-Out Water Immersion

Kruel, Luiz F. M.1; Peyré-Tartaruga, Leonardo A.1; Coertjens, Marcelo2; Dias, Adriana B. C.1; Da Silva, Rafael C.1; Rangel, Antônio C. B.1

Journal of Strength and Conditioning Research: January 2014 - Volume 28 - Issue 1 - p 281–289
doi: 10.1519/JSC.0b013e318295d534
Original Research
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Kruel, LFM, Peyré-Tartaruga, LA, Coertjens, M, Dias, ABC, Da Silva, RC, and Rangel, ACB. Using heart rate to prescribe physical exercise during head-out water immersion. J Strength Cond Res 28(1): 281–289, 2014—The purpose of this study was to compare and correlate the effect of age group, sex, depth of water immersion, and the heart rate (HR) assessed out of the water on the HR behavior in individuals subjected to head-out water immersion. A total of 395 healthy individuals of both sexes, aged between 07 and 75 years, underwent vertical head-out water immersion. Heart rate was assessed out of the water in the supine and orthostatic (OHR) positions and at immersion depths corresponding to the ankle, knee, hip, umbilicus, xiphoid process, acromion, neck, and also the neck with the arms out of the water. The formula (ΔHR = OHR − HR immersion depth) was used to calculate the reduction in HR at each immersion depth. No age-based or sex-based differences in HR were found. The greater the depth of the water, the greater was the decrease in HR (p < 0.05); however, no differences were found between the HR values obtained below the depth corresponding to the umbilicus. Similarly, there was a significant relationship between OHR and ΔHR measured at levels below the depth corresponding to the umbilicus (e.g., xiphoid process level: r = 0.62; p < 0.05). Therefore, this study suggests to appropriately prescribe the intensity of water-based exercise intensity performed during vertical immersion: OHR should be measured before the individual entering the aquatic environment; ΔHR should be measured according to the depth at which exercise is to be performed, and we suggest an adaptation to Karvonen’s HRmax prediction formula (predicted HRmax: 220 − age − ΔHR) to prescribe and control the intensity of the exercise performed during vertical immersion.

1Exercise Research Laboratory, School of Physical Education, Federal University of Rio Grande do Sul, Porto Alegre, Brazil; and

2College of Physiotherapy, Federal University of Piauí, Parnaíba, Brazil

Address correspondence to Luiz F. M. Kruel, kruel@esef.ufrgs.br.

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Introduction

During immersion in an aquatic environment, a large volume of blood is displaced from the peripheral to the central regions of the body. Arborelius et al. (4) found that when a sitting subject is immersed, 700 ml of blood from the peripheral veins is displaced, thus increasing the central blood volume. A quarter of this volume is stored in the heart and the remainder is distributed throughout the pulmonary vascular system. Risch et al. (26) found that when a standing subject is immersed up to the neck, the volume of the heart increases 180–250 ml on average and remains elevated and constant with a maximum distension of 31%, which is reached in about 6 seconds (27). This means that, of the 700 ml of blood displaced to the chest, hundreds of milliliters of excess blood flow is rapidly forced through the right ventricle. Increases in central venous pressure of about 12–17 mm Hg have also been recorded (9).

This redistribution of blood causes a significant burden on the heart, which leads to a decrease in heart rate (HR) because of the vagal tone triggered by stimulation of the atrial and arterial reflex mechanisms (26). Similarly, a significant decrease in sympathetic impulses is seen during vertical immersion (20). In this regard, the decreased HR response during immersion has been attributed to the action of hydrostatic pressure (compressive force) (27) and thermoregulatory adjustments (5,30), which increases venous return, especially from the lower limbs and abdomen to the chest, causing adjustments in sympathetic-parasympathetic balance (5,6). Equally, other associated factors also appear to influence the HR behavior in the water, such as the depth at which the individual is immersed (20,21) and the HR assessed out of the water (27).

Aspects, such as age group and sex (21,37), the standing position of the individual in the water and the immersion depth (20,21) and hydrostatic weight (3,14), and HR assessed out of the water (27), have not been taken into consideration as parameters for use in the prescription and the control of intensity based on HR in physical exercises performed in water (34,38). Normally, the procedure for calculating the HR target areas in an aquatic environment, such as in water aerobics, walking and deep water running programs, consists, first, in subtracting fixed values based on the result of Karvonen’s HRmax prediction formula (predicted HRmax: 220 − age), which predicts an average discount in the HRmax measured in the water to, later, calculate the training HR using a percentage of HR reserve (29). However, there is a problem with the use of these fixed values, because they were obtained based on research conducted with swimmers in the supine position (predicted HRmax: 220 − age − 13) (17) or without any scientific justification (predicted HRmax: 220 − age − 17) (29). For example, these formulas do not take into consideration the fact that the xiphoid process corresponds to the depth of immersion most widely adopted for exercises taught during vertical immersion, such as water-based exercises (1,2,8,25).

The appropriate prescription of exercise performed during vertical immersion is justified because water aerobics, walking, and deep water running, for example, are being chosen by a growing number of people in different parts of the world, whether for fitness, recreation, or rehabilitation (1,2,8,22,24,25,28,38). Therefore, the purpose of this study was to compare and correlate the effect of age group, sex, depth of water immersion, and the HR assessed out of the water on the HR behavior in people subjected to head-out water immersion. We hypothesized that the behavior of the HR during immersion would be influenced by age and sex and that the deeper the immersion the lower the HR and the higher the measured out of the water HR the greater would be the reduction in HR during immersion. Based these assumptions, these parameters would be needed to obtain an individualized prescription of exercise performed during vertical immersion.

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Methods

Experimental Approach to the Problem

Heart rate is a physiological parameter that is widely used to prescribe and control the intensity of exercise performed during vertical immersion. However, HR may change significantly due to effects of the liquid medium. Among the changes observed, we can highlight the reductions in resting and training HR when compared with similar exercise performed out of the water (34). This phenomenon is observed in situations where the water temperature is below thermoneutral temperatures (5,30), with the exception of very cold (30) and warm temperatures (5). Therefore, this feature hinders the prescription of exercise during vertical immersion based on HR. To correct this problem, HRmax prediction equations that take into account the reduction that HR suffers in the water have been proposed (17,29). These formula, however, do not take into consideration factors such as the sex and age of the individual, nor the vertical position or depth of immersion (20,21,26,37). Furthermore, there is evidence to show that individuals with elevated HR out of the water undergo a large reduction in HR upon entering the water (27). These factors could influence the prescription of water-based exercise when the prescription is based on percentages of HRmax. This cross-sectional study aimed to compare the effect of three factors (intergroup): sex; age group; and HR assessed out of the water in the orthostatic position range (OHR range) on HR and its reduction during immersion. Comparisons of HR from the same individuals (intragroup) in different positions (supine and orthostatic) in different environments (in and out of the water) and at different immersion depths were also performed. Furthermore, the relationship between the OHR range and the reduction in HR during immersion (ΔHR) was investigated. The dependent variables are the HR assessed out of the water in the supine (SHR) and orthostatic positions (OHR), HR assessed in the water at different immersion depths ankle HR (AnHR); knee HR (KHR); hip HR (HHR); umbilicus HR (UHR), xiphoid process of the sternum HR (XHR); acromion HR (AHR); neck HR with the arms in the water (NHR_in) and neck HR with the arms out of the water (NHR_out)) and the reduction in HR during immersion (ΔHR = OHR − HR immersion depth). The independent variables are sex, age group, the supine and orthostatic positions out of the water, the orthostatic position at different immersion depths and the OHR ranges. A new proposal for predicting HRmax during vertical immersion is suggested based on an adaptation of Karvonen’s HRmax prediction formula.

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Subjects

The study sample was composed of physically active individuals who had regularly performed physical exercises in water for at least 1 year before the experiment. It was made up of pupils from water aerobics, deep water running, and swimming programs undertaken in university extension projects. Of the subjects, 277 were women and 118 men, totaling 395 people, aged between 7 and 75 years and with a mean body mass of 62.1 ± 15.0 kg. They were divided into 6 age groups, according to sex, as follows: 07–13 years, 15 females and 24 males; 14–17 years: 23 females and 13 males; 18–25 years, 39 females and 46 males; 26–35 years, 19 females and 11 males; 36–55 years, 120 females and 16 males; and 56–75 years, 61 females and 08 males. All subjects received prior individual guidance about the objectives, procedures, and risks of the study and agreed to sign a consent form approved by the institutional ethics committee that evaluates research involving human beings and their fundamental scientific and ethical requirements. In accordance with the established ethical procedures, for those subjects younger than 18 years, prior approval from the parents or guardians was sought after they had been informed of the objectives and procedures of this research.

All the subjects were healthy, having undergone prior medical and cardiorespiratory assessment. None of the subjects were using any drug that could directly or indirectly alter cardiac function. The subjects were instructed not to eat a heavy meal or consume stimulants for at least 4 hours before data collection. They were expected to have fasted for 3 hours. Moderate consumption of water was limited to 1 hour before testing. They were also instructed not to perform physical exercise during the 48 hours before the test and to allow sufficient time for at least 8 hours of peaceful sleep the night before the test.

Samples were collected between 3:00 PM and 5:30 PM from October to December representing the seasons of spring and summer in the subtropical southern hemisphere. All subjects were accustomed to the water, because they regularly participated in water-based physical exercise programs. Moreover, 3 days before data collection, they attended a test familiarization session. No data were collected during the familiarization session.

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Procedures

For the data collection, anatomical landmarks were marked: the ankle; knee; hip; umbilicus; xiphoid process of the sternum, shoulder, and neck. Marking these points was important to identify the depth of immersion of the individuals. Once the anatomical points had been marked, the subjects lay on a mattress next to the immersion tank. Before the subjects entered the water, HR measurements were obtained in the supine (SHR) and in the standing position (OHR). SHR was measured after 10 minutes at rest out of the water in the supine position, and OHR was measured after the subject had remained at rest for 2 minutes in the standing position inside the immersion crane. In total, there were 7 immersion depths (ankle, knee, hip, umbilicus, xiphoid process, acromion, and neck) and the subjects remained at each depth for 1 minute after which the HR was measured. Once the HR was measured at one depth, the subject was immediately lowered to the next level. At the seventh level (neck), the subjects remained with the arms out of the water for 1 minute with the arms in water for 1 minute. The length of time spent at each depth was defined based on the work of Risch et al. (27) who noted maximal heart dilatation and stabilization of the HR after 6 seconds during rapid immersion to the neck and stabilization of central venous pressure in less than 1 minute at each immersion depth (26). In our study, it was decided to use a longer period in order to ensure the acute cardiovascular adaptations due to immersion. Thus, the subjects remained immersed for a total of 8 minutes. The denomination of each HR measurement performed during vertical immersion corresponded to the depth at which the collection was performed: ankle HR (AnHR); knee HR (KHR); hip HR (HHR); umbilicus HR (UHR), xiphoid process of the sternum HR (XHR); and acromion HR (AHR), HR neck with your arms in the water (NHR_in) HR neck with your arms out of the water (NHR_out) (Figure 1). The values for the reduction in HR that occurred during immersion (ΔHR) were determined by subtracting the OHR from HR obtained at a given depth (ΔHR = OHR − HR immersion depth). Subsequently, OHR values were arranged in bands of 10 b·min−1, ranging from 50 to 129 b·min−1. Thus, it was possible to analyze the reductions in HR at different immersion depths for the individuals who exhibited a particular range of OHR before entering the water.

Figure 1

Figure 1

The measuring instruments used were an immersion tank and an immersion crane developed in our laboratory (13) (LAPEX, Porto Alegre, Brazil), which was used to lower the subjects into the immersion tank and an HR monitor (Vantage NV; Polar Electro-Oy, Kempele, Finland). The platform of the immersion crane was firm, and a mechanism coupled to the immersion tank ensured that immersion occurred without physical effort or altered HR because of physical instability. The temperature of the water used in the immersion tank varied between 29 and 30° C and was controlled by a gas water heater. This temperature was chosen because of its wide use water-based exercises (1,2,8,25), as it was considered interesting to establish relations with such exercises. Srámek et al. (30) found that during prolonged immersion of up to 1 hour, a decrease in HR occurred at temperatures of 32 and 20° C. Over the same period, HR only rose as from temperatures of 14° C (very cold). The increased HR at lower temperatures was correlated with the increased sympathetic activity during immersion. This phenomenon was not observed at temperatures of 32 and 20° C, which exhibited reduced sympathetic activity. The temperature outside the water varied from 26 to 29° C.

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Statistical Analyses

The statistical analysis was performed with the aid of software (version 16.0; SPSS, IBM Corporation, Armonk, NY, USA), and the variables are expressed as mean and standard error. The normal Gaussian’s distribution of the data was confirmed using the Kolmogorov-Smirnov’s (Lilliefors’) test. All data from this study were analyzed using a simple repeated measures analysis of variance (ANOVA) to compare the mean HRs collected at the different immersion depths and out of the water. A between-group analysis was carried out using 3-factor ANOVA for the comparisons of the mean HRs collected between the age groups and sexes and comparisons of the mean reduction in HR during immersion (ΔHR) for each OHR range. The Tukey’s post hoc test was used to analyze the differences between HRs (HR assessed out of the water and at different immersion depths) and between the ΔHR. Pearson’s linear product-moment correlation was calculated to determine the relationships between the values for the reduction in HR during immersion (ΔHR) with the different OHR ranges. The statistical power for the n size ranged from 0.80 to 0.90, and differences were considered significant at an alpha level of p ≤ 0.05 for all analyses.

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Results

Figure 2 shows the data relating to HR in relation to age group of the individuals. We can see that the average HR behavior was quite similar among the age groups, because no statistical difference was found between the different age groups, ranging from 07 to 75 years, for both sexes (p > 0.05).

Figure 2

Figure 2

As the different age groups presented no significant difference in terms of HR values, they were grouped into mean values according to sex, regardless of age. Figure 3 shows the decrease in HR observed at the different immersion depths according to sex. As with the age groups, there was no statistically significant difference in the HR values between the sexes (p > 0.05). In light of this result, the HR data from the males and females were pooled and analyzed as a single group (Figure 3). When analyzing the HR behavior at different immersion depths, a significant decrease was found to occur as the depth increased (p < 0.05).

Figure 3

Figure 3

Another analysis carried out in the present study was to correlate the reduction in HR during immersion (ΔHR) with the OHR range. This analysis showed that the higher the OHR range, the greater the ΔHR, especially as from the depth corresponding to the umbilicus (Figure 4). Significant correlation coefficients were found between the ΔHRs obtained at the greater depths corresponding to the xiphoid process (r = −0.62), acromion (r = −0.60), and neck (r = −0.63), with p < 0.001.

Figure 4

Figure 4

In light of these results, we compared the ΔHR values between the different OHR ranges. According to Figure 4, it can be seen that significant reductions of about 9 b·min−1 are observed as from the OHR range of 70–79 b·min−1 (p < 0.05). The presented values only refer to the ΔHRs obtained at the depth corresponding to the xiphoid process (ΔHR = OHR − XHR), as this depth is widely used in water-based exercise programs (1,2,8,25).

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Discussion

No significant differences in the HR values were found between the age groups throughout the different experimental conditions (Figure 2). Studies have found that in older individuals, the cardiovascular adjustments triggered during situations involving hemodynamic changes, such as immersion, are more impaired. Miwa et al. (21) found a less pronounced decrease in the muscle sympathetic nerve activity in older subjects (53.5 ± 9.1 years) undergoing immersion when compared with younger subjects (22.1 ± 4.0 years). As a consequence, they also found a smaller reduction in HR and an increased response in the mean blood pressure in older individuals compared with young people throughout the immersion. Similar results for blood pressure have been reported in other studies involving immersion vertical (33,35). These responses have been attributed to decreased baroreflex response and/or baroreceptor sensitivity with advancing age, reducing the adjustment range of the sympathetic-parasympathetic balance (10,12).

However, these assumptions are not definitive. The results from Miwa et al. (21) show no significant differences in the decrease in the muscle sympathetic nerve activity or in the increase in the mean blood pressure, between the age groups during vertical immersion. In relation to HR, significant differences were only found in the depth corresponding to the umbilicus, unlike the depth corresponding to the shoulder. The correlation found between the decrease in suppressive response of muscle sympathetic nerve activity and age was r = 0.53 (p < 0.05). Other studies, however, have found that the autonomic response presents no significant difference with advancing age (23,36).

In relation to sex, it can be seen that there were no significant differences in the mean HRs (Figure 3). Other studies have also found similar cardiovascular outcomes when comparing men and women during immersion (37). However, in female patients, the mean HRs during immersion tend to be higher than those found in males. This is probably because of a higher concentration of body fat in females, mainly located around the hips and chest, which attenuates the peripheral vasoconstriction generated by thermoregulation and its effects on the reflexive reduction of the HR (7).

Given this situation, we may consider that, in general, in subjects acclimated to the aquatic environment, the behavior of the HR is similar during vertical immersion, that is, as the depth of immersion increases, the HR decreases significantly, independent of the age group or sex of the individuals (Figure 3). In the literature, this behavior pattern has been previously described in studies comparing different depths of immersion (21,26), although those studies did not include such a large sample composed of different age groups and sexes. Thus, in our study, the individuals from the different age groups and sex group were later analyzed as a single group (both) (Figure 3).

The reasons for this behavior are well documented. Mano et al. (20) found a significant and progressive reduction in the HR, in the systemic blood pressure, and in the diameter of the leg and the thigh as the immersion depth increased until it reached the level of the neck. It is known that, during immersion in an aquatic environment, hydrostatic pressure (compressive force) causes blood to be displaced from the peripheral regions to the chest and heart, causing a significant increase in central venous pressure and consequently the volume of the heart (9,15). This mechanism promotes a reduction in the sympathetic impulse and, probably, an increase in vagal tone, reducing the HR because of stimulation of the carotid and aortic receptors, which is seen to be proportional to the immersion depth (18,19).

It can also be seen that the behavior of the HR showed statistically significant differences between 3 levels of immersion depth, consisting of (a) ankle and knee, (b) hip, and (c) umbilicus, xiphoid process of the sternum, acromion, neck with your arms in the water, and neck with your arms out of the water (Figure 3). It can also be seen that the HR measured in the early stages of immersion, corresponding to depths of the ankle and knee, showed no statistically significant differences with that found in OHR. This may be because of the fact that the blood supply in these regions is less than that found, for example, in the thighs, abdomen, and chest, thus providing a smaller volume of blood to be shunted to the central regions of the body (11). Therefore, there would be less incentive for the reduction of the HR reflex.

This result became evident as from the point corresponding to the hip HR. At this point, there was a significant decrease in HR in relation to the previous measures, probably because of the greater availability of muscle tissue and consequently of blood. The decrease in HR was further accentuated when the level of immersion reached the point corresponding to the umbilicus HR. From this point on, the measures of HR found (xiphoid process, acromion and neck) were even smaller and there was no statistical difference between them. The existence of larger vessels and viscera with greater blood supply could explain the hemodynamic changes that would have induced this sharp decline in HR (11).

Another interesting finding was the similarity found between the HR collected in the supine position out of the water with those obtained as from the depth corresponding to the umbilicus, xiphoid process, acromion, and neck. Apparently, the volume of blood displaced to the center of the body at these immersion depths is similar to that obtained during supine position out of the water. Studies have found that the volume of the heart measured in supine position is similar to that obtained during immersion at the depth of the xiphoid process (26). This evidence confirms that the redistribution of blood flow observed with immersion in water as from the umbilicus promotes hemodynamic changes similar to those found in the supine position out of the water (Figure 3).

Furthermore, the reduction in HR during immersion (ΔHR) was found to be influenced by HR assessed out of the water in the orthostatic position (OHR) (Figure 4). Although individuals who began the immersion with 50–59 b·min−1 suffered minor reductions in HR, regardless of the depth of immersion, those who started in the range of 120–129 b·min−1 suffered major changes, dependent of the depth of immersion. Marked decreases in HR were observed as from the range of 70 to 79 b·min−1, from the depth corresponding to the hip, achieving significance at higher OHRs and at greater depths. At the depths corresponding to the ankle and knee, there were no significant differences in HR, regardless of the OHR range. Of particular note, we can see the comparisons of the ΔHRs measured only at the depth of the xiphoid process among the different OHR ranges (Figure 4).

Accordingly, it can be seen that as from the immersion depth corresponding to the hip, HR behavior in the water is more strongly influenced by the OHR. Although for those individuals who had lower OHRs, the effect of immersion on the reduction in HR appears to be minimal, for those who had higher OHRs the effect was quite significant. It is known that both the decrease and the increase in HR that occur in the resting position are controlled mainly by the parasympathetic balance. However, as from resting HRs greater than approximately 100 b·min−1, sympathetic control of HR becomes more pronounced (16). Thus, the large reduction found in the HR of individuals with OHRs above 100 b·min−1 appears to represent a more pronounced reflection of the effect of immersion in an aquatic environment on the reduction of sympathetic activity. On the other hand, with the lower OHRs, the effect of immersion on the reduction in HR was small, possibly because of the reduced sympathetic activity and increased parasympathetic activity out of the water. These results are in agreement with significant correlation values found for these depths.

These results also seem to suggest a probable relationship between physical fitness and the magnitude of the reduction in HR during immersion, because the individuals with higher OHR showed higher ΔHR. Previous studies, however, contradict these assumptions (31,32). Although the physical fitness of the individuals participating in the present study was not evaluated, they all regularly attended water-based exercise programs.

Thus, these results provide some important insights for the prescription of water-based exercise programs, that is, because the procedures currently used to determine training HR during vertical immersion in an aquatic environment, such as water aerobics, walking, and deep water running programs, do not take into account the standing position of the individual in the water, the immersion depth, or his/her OHR. Normally, the procedure for calculating the HR target areas in an aquatic environment, such as in water aerobics, walking, and deep water running programs, consists, first, in subtracting fixed values based on the result of Karvonen’s HRmax prediction formula (predicted HRmax: 220 − age), which predicts an average discount in the HRmax measured in the water to, later, calculate the training HR using a percentage of HR reserve (29). However, there is a problem with the use of these fixed values, because they were obtained based on research conducted with swimmers in the supine position (predicted HRmax: 220 − age − 13) (17) or without any scientific justification (predicted HRmax: 220 − age − 17) (29). For example, these formulas do not take into consideration the fact that the xiphoid process corresponds to the depth of immersion most widely adopted for exercises taught during vertical immersion, such as water-based exercises (1,2,8,25). To appropriately prescribe exercises performed during vertical immersion, we suggest that HR should be measured in the orthostatic position (OHR) before the individual entering the aquatic environment; ΔHR should be measured according to the depth at which the exercise is to be performed; and we suggest adapting Karvonen’s HRmax prediction formula by subtracting the ΔHR effect (predicted HRmax: 220 − age − ΔHR) to prescribe and control the intensity of the exercise performed during vertical immersion. This subtraction takes into account the fact that the HRmax obtained in water is significantly lower than that obtained out of the water (34), respecting the parameters of biological individuality such as the vertical position, the immersion depth, and the individual’s OHR.

As a result of this study, we can conclude that an individual’s HR during head-out water immersion in an aquatic environment is influenced by both the level of the water’s depth and the individual’s OHR. These alterations are mainly observed at greater depths and higher OHRs. Apparently, depths of vertical immersion below the knee and OHR of up to 70 b·min−1 have little influence on the reduction in HR; by contrast, immersion depths as from the hip and OHR over 70 b·min−1 have considerable influence on the reduction in HR. Furthermore, both the age and sex of the individuals do not significantly interfere in HR behavior inside the water. These results represent an important discovery that should lead to more appropriate prescription for exercises in the vertical immersion in an aquatic environment, taking into account the depth of immersion and the ΔHR based on prior evaluation of the OHR.

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Practical Applications

The findings of this study are important both for the prescription and for the control of physical exercises performed in water such as water aerobics and deep water running performed during head-out water immersion. To obtain an individualized prescription based on the percentage of HRmax, the trainer needs to consider the standing position, the depth at which the individual is immersed, and the HR assessed out of the water. These parameters influence the behavior and reduction of the HR in the water and are included in the formula “ΔHR = OHR − HR immersion depth,” where ΔHR represents the reduction in HR that occurred during immersion, OHR is the HR measured out of the water after 2 minutes at rest in the orthostatic position preceded by 10 minutes in the supine position, and the HR immersion depth is the HR obtained at a given depth at rest in the orthostatic position after 1 minute. To predict the HRmax in the water, the literature recommends adapting Karvonen’s HRmax prediction formula based on research conducted with swimmers in the supine position or without any scientific justification (17,29). The results of our study contradict those adaptations. Therefore, as a new adaptation to the formula for predicting maximal HR, we recommend using the formula “predicted HRmax: 220 − age − ΔHR” to prescribe and control the intensity of the exercise performed during vertical immersion based on percentages of the HRmax. These recommendations will be relevant for physical exercises performed in vertical immersion depths as from the hip and the HR of over 70 b·min−1 collected out of the water while standing at rest.

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Acknowledgments

The authors wish to thank the subjects for participation in the study.

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Keywords:

age; sex; cardiovascular regulation; training; aquatic environment

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