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International Journal of Aquatic Research and Education
#3091) >91&)6 68-'0)
Biophysiologic E!ects of Warm Water Immersion
Bruce E. Becker
Washington State University
Kasee Hildenbrand
Washington State University
Rebekah K. Whitcomb
Washington State University
James P. Sanders
Washington State University
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International Journal of Aquatic Research and Education, 2009, 3, 24-37
© 2009 Human Kinetics, Inc.
Biophysiologic Effects
of Warm Water Immersion
Bruce E. Becker, Kasee Hildenbrand, Rebekah K. Whitcomb,
and James P. Sanders
Physiologic change associated with aquatic activity has been found to profoundly
affect human function and health-related biologic alterations. Similar to sleep
research, aquatics has emerged as an area ripe with human health and performance
implications. Aquatic activity impacts the cardiovascular, musculoskeletal, autonomic
nervious system (ANS) and endocrine systems in ways that have positive public
health implications for issues confronting the nation, including obesity, diabetes and
arthritis (Becker, 2004). Aquatic activity has tremendous application in the area of
sports medicine and has great potential value to student athletes in both training and
rehabilitation. The aquatic environment is a research area just emerging as a focus of
physiologic importance with many health benets that apply across the age span and
could be widely accessed by the American public if both research support and under-
standing by the health professionals were to increase.
Water Immersion and the Body
Immersion produces a dramatic shift of blood from the extremities to the chest,
with approximately 2/3 of this volume in lung circulation and 1/3 within the heart
(Arborelius, Balldin, Lilja, & Lundgren, 1972; Begin et al., 1976; Christie et al.,
1990). This creates a major increase in cardiac lling volume, resulting in increased
stroke volume and cardiac output (Begin et al., 1976; Christie et al., 1990). The
resulting effect of immersion is that the heart pumps effectively the same amount
of blood per minute at rest as it does during the initiation of aerobic exercise.
Therefore immersion may be a useful way of beginning cardiac rehabilitation or
recovery from severe debility (Cider, Svealv, Tang, Schaufelberger, & Andersson,
2006). At the same time, immersion decreases peripheral resistance, reducing the
amount of work the heart must do to move this volume of blood, so the effort
required to circulate blood decreases while cardiac efciency increases (Arbore-
lius et al., 1972; Gabrielsen, Johansen, & Norsk, 1993). This supports the ratio-
nale that aquatic exercise may be benecial for cardiac rehab following ischemic
Bruce Becker, Kasee Hildenbrand, and Rebekah Whitcomb are with the Educational Leadership and
Counseling Psychology Department and James Sanders is with the Sociology Department, all at
Washington State University in Pullman. E-mail khildenbrand@wsu.edu.
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Warm Water Immersion 25
heart injury (Gabrielsen et al., 2001; Gabrielsen, Sorensen et al., 2000; Hanna,
Sheldahl, & Tristani, 1993; Heigenhauser, Boulet, Miller, & Faulkner, 1977; Jiang
et al., 1994; Magder, Linnarsson, & Gullstrand, 1981; McMurray, Avery, & Sheps,
1988; Meyer & Leblanc, 2008).
Circulation to deep muscle structures is also increased signicantly in water
immersion, improving oxygen ow to tissues and potentially facilitating healing
of muscle, bone, and joint injuries (Balldin & Lundgren, 1972; Balldin, Lundgren,
Lundvall, & Mellander, 1971). Improved blood ow is also relevant to processes
that alter tissue circulation, including diabetes and some auto-immune diseases.
Neck-depth immersion may enhance brain blood ow through reduction in periph-
eral vascular resistance combined with increased cardiac output. This may improve
brain functions, including cognition and memory, which could potentially aid in
head trauma recovery or stroke rehabilitation (Bonde-Petersen, Schultz-Pedersen,
& Dragsted, 1992). Renal efciency also improves, producing diuresis through
increased excretion of sodium and potassium, aiding in reduction of edema when
present (Epstein, 1976, 1992).
The autonomic nervous system (ANS) is an important homeostatic mecha-
nism in several of the body’s regulatory functions. The ANS is the major control
mechanism for cardiovascular regulatory activity, including heart rate and arterial
pressure. In addition, it controls the gastrointestinal motility and secretion, renal
and bladder function, visual alterations, thermoregulation, and a number of mental
processes. Essentially, it functions as the motherboard for human bioregulation.
The two major subdivisions of the ANS are the sympathetic and the parasympa-
thetic systems. The functions of the sympathetic nervous system (SNS) are to
control the ght or ight responses of the body while the functions of the para-
sympathetic nervous system (PNS) are to control the relaxation and repose
responses. The anatomic location of these systems is in the brainstem, the spinal
cord, and the hypothalamus.
The anatomy, interconnections, and neural regulatory mechanisms are quite
complex, with many reex triggers and feed-back mechanisms. The response
speed is dramatic, potentially capable of doubling heart rate in a matter of a few
seconds. There are two major neurotransmitters activated by the ANS: acetylcho-
line and norepinephrine, categorized as catacholamines. A number of methods
have been used to measure the function of the ANS components, including blood
hormones such as corticosteroid and catecholamine levels, galvanic skin responses
(polygraphs), salivary cortisols, and heart rate variability (HRV). While measure-
ment of blood hormones is useful, because ANS changes are so instantaneous, a
running measurement of sympathetic/parasympathetic inuence is technically
difcult outside of a laboratory. Polygraphs are used in legal work but not com-
monly in research applications. HRV has emerged as a major method of assessing
autonomic activity because it is noninvasive, inexpensive, and offers real-time
information about the inuence of the two subdivisions of the ANS (Lombardi,
2002; Rajendra Acharya, Paul Joseph, Kannathal, Lim, & Suri, 2006; Stauss,
2003; Thayer & Brosschot, 2005). Research has shown that stress and fear
increases SNS activity, whereas relaxation, meditation, and neutral water immer-
sion decrease SNS activity and increase PNS (Ditto, Eclache, & Goldman, 2006;
Mano, Iwase, Yamazaki, & Saito, 1985; Perini & Veicsteinas, 2003; Ziegelstein,
2007). Increased SNS activation is associated with adverse cardiac events, includ-
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26 Becker et al.
ing arrhythmias, whereas increased vagal activation (PNS) is associated with a
decrease in adverse cardiac events (Lombardi, 2002; Thayer & Brosschot, 2005;
Thayer & Lane, 2007). As a result of these ndings, HRV has become a major tool
in the assessment of ANS activity and is commonly used in coronary care units for
this purpose.
HRV analysis is based upon the understanding that a normal heart beats regu-
larly, but with instantaneous variation. This variation is dependent upon respira-
tory frequency and ANS activity, including the interplay between the SNS and
PNS subdivisions. By studying the variation using mathematical analysis, the
variation may be broken into frequency spectra. By measuring the power of these
various spectra, the inuence of the two subdivisions may be assessed. Typically,
it is believed that low and very low frequency spectral power in the rage from 0.15
to 0.4 Hz represents SNS, and that high frequency spectral power in the 0.15–0.4
Hz represents PNS inuence. Further analysis of these variables can be used to
examine the relationship between these two subdivisions, referred to as sympa-
thovagal balance (Lombardi, 2002; Rajendra Acharya et al., 2006; Stauss, 2003;
Thayer & Brosschot, 2005).
Comparable to meditation, aquatic immersion in warm water temperatures
has been shown to exert an effect upon the ANS, decreasing sympathetic power
while increasing vagal inuence (Miwa, Sugiyama, Mano, Iwase, & Matsukawa,
1997; Nishimura & Onodera, 2000, 2001; Perini & Veicsteinas, 2003). A limited
amount of research has been done to assess the effects of immersion temperature
upon autonomic bioregulation. Most of the current literature has subjects in a
supine oating position, rather than in the common seated position used while
bathing or hot-tubbing (Nishimura & Onodera, 2000, 2001). This study examines
water immersion impacts on the sitting position by contrasting ANS regulation
measures in warm water to those in cool and neutral temperatures.
Immersion in warm water is generally found to be pleasurable, creating an
almost universal feeling of relaxation. The ANS is the most rapidly responsive
bioregulatory control function within the body. Using HRV, these changes can be
clearly measured. It is known that positive emotional states are associated with
increased sympathovagal balance, while negative emotions and stress will decrease
HRV and sympathovagal balance (Brosschot, Van Dijk, & Thayer, 2007; Lane et
al., 2008; Thayer & Lane, 2008; Thayer & Sternberg, 2006). The purpose of this
study was to address whether the ANS would show changes that mirrored these
positive emotion responses in HRV. In addition, we examined physiologic changes
that are ANS-mediated, including blood pressure, heart rate, and core
temperature.
Methods
This study protocol was reviewed and approved by the Institution’s Investiga-
tional Review Board. Sixteen healthy, college-aged participants volunteered for
this study, consisting of eight males and females. Resting measurements of heart
rate and blood pressure were taken using a standard automated plethysmometer
(OMRON HEM-755, Omron Healthcare, Inc, Bannnockburn IL). Participants
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Warm Water Immersion 27
ingested a radiofrequency core temperature transmitter (CorTemp, HQInc, Pal-
metto, FL) that continuously monitored core temperature during the study. To col-
lect HRV data, subjects were connected to a BioPac biologic monitoring system
(BioPac Systems Inc, Goleta CA) that continuously measured heart rate and elec-
trocardiogram (ECG) with electrodes placed on right supraclavicular, right iliac,
and the left apex.
Participants rested poolside for six minutes before initial measurements were
taken. Data collection began with 6 minutes of baseline data and then subjects
were immersed in the cool (31.1 °C) tub for 24 minutes (minutes six through 29).
Vital signs (i.e., heart rate, core temperature, and blood pressure) were measured
four times while in the cool tub: at the tail-end of the 11th, 17th, 23rd, and 29th
minutes. Afterward, subjects exited the cool water and rested poolside for 12 min-
utes (minutes 30–41). Vitals were recollected half way through this rst recovery
period. Participants then immersed in neutral (36 °C) water for 24 minutes (min-
utes 42–65), followed by poolside recovery for 12 minutes (minutes 66–77). As
before, vitals were retaken at the half-way point of the recovery period. Finally,
participants immersed in the warm (39 °C) tub for 24 minutes (minutes 78–101)
before sitting at poolside for a nal 12 minutes (minutes 102–114). Vitals were
taken at both the half-way point and end of the third recovery period.
BioPac data were cleaned before employing a fast-Fourier transform of the
ECG into HRV. HRV data contained power spectrum data in very low frequency
(VLF = 0.04HZ), low frequency (LF = 0.04–0.15HZ), and high frequency (HF =
0.15–0.4HZ), as well as sympathetic power, vagal power, and autonomic balance.
Like vital measures, HRV data were assessed for the baseline and recovery peri-
ods and at the same time segments of the immersion periods. Table 1 provides
descriptive statistics. Values listed for immersion and recovery periods represent
the average of the measures taken.
As the underlying goal of the paper is to further explore potentially unique
physiological effects of warm water immersion, paired-samples T tests were con-
ducted. Paired-samples T tests are an appropriate statistical method because data
were derived from the same subjects experiencing different conditions at different
time points. Moreover, T tests allow for the inference of signicance when sample
size (and consequently statistical power) is low—a valuable feature considering
the limited sample size of this study. Changes over time are graphed as a means to
visually represent the relationships between water temperature and physiological
response to immersion.
Results
Table 2 presents results of paired-samples T tests analyses that compare vital and
ANS measures taken while in warm water to those taken in other immersion
states. Results strongly suggest that warm water has a signicant effect on human
bioregulatory processes. Physiological responses to warm water not only tend to
signicantly differ from baseline and recovery values, but the effects of warm
water immersion also tend to differ from cool and neutral water immersion in
meaningful ways. Key ndings of the analyses are reviewed below.
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28 Becker et al.
Heart Rate
Heart rate increased signicantly in warm water when compared with cool and
neutral immersion temperatures. Additional analyses (not shown here) reveal that
immersion in neutral water did not signicantly change heart rate from the rst-
recovery period. Immersion in cool water had the reverse effect of warm water
because it lowered heart rate by an average of 4.735 beats per minute, but the
magnitude of cool water’s impact on heart rate is not nearly as great as warm
water. Immersion in warm water increased participant’s heart rate by an average
of 21.573 beats per minute. As evidenced in Figure 1a, it appears that heart rate
recovers signicantly from its elevated warm water state once removed for even a
few minutes, returning to a level close to those measured in baseline and recovery
periods.
Core Temperature
Core temperature also appears to increase signicantly in warm water compared
with cool or neutral temperatures. Analyses not shown here indicate that immer-
sion in cool or neutral water did not signicantly alter participants’ body tempera-
ture, though limited sample size may partially account for the nonnding. In any
case, immersion in warm water increased participants’ core temperature by 0.45
°C. Figure 1b provides a visual presentation of the change. Lastly, unlike for heart
rate, removal from warm water did not signicantly reduce core temperature.
Thus, it appears the body requires a longer period of time to return to baseline
core temperature than it does to restore baseline heart rate once removed from
warm water.
Systolic Blood Pressure
Warm water immersion signicantly lowered participants’ systolic blood pres-
sure. On average participants’ systolic blood pressure decreased by 11.596 mmHg,
while rising slightly toward the end of the warm immersion period. During immer-
sion periods water temperature alone does not seem to be the major factor in this
effect, due to systolic blood pressure dropping similarly in cold, neutral, and warm
water. During recovery immersion temperature does appear to affect systolic pres-
sure either. During the cool and neutral immersion recovery periods, systolic pres-
sures rose signicantly, while following warm water immersion the recovery rise
was far smaller. Figure 2 presents a summary of the pattern.
Diastolic Blood Pressure
Participants’ diastolic blood pressure responded similarly to systolic pressure. On
average participants’ diastolic blood pressure decreased by 25.826 mmHg in
warm water. Unlike subjects’ systolic pressure response however, warm water
lowered diastolic blood pressure signicantly compared with cool and neutral
water. As with systolic pressure, diastolic blood pressures increase was signi-
cantly less in the third recovery period. This again implies that diastolic blood
pressure responds differently based on whether one exits warm water as opposed
to cool or neutral temperatures. Figure 2 provides a visual summary of systolic
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Warm Water Immersion 29
Table 1 Descriptive Statistics
Variable Minimum Maximum Mean sd
Baseline vitals heart rate 48 83 67.412 10.000
Core temp [n = 8] 36.44 37.34 37.05 0.296
Systolic BP 98 130 112.118 8.690
Diastolic BP 56 86 72.235 8.164
ANS VLF 6.608 16.620 9.910 3.060
HF 0.631 2.598 1.079 0.584
SVB 2.011 4.280 3.602 0.611
Cool vitals heart rate 48 76.5 62.694 9.068
Core temp [n = 9] 36.80 37.648 37.212 0.281
Systolic BP 91 118 102.529 7.293
Diastolic BP 51.5 70.5 61.382 5.053
ANS VLF 6.608 16.620 9.910 3.060
HF 0.636 4.992 1.991 1.253
SVB 1.131 3.875 2.636 0.819
1st recovery vitals heart rate 45 76 60 9.738
Core temp [n = 9] 36.79 37.63 37.269 0.292
Systolic BP 94 134 113.118 10.487
Diastolic BP 61 84 75.235 6.440
ANS VLF 4.212 22.228 12.233 4.576
HF 0.684 2.421 1.344 0.568
SVB 2.684 5.765 3.791 0.781
Neutral vitals heart rate 46.3 75.3 60.406 7.933
Core temp [n = 9] 36.58 37.448 37.111 0.261
Systolic BP 87.8 112.80 98.176 6.883
Diastolic BP 48 67.5 56.632 5.545
ANS VLF 19.695 48.321 30.560 8.190
HF 0.596 3.542 1.563 0.888
SVB 1.608 3.903 2.864 0.700
2nd recovery vitals heart rate 50 69 59.882 5.529
Core temp [n = 8] 36.79 37.41 37.167 0.211
Systolic BP 100 132 113.647 8.485
Diastolic BP 67 94 79 6.748
ANS VLF 8.282 20.159 13.335 3.955
HF 0.794 3.077 1.533 0.678
SVB 1.947 4.277 3.553 0.644
Warm vitals heart rate 68 101 81.588 8.711
Core temp [n = 10] 37.335 38.05 37.643 0.205
Systolic BP 83.5 120 102.610 9.014
Diastolic BP 40.75 65.5 53.176 6.109
ANS VLF 9.637 19.580 15.394 2.982
(continued)
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30 Becker et al.
and diastolic blood pressure graphed together. This allows comparisons between
pulse bandwidth, and shows it increased successively during neutral and warm
water immersion.
Very Low Frequency Power Spectral Data (VLF)
Warm water immersion signicantly raises VLF HRV, though not nearly to the
degree of cool and neutral water. Warm water immersion increased VLF power by
an average of 2.060HZ. While this is a signicant change, warm water’s impact is
far smaller than cool and neutral water, which raised participants’ VLF values by
22.441HZ and 18.328 HZ, respectively. Figure 3a presents the difference.
Table 1 (continued)
Variable Minimum Maximum Mean sd
HF 0.271 1.323 0.587 0.291
SVB 1.857 4.782 3.893 0.710
3rd recovery vitals heart rate 51 92 67.941 9.384
Core temp [n = 9] 37.475 38.075 37.728 0.189
Systolic BP 92.5 131 108.559 10.484
Diastolic BP 56 80 66.941 6.169
ANS VLF 7.032 18.033 10.548 2.894
HF 0.619 2.114 1.038 0.387
SVB 2.364 4.956 3.920 0.685
Table 2 Paired-Samples T Test Analyses of Warm Water Effects by
Water Immersion Status+
Variable Baseline Cool
1st
Recovery Neutral
2nd
Recovery
3rd
Recovery
Heart rate +14.043*** +18.779*** +21.455*** +21.073*** +21.573*** +13.69***
Core temp +0.700** +0.423** +0.367** +0.524*** +0.450*** −0.093
Systolic
BP −4.890 −0.434 −11.066*** +3.904 −11.596*** −6.508***
Diastolic
BP −19.059*** −8.206*** −22.059*** −3.456* −25.826*** −13.588***
VLF +5.484*** −16.956*** +3.162** −15.166*** +2.060* +4.847***
HF −0.492*** −1.405*** −0.757*** −0.976*** −0.947*** −0.451**
SVB +0.291* +1.246*** +0.101 +1.029*** +0.339* −0.028
+Horizontal axis is reference category.
*p <.05; **p <.01; ***p <.001, 2-tailed.
n = 8–18
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Figure 1 — Vitals (a. heart rate; b. core temperature).
(1a.)
(1b.)
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32 Becker et al.
High Frequency Power Spectral Data (HF)
HF HRV power is heavily inuenced by water immersion status. Unlike cool and
neutral water, both of which appear to signicantly increase the response in HF
power spectrum, exposure to warm water is associated with a signicant decline
in HF power. Figure 3b shows warm water immersion decreased participants’ HF
power by 0.947HZ.
Sympathovagal Balance (SVB)
Results for SVB reverse those of HF power spectral analysis. Cool and neutral
water signicantly decreased SVB, but warm water immersion signicantly
increased SVB. Figure 3c shows warm water immersion increased SVB by
0.339HZ.
Discussion
The purpose of this study was to address whether the ANS would show changes
in HRV. In addition, we examined physiologic changes that are ANS-mediated
including blood pressure, heart rate, and core temperature. HRV has been used
extensively to monitor ANS function, because it is safe, noninvasive, and rela-
tively inexpensive (Sinski, Lewandowski, Abramczyk, Narkiewicz, & Gaciong,
Figure 2 — Systolic and Diastolic blood pressure.
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Figure 3 — ANS (a. VLF power spectral data; b. HF power spectral data; c. Sympathova-
gal balance).
(3a.)
(3b.)
(3c.)
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34 Becker et al.
2006). The impact of autonomic dysfunction has been associated with a great
number of diseases and health issues (Thayer & Lane, 2007; Thayer & Siegle,
2002; Thayer & Sternberg, 2006). Methods of ANS alteration to increase HRV
and to decrease the inuence of the SNS in particular have been shown to have
positive effects on critical elements of bioregulation. This includes a number of
important mood and cognitive processes (Eskandari & Sternberg, 2002; Lane et
al., 2008; Thayer & Brosschot, 2005; Thayer & Siegle, 2002; Thayer, Newman, &
McClain, 1994; Tiller, McCraty, & Atkinson, 1996; Ziegelstein, 2007). Safe,
easily available nonpharmacologic methods of achieving this autonomic adjust-
ment could have potential utility and applicability over a range of health care
issues. Warm water immersion has been shown in some previous studies to have
some signicant effects upon the ANS, reducing SNS activity, and increasing
PNS inuence upon the ANS (Mourot et al., 2008; Mourot et al., 2007; Nagasawa
et al., 2001; Nishimura & Onodera, 2000, 2001). The mood and cognitive effects
of these autonomic adjustments reduce anxiety, increase working memory,
increase executive function (a complex group of cognitive skills), and attentional
regulation (Thayer & Brosschot, 2005). It is perhaps of note in this context that
Winston Churchill, a prolic writer but also someone who suffered from depres-
sion, was known for doing a great deal of his writing in the bathtub.
In healthy college-aged adults immersion in water produced a signicant
number of important physiologic changes that may provide health benets. These
include changes in blood pressure, HRV, and core temperature. The authors
believe that the cascade of these changes is intimately involved with ANS bioreg-
ulation. Further, these changes seem to be inuenced by immersion temperatures,
as a statistically signicant relationship between ANS activity manifested by HRV
and water temperatures was found. Cool water produced a rise from baseline in
SNS activity, with a drop in sympathovagal balance. This likely represents a phys-
iologic stress response. Somewhat surprisingly, when compared with cool and
neutral immersion, warm water immersion still produced a rise in sympathetic
power (while smaller) with a small drop in sympathovagal balance from baseline.
This elevation of sympathovagal balance lasted throughout the postimmersion
period of study. A rise in sympathovagal balance is associated with stress reduc-
tion, positive emotions, relaxation, and meditation (Thayer & Lane, 2000; Thayer
& Siegle, 2002; Thayer et al., 1994). Such a physiologic change causes a decrease
in cardiac irritability, a reduction in blood pressure, and a decrease in anxiety
(Thayer & Brosschot, 2005; Thayer & Lane, 2000; Thayer & Siegle, 2002; Thayer
& Sternberg, 2006; Ziegelstein, 2007).
We observed a decrease in both mean blood pressure and diastolic pressures
during the immersion period, most pronounced during the warm water cycle and
subsequent to it. This has been seen in a number of prior studies (Allison & Reger,
1998; Arborelius et al., 1972; Coruzzi, Musiari, Mossini, Ceriati, & Novarini,
1993; Gabrielsen, Warberg et al., 2000; Nishimura & Onodera, 2000; Park, Choi,
& Park, 1999; Robiner, 1990).
This study assessed 16 college-aged individuals, a relatively small number of
subjects. The subjects were all healthy and none took regular medications. A
larger sample might have given somewhat different results as would have an older
group of subjects. For this study, we chose water temperatures that were within a
narrow range. During initial work we attempted a lower temperature but found
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Warm Water Immersion 35
that subjects rst chilled and then shivered by the conclusion of the 24-min immer-
sion period. This greatly affects the ECG pickup, masking the signal with muscle
artifact. Shivering often persisted into the second tank, thus creating signal artifact
that made the HRV measurement unusable. We initially attempted 40 °C for the
warm temperature, but found that none of the subjects were able to remain in the
water for the desired 24 min immersion cycle, as core temperatures were elevated
as has been found in prior research (Allison & Reger, 1998). The 24-min immer-
sion cycle was chosen because HRV measurement requires at least a 5 min steady
state cycle, and we were interested in assessment over a period of initial accom-
modation, followed by physiologic responses during full equilibration. The four
6-min periods left us a margin of error for data cleaning.
Conclusions
We believe this to be the rst study examining the effects of water immersion
upon the ANS using various temperatures. There were striking differences between
the three immersion states, with a pronounced increase in sympathovagal balance
during the warm water immersion period, with a reduction in both diastolic and
mean blood pressure. Core temperature also increased signicantly in warm water
compared with cool or neutral temperatures. The results showed substantial indi-
vidual variation in magnitude, but not in direction. There may be clinical utility
for the effects seen during warm water immersion.
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