medicina
Article
Comparison of Subjective Workout Intensities
between Aquatic and Land-based Running in Healthy
Young Males: A Pilot Study
Chang-Hyung Lee 1, Jun Hwan Choi 2
and Soo-Yeon Kim 1,*
1
Rehabilitation Medicine, Pusan National University School of Medicine and Research Institute for
Convergence of Biomedical Science and Technology, Pusan National University Yangsan Hospital,
Yangsan 50612, Korea; aarondoctor@gmail.com
2
Department of Rehabilitation Medicine, Jeju National University Hospital, Jeju National University School of
Medicine, Jeju 63241, Korea; miraerojh0728@gmail.com
*
Correspondence: drkimsy@gmail.com
Received: 17 February 2020; Accepted: 26 March 2020; Published: 28 March 2020


Abstract: Background and objectives: Aquatic exercises have demonstrated several advantages over
land-based exercise, but only a few studies have compared the workout intensities and eﬃciencies
in a stage-speciﬁc manner. This study aimed to investigate workout intensity during aquatic and
land-based running, based on the rating of perceived exertion (RPE). Materials and Methods: Twenty
healthy young male subjects underwent a land-based running test (LRT) and an aquatic running test
(ART), in the form of a cardiopulmonary exercise treadmill test and a shallow-water running test. The
seven stages of the ART were composed of 3 minutes each of the Bruce protocol performed during
the LRT. In the ART, the participants were instructed to run in a swimming pool with matching RPE
to that obtained at each stage of the LRT. Results: Heart rate (HR) during both LRT and ART exhibited
a linear relationship (r = 0.997 and 0.996, respectively; p < 0.001). During the initial and middle
period, HR was higher in the ART than in the LRT. However, in the ﬁnal period, HR was higher in
the LRT than in the ART. Conclusions: In aquatic exercises based on the RPE obtained from the LRT,
HR exhibited a linear relationship in both the ART and the LRT. The ART appears to increase cardiac
loading more eﬃciently in the initial period and does not increase cardiac loading abruptly at a later
period. Although there is no precise, objective, controlled parameter to compare the ART and the
LRT, the RPE may be used as a convenient measurement for workout intensity in aquatic running.
Keywords: aquatic exercise;
cardiorespiratory response;
shallow-water running;
rating of
perceived exertion
1. Introduction
Aquatic exercises are popular activities in the context of ﬁtness, therapy, and rehabilitation [1].
Aquatic exercises have several advantages over land-based exercises; they lower risks of injury by
reducing musculoskeletal loading [2], provide a sense of comfort, safety and psychological beneﬁt due
to freedom from concerns of falling down [3], enable aerobic and resistance exercises to be combined
by utilizing the resistance of water [4,5], and, ﬁnally, aid in weight reduction by increasing energy
expenditure [6]. Accordingly, aquatic exercises have recently been prescribed for cardiorespiratory
ﬁtness, injured athletes, and the elderly [3,7,8].
Aquatic running methods include deep-water running (DWR) and shallow-water running
(SWR) [9]. During DWR, participants run while ﬁtted with a buoyant belt or vest across a swimming
pool. DWR diﬀers from land running in terms of kinematics and lower limb muscle recruitment [10],
due to the absence of a ground support phase and the involvement of water resistance. On the other
Medicina 2020, 56, 151; doi:10.3390/medicina56040151
www.mdpi.com/journal/medicina
Medicina 2020, 56, 151
2 of 9
hand, during SWR, participants run without any buoyant device and are typically immersed between
the waist and xiphoid process. SWR diﬀers from DWR in terms of the presence of ground reaction
force, which is lower compared to land running and depends on the depth of immersion. Regardless
of water depth, aquatic exercise has been suggested as a good therapeutic method, as it decreases axial
loading on the musculoskeletal structure [1,11]. One can exercise safely in water without inﬂicting
unnecessary load on the musculoskeletal structure due to weight [7].
However, the eﬃciency of aquatic exercise has not been clearly demonstrated in previous studies.
A majority of the studies on this topic have demonstrated that aquatic running is characterized by lower
workout intensity than land-based treadmill running, as indicated by lower cardiorespiratory responses
such as peak oxygen consumption (VO2, mL/kg/min) and heart rate [3,12,13]. This observation can be
explained by several factors such as the absence of, or a low, ground reaction force [14], diﬀerent water
temperatures [15,16], no objective measurement method such as self-selected exercise intensity [2],
and lack of familiarity with aquatic activities [17]. Unlike other exercises, methods for conventional
exercise prescription and the measurement of workout intensity for aquatic exercises are rather limited.
The most frequently used method of measuring results from aquatic exercises depends on ratings
of perceived exertion (RPE) and heart rate (HR, bpm) [1,5,18]. The former is generally rated on the
Borg CR10 Scale [19] in clinics and could be deﬁned as a subjective rating of the intensity of a speciﬁc
exercise. Physicians usually prescribe water-based exercises using an appropriate intensity scale of 0 to
10, in accordance with the workout capacity of the patients. However, the measurement of the precise
workout intensity in a controlled manner could enable an objective comparison between aquatic and
land-based exercises.
Although there has been a growth in interest in the use of aquatic exercises for therapy or training,
only a few studies have examined means of measuring or controlling the workout intensities associated
with aquatic exercises. On the basis of their ﬁndings, and on its strong correlation with heart rate
during aquatic running, Wilder et al. suggested a cadence that provided subjects with a rhythm for
regular limb movements as a measure of the intensity of aquatic running [18]. In a study on the exercise
intensities of water-based activities, Raﬀaelli et al. demonstrated that various workout intensities can
be obtained by changing movement frequencies and exercise types, such as jumping, running, and
kicking [20]. However, previous studies on aquatic exercises with buoyancy devices or anaerobic
exercises showed that these conditions could inﬂuence workout intensities. In addition to the simple
comparison of total workout intensity, there are no previous studies demonstrating the diﬀerences
in terms of intensity increment. Apparently, the above-mentioned results did not provide a precise
comparison between aquatic and land-based exercise. To compare the precise workout intensity
between the two methods, the standard protocol for exercise method and stage evaluation should be
compared using the same standard measurement methods. In addition, although aquatic exercises
have demonstrated several advantages over land-based exercises, only a few studies have compared
the workout intensities and eﬃciencies in a stage-speciﬁc manner [3,7,13]. In previous studies of
aquatic exercises, aquatic treadmills were used to control walking speed and workout intensities (HR,
VO2, and RPE) for comparison with land-based running. However, measuring cardiorespiratory
responses, such as VO2,, during running in a swimming pool in clinical practice is challenging. The aim
of this study was to assess whether subjective perception reﬂects exercise workout intensity through a
comparison between aquatic and land-based running based on the RPE.
2. Materials and Methods
Twenty healthy young males with no history of musculoskeletal injury and not undergoing any
pharmacological therapy were recruited for this study. To rule out age and sex diﬀerences, we recruited
only young males regardless of prior experience in aquatic running. To determine inclusion and
exclusion criteria, the participants were asked to respond to a questionnaire. The inclusion criteria
were as follows: (1) age ranging from 20 to 39 years (mean, 26.0 ± 7.3 years); (2) stature over 160 cm
(mean, 172.1 ± 3.7 cm); (3) body weight ranging from 60 to 75 kg (mean, 66.2 ± 7.3 kg); and (4) body
Medicina 2020, 56, 151
3 of 9
mass index (BMI) ranging from 20 to 26 kg/m2 (mean, 22.3 ± 2.7 kg/m2). All subjects received written
and oral instructions before the test and provided informed consent. This study was approved on 14
Jan 2014 by the institutional review board (IRB 05-2012-018).
2.1. Experimental Setting for Aquatic Running Test (ART)
We suggested an ART protocol for subjective exercise intensity that can be conveniently used
in clinical practice. The water level was adjusted to be between the xiphoid process and the jugular
notch. No buoyancy device was utilized to prevent any possible inﬂuence on the workout intensities.
ART was performed in the form of SWR to gain optimal exercise eﬃciencies with relatively moderate
ground reaction force and much lower physical burden compared to the land-based running test (LRT).
The workout intensities of LRT were assessed by measuring cardiorespiratory responses such as HR,
VO2, and RPE, which provide objective and subjective measures of exercise intensity. Additionally, the
workout intensities of ART were assessed by measuring HR and RPE.
2.2. Aquatic Running and Land-Based Running Procedures
All 20 subjects participated in two running tests. One was a land-based running test in the form of
a cardiopulmonary exercise treadmill test (CPET), and the other was an aquatic running test (Figure 1).
Medicina 2020, 56, x FOR PEER REVIEW 
3 of 9 
 
stature over 160 cm (mean, 172.1 ± 3.7 cm); (3) body weight ranging from 60 to 75 kg (mean, 66.2 ± 7.3 
kg); and (4) body mass index (BMI) ranging from 20 to 26 kg/m2 (mean, 22.3 ± 2.7 kg/m2). All subjects 
received written and oral instructions before the test and provided informed consent. This study was 
approved on 14 Jan 2014 by the institutional review board (IRB 05-2012-018). 
2.1. Experimental Setting for Aquatic Running Test (ART) 
We suggested an ART protocol for subjective exercise intensity that can be conveniently used 
in clinical practice. The water level was adjusted to be between the xiphoid process and the jugular 
notch. No buoyancy device was utilized to prevent any possible influence on the workout 
intensities. ART was performed in the form of SWR to gain optimal exercise efficiencies with 
relatively moderate ground reaction force and much lower physical burden compared to the 
land-based running test (LRT). The workout intensities of LRT were assessed by measuring 
cardiorespiratory responses such as HR, VO2, and RPE, which provide objective and subjective 
measures of exercise intensity. Additionally, the workout intensities of ART were assessed by 
measuring HR and RPE. 
2.2. Aquatic Running and Land-Based Running Procedures 
All 20 subjects participated in two running tests. One was a land-based running test in the form 
of a cardiopulmonary exercise treadmill test (CPET), and the other was an aquatic running test 
(Figure 1). 
 
Figure 1. The land-based running test (LRT) (A) and the aquatic running test (ART) (B). LRT was 
performed on an incline-adjustable treadmill with continuous vital sign and electrocardiographic 
monitoring. ART was performed in a swimming pool with a water level between the xiphoid-process 
and the jugular notch, with monitoring of heart rate (HR) using a water-resistant chest-strap 
transmitter. 
The LRT was performed before the ART and participants had a rest interval between the two 
testing sessions of at least 72 hours to maximize performance in each protocol.  
Each running test included a warm-up exercise for 5 minutes. The peak VO2, HR, and RPE 
during LRT were measured during the test. LRT was performed on a calibrated, incline-adjustable 
treadmill (STEX 8100T, TaeHa, Korea) with real-time recording 12-channel electrocardiographic 
monitoring (Philips Health Care 3000 Minuteman Rd., Andover, MA, USA) and vital sign 
monitoring based on the Bruce protocol. The Bruce protocol is a standard test in cardiology and 
comprises multiple exercise stages that each last 3 minutes. At each stage, the gradient and speed of 
the treadmill are elevated to increase work output, called METs (metabolic equivalent of task). 
Figure 1. The land-based running test (LRT) (A) and the aquatic running test (ART) (B). LRT was
performed on an incline-adjustable treadmill with continuous vital sign and electrocardiographic
monitoring. ART was performed in a swimming pool with a water level between the xiphoid-process
and the jugular notch, with monitoring of heart rate (HR) using a water-resistant chest-strap transmitter.
The LRT was performed before the ART and participants had a rest interval between the two
testing sessions of at least 72 hours to maximize performance in each protocol.
Each running test included a warm-up exercise for 5 minutes. The peak VO2, HR, and RPE during
LRT were measured during the test. LRT was performed on a calibrated, incline-adjustable treadmill
(STEX 8100T, TaeHa, Korea) with real-time recording 12-channel electrocardiographic monitoring
(Philips Health Care 3000 Minuteman Rd., Andover, MA, USA) and vital sign monitoring based on the
Bruce protocol. The Bruce protocol is a standard test in cardiology and comprises multiple exercise
stages that each last 3 minutes. At each stage, the gradient and speed of the treadmill are elevated
to increase work output, called METs (metabolic equivalent of task). Stage 1 of the Bruce protocol is
performed at 1.7 miles per hour and at a 10% gradient. VO2 was determined by analyzing expired
air through a breath-by-breath method using a portable telemetric system (Ultima Series™ metabolic
stress-testing system, MGC Diagnostics, Saint Paul, Minnesota). For LRT, all the participants were
Medicina 2020, 56, 151
4 of 9
instructed to increase their workout intensity on the treadmill test until the achievement of submaximal
threshold (80% of maximal heart rate or an RPE of 8~9). The exercise test was terminated on the
participant’s request or according to the guidelines of the American College of Sports Medicine [20].
RPE was assessed using a numerical rating scale, the Borg CR10 Scale (0–10).
ART was performed in a swimming pool, while heart rate was monitored using a water-resistant
chest-strap transmitter (Polar T34, Polar Electro, Inc, Kempele, Oulu, Finland), with the water level
reaching between the xiphoid process and the jugular notch (water temperature, 31 ◦C). During ART, all
participants were instructed to run by moving their arm back and forth without swimming while their
legs continued to run in the pool for 3 minutes in each stage, similar to the Bruce protocol performed in
the LRT. To gradually and constantly increase the workout intensity in water, we deﬁned the workout
intensity as HR of ART as the matching RPE that was obtained at each stage of LRT. After acquiring all
seven stages, the stages were classiﬁed as follows: initial (stages 1 and 2), middle (stages 3 to 5), and
ﬁnal (stages 6 and 7). During ART, heart rate was measured at rest and at each stage.
2.3. Statistical Analysis
Kolmogorov–Smirnov veriﬁcation was used to prove the normality of the data, which were found
to be not normally distributed. A Wilcoxon signed-rank test was used to assess the diﬀerence in
heart rate between the ART and the LRT at each stage. Spearman’s correlation was used to analyze
the stage-associated diﬀerences. Statistical analyses were performed using SPSS version 21.0 for
Windows (SPSS Inc., Chicago, IL, USA). Statistical signiﬁcance was accepted for p values < 0.05 and
<0.001 respectively.
3. Results
Twenty male subjects completed LRT and ART on separate days. At the end of each exercise,
the peak VO2, HR, and RPE in LRT were 43.8 ± 3.9 mL/kg/min, 179.5 ± 9.7 bpm, and 8.70 ± 0.82,
respectively. The peak HR for ART at stage 7 were 172.2 ± 4.7 bpm. The ﬁnal workload for the LRT
(which is equivalent to stage 7 of ART) was at a speed of 6.0 mph and at an inclination of 22% (Table 1).
Table 1. Heart rate (HR), rating of perceived exertion (RPE), and oxygen consumption (VO2) measured
at rest state and during the seven stages of the land-based running test (LRT).
Stage
HR (bpm)
RPE
VO2 (mL/kg/min)
Rest
74.9 ± 9.6
0
3.3 ± 0.8
Stage 1
91.8 ± 10.4
0.3 ± 0.2
8.9 ± 1.0
Stage 2
103.8 ± 8.5
1.2 ± 0.6
13.2 ± 1.3
Stage 3
117.3 ± 9.9
2.5 ± 0.7
18.0 ± 1.4
Stage 4
128.5 ± 9.2
3.7 ± 0.6
24.1 ± 2.4
Stage 5
144.2 ± 10.1
5.1 ± 0.8
30.3 ± 3.9
Stage 6
161.4 ± 11.2
6.8 ± 1.1
36.3 ± 4.3
Stage 7
179.5 ± 9.7
8.7 ± 0.8
43.8 ± 3.9
RPE scores were obtained using the Borg CR10 Scale (0 to 10); Values are presented as mean ± standard deviation
(SD); bpm, beats per minute.
HR during both LRT and ART exhibited a linear relationship (r = 0.997 and 0.996, respectively; p
< 0.001) (Figure 2). During the initial and middle period, HR was higher in ART than LRT; however, in
the ﬁnal period, HR was higher in LRT than ART. Statistically signiﬁcant diﬀerences were observed
between LRT and ART for HR during stages 2, 3 and 7 (p < 0.05) (Table 2).
Medicina 2020, 56, 151
5 of 9
Medicina 2020, 56, x FOR PEER REVIEW 
5 of 9 
 
in the final period, HR was higher in LRT than ART. Statistically significant differences were 
observed between LRT and ART for HR during stages 2, 3 and 7 (p < 0.05) (Table 2). 
 
Figure 2. Increase in heart rate (HR) observed during the seven stages of the land-based running test 
(LRT) (A) and the aquatic running test (ART) (B) from the rest state. Linear relationship between HR 
and the seven stages in LRT (A) and ART (B) (r = 0.997 and 0.996, respectively, p < 0.001, Spearman’s 
correlation test); bpm, beats per minute. 
Table 2. Comparison of heart rate (HR) between land-based running test (LRT) and aqua-based 
running test (ART) according to stages. 
Stage 
HR (bpm) 
Z-score 
p Value 
LRT 
ART 
 
(Mean ± SD) 
(Median) 
(Mean ± SD) 
(Median) 
Rest 
74.9 ± 9.6 
77.5 
76.1 ±6.6 
78.0 
−1.029 
0.304 
Stage 1 
91.8 ± 10.4 
91.5 
93.9 ± 7.7 
93.5 
−1.188 
0.235 
Stage 2 
103.8 ± 8.5 
106.0 
112.7 ± 11.1 
113.0 
−2.298 
0.022 * 
Stage 3 
117.3 ± 9.9 
116.5 
123.4 ± 7.5 
124.0 
−1.989 
0.047 * 
Stage 4 
128.5 ± 9.2 
129.0 
136.4 ± 8.4 
135.5 
−1.888 
0.059 
Stage 5 
144.2 ± 10.1 
144.0 
149.4 ± 7.6 
145.5 
−1.071 
0.284 
Stage 6 
161.4 ± 11.2 
161.0 
160.4 ± 6.3 
160.0 
−0.358 
0.721 
Stage 7 
179.5 ± 9.7 
179.5 
172.2 ± 4.7 
172.5 
−2.052 
0.040 * 
* p < 0.05, Wilcoxon signed-rank test; bpm, beats per minute; Z-score using the normal 
approximation to the binomial distribution. 
4. Discussion 
To prescribe exercise intensity or predict the resulting outcome, precise and controlled exercise 
protocols should be prepared. Although the beneficial effects of aquatic exercises have been widely 
accepted, the workout intensity should be considered in exercise prescription for therapy or 
rehabilitation to obtain optimal positive effects while avoiding possible injury [7,21]. 
Measurements of total calorie loss or VO2max are useful for acquiring an objective measure of 
workout intensity. However, in the absence of appropriate respiratory aquatic analysis equipment, 
obtaining such data is challenging, especially in a swimming-pool-based exercise. Theoretically, 
subjective parameters used to measure workout intensity have an advantage over objective 
parameters in reflecting the actual 'perceived intensity' of individuals. Although the same protocol 
Figure 2. Increase in heart rate (HR) observed during the seven stages of the land-based running test
(LRT) (A) and the aquatic running test (ART) (B) from the rest state. Linear relationship between HR
and the seven stages in LRT (A) and ART (B) (r = 0.997 and 0.996, respectively, p < 0.001, Spearman’s
correlation test); bpm, beats per minute.
Table 2. Comparison of heart rate (HR) between land-based running test (LRT) and aqua-based running
test (ART) according to stages.
Stage
HR (bpm)
Z-score
p Value
LRT
ART
(Mean ± SD)
(Median)
(Mean ± SD)
(Median)
Rest
74.9 ± 9.6
77.5
76.1 ±6.6
78.0
−1.029
0.304
Stage 1
91.8 ± 10.4
91.5
93.9 ± 7.7
93.5
−1.188
0.235
Stage 2
103.8 ± 8.5
106.0
112.7 ± 11.1
113.0
−2.298
0.022 *
Stage 3
117.3 ± 9.9
116.5
123.4 ± 7.5
124.0
−1.989
0.047 *
Stage 4
128.5 ± 9.2
129.0
136.4 ± 8.4
135.5
−1.888
0.059
Stage 5
144.2 ± 10.1
144.0
149.4 ± 7.6
145.5
−1.071
0.284
Stage 6
161.4 ± 11.2
161.0
160.4 ± 6.3
160.0
−0.358
0.721
Stage 7
179.5 ± 9.7
179.5
172.2 ± 4.7
172.5
−2.052
0.040 *
* p < 0.05, Wilcoxon signed-rank test; bpm, beats per minute; Z-score using the normal approximation to the
binomial distribution.
4. Discussion
To prescribe exercise intensity or predict the resulting outcome, precise and controlled exercise
protocols should be prepared.
Although the beneﬁcial eﬀects of aquatic exercises have been
widely accepted, the workout intensity should be considered in exercise prescription for therapy or
rehabilitation to obtain optimal positive eﬀects while avoiding possible injury [7,21].
Measurements of total calorie loss or VO2max are useful for acquiring an objective measure of
workout intensity. However, in the absence of appropriate respiratory aquatic analysis equipment,
obtaining such data is challenging, especially in a swimming-pool-based exercise. Theoretically,
subjective parameters used to measure workout intensity have an advantage over objective parameters
in reﬂecting the actual ’perceived intensity’ of individuals. Although the same protocol for workout
intensity is performed in water, the output (HR in this study) could be inconsistent because exercise
output could be strongly inﬂuenced by individual physical characteristics (e.g., running performance in
water) [10] and environmental factors (e.g., water temperature, relative depth of water) [16,22]. In this
study, we measured the subjective RPE value, and the gradual increase in its intensity in accordance
with each stage of the LRT. Due to the viscosity and consistency of water, cadence should not be used
Medicina 2020, 56, 151
6 of 9
as the standard measurement method as speed increases. Diﬃculty perceptions in the ART increased
with increasing intensity, which corresponded to the RPE in the LRT. HR in both the LRT and the ART
with matching RPE exhibited a linear relationship (r = 0.997 and 0.996, respectively; p < 0.001), and
there were no statistical diﬀerences, except in stages 2, 3, and 7. Although there is no precise objective
parameter compared between the ART and the LRT, the RPE may potentially provide a fundamental,
convenient, and meaningful parameter in aqua that reﬂects an “actual perceived workout intensity”
measure in individuals.
In the present study, HR as exercise workout of ART and LRT were assessed and compared
(Table 2). The plots shown in Figure 2 show an increase in HR and during the three deﬁned test
periods for both ART and LRT. Initially, ART exhibited higher HR values than LRT during the initial
and middle periods. Conversely, HR was lower in ART than LRT during the ﬁnal period (stage 5 to
6). Theoretically, the ground reaction force due to counter dragging force can be controlled in our
method, thus enabling exercise intensity to be increased gradually during the course of the test. The
maximum kinetic eﬀort was observed during the early stage of the ART, presumably due to the higher
metabolic energy required to overcome the drag force on lower limbs to initiate aquatic running [14],
and at this stage HRs were higher for the ART than in the LRT. Secondly, the mechanical counterforce
corresponding to the water depth should be considered. If a person moves more vigorously in water,
more counterforce is imposed, thus leading to an increase in HR and RPE. However, the kinetic eﬀort
could not increase exponentially, due to the enhanced counterforce at a later period. Despite the greater
eﬀort at a later period, the actual increase in cardiac loading turned out to be small compared to LRT.
It was hypothesized that precise measurement of the workout intensity and eﬀectiveness of aquatic
exercise in a stage-speciﬁc manner could allow objective comparisons, thus enabling prescription of
aquatic exercises in a more objective and safe manner in the clinical setting. In the present study,
comparison of the aquatic workout intensity with LRT using RPE as a parameter provided subjective
data which could be useful in clinical practice. A higher HR in ART compared to LRT was observed
until the middle exercise period, therefore suggesting a higher magnitude of enhancement of workout
intensities during the initial and middle period. Aquatic running, a higher-intensity exercise workout
that is less physically burdening than land-based running, is likely to be especially advantageous for
elderly, obese, and severely ill patients. Consequently, aquatic exercise can be recommended over land
exercise for patients with a deconditioned or weakened physical status (for example, patients suﬀering
from osteoarthritis or rheumatoid arthritis) [23]. Furthermore, the lower HR during the ﬁnal period of
ART relative to LRT provides a safer exercise window. Considering these aspects, ART could be safely
recommended in clinical settings, as it is also not as demanding as LRT with regard to its physical
burden, especially in patients with a low cardiac ejection fraction below 55% (e.g. heart failure) [24].
There are several possible explanations for the observed workout diﬀerences. The physiological
adaptation to water should be considered. In a cross-sectional study, it was demonstrated that
subjects who participated in a session of aquatic exercise achieved acute adaptation. For example, as
pool water temperature decreased, HR increased rapidly in response to sympathetic nervous system
stimulation [16,22]. The increase in peripheral resistance with vasoconstriction is due to the blood being
redirected from the periphery to maintain core temperature [25]. It has been shown that immersion at
neutral temperature (32 ◦C) lowers HR by 15%, but immersion in cold water (14 ◦C) increases HR by
5% [16]. In the present study, the swimming pool temperature was maintained at 31 ◦C; therefore, we
believe that the eﬀect of water temperature on HR was minimal during the early stage of exercise. On
the other hand, RPE has been reported to decrease with the depth of immersion [26], and, in another
study, high RPE appeared to be positively related to ground reaction force, the amount of drag force
on lower limbs [14], and changes in the neuromuscular patterns of active muscles [27]. As ground
reaction forces are always lower during shallow water running (SWR) than during land running,
musculoskeletal burden and the risk of injury could be reduced during SWR.
However, previous studies have demonstrated a decrease in HR with an increase in the water
depth [17,26], and that the decline in HR is associated with the inﬂuence of hydrostatic pressure
Medicina 2020, 56, 151
7 of 9
and buoyancy on the stroke volume of heart and consequent alteration in blood distribution in the
body [18]. In the present study, the water level was adjusted to a level between the xiphoid process
and the jugular notch. This depth was more than the speciﬁc level used for SWR but shallower than
that used for DWR. Consequently, it was hypothesized that the lower HR observed during the later
period of ART compared to LRT was due to the water depth employed in this study.
Generally, the water depth in swimming pools is usually at adult chest height, and a similar level
was employed in the present study. Apparently, it is hypothesized that our ﬁndings are meaningful,
based on the observation that the workout intensities of aquatic exercises reﬂect real situations, and are
applicable to aquatic exercises in clinical practice.
There are several results of clinical importance in our study. As there was previously no exact
guideline on intensity in aquatic therapy, these ﬁndings might suggest a possible guideline for aquatic
exercise. Initially, ART can increase the heart rate eﬃciently, which appears to be a good choice for
patients in a deconditioned state or unable to walk. Meanwhile HR during the ART rises slower than
during the LRT at later stages. This relatively safe ‘window period’ could be safely suggested in
patients with cardiac problems. In general, heart rate increases linearly in relation to RPE. However,
the heart rate did not increase linearly in later stages of ART. This might reﬂect the increasing viscosity
and resistance with the increase in running eﬀort, increasing demand for endurance and muscle fatigue
in the water, and hence increasing thermal loss in ART.
There are several limitations of this study. First, all study subjects were young male adults; thus,
our results are applicable primarily to this population and not to female subjects, the elderly, or patients
with cardiac problems. Second, we only considered the partial inﬂuence of cardiorespiratory response
on the workout intensity of the ART by measuring HR with controlled RPE, but not VO2, because of
the absence of an appropriate respiratory gas analysis system at the swimming pool. In future studies,
it will be necessary to conﬁrm whether RPE, a subjective parameter, is a meaningful indicator that can
reﬂect and objectively measure exercise intensity in aquatic exercises in various conditions.
5. Conclusions
This study demonstrates the workout intensities of aquatic and land-based running based on
RPE. The workout diﬃculty perceptions in aquatic running increased with increasing intensity, which
corresponded to RPE in the land-based running test. Although there is no precise comparison of
controlled cardiorespiratory measurement to compare aquatic and land-based running, RPE may be
used as a meaningful measurement for workout intensity in aquatic running.
Author Contributions: Writing original draft, C.-H.L.; writing-review & editing, S.-Y.K.; methodology, J.H.C. All
authors have read and agreed to the published version of the manuscript.
Funding: This research was supported by a grant of the Korea Health Technology R&D Project through the Korea
Health Industry Development Institute (KHIDI), funded by the Ministry of Health & Welfare, Republic of Korea
(grant number: HI17C2397).
Conﬂicts of Interest: The authors declare no conﬂict of interest.
References
1.
Barker, A.L.; Talevski, J.; Morello, R.T.; Brand, C.A.; Rahmann, A.E.; Urquhart, D.M. Eﬀectiveness of aquatic
exercise for musculoskeletal conditions: A meta-analysis. Arch. Phys. Med. Rehabil. 2014, 95, 1776–1786.
[CrossRef]
2.
Colado, J.C.; Triplett, N.T. Monitoring the intensity of aquatic resistance exercises with devices that increase
the drag force: An update. Strength Cond. J. 2009, 31, 94–100. [CrossRef]
3.
Choi, J.H.; Kim, B.R.; Joo, S.J.; Han, E.Y.; Kim, S.Y.; Kim, S.M.; Lee, S.Y.; Yoon, H.M. Comparison of
cardiorespiratory responses during aquatic and land treadmill exercise in patients with coronary artery
disease. J. Cardiopulm. Rehabil. Prev. 2015, 35, 140–146. [CrossRef] [PubMed]
Medicina 2020, 56, 151
8 of 9
4.
Valtonen, A.; Pöyhönen, T.; Sipilä, S.; Heinonen, A. Eﬀects of aquatic resistance training on mobility limitation
and lower-limb impairments after knee replacement. Arch. Phys. Med. Rehabil. 2010, 91, 833–839. [CrossRef]
[PubMed]
5.
Colado, J.C.; Tella, V.; Triplett, N.T. A method for monitoring intensity during aquatic resistance exercises.
J. Strength Cond. Res. 2008, 22, 2045–2049. [CrossRef] [PubMed]
6.
Barbosa, T.M.; Marinho, D.A.; Reis, V.M.; Silva, A.J.; Bragada, J.A. Physiological assessment of head-out
aquatic exercises in healthy subjects: A qualitative review. J. Sports Sci. Med. 2009, 8, 179–189.
7.
Kim, E.; Kim, T.; Kang, H.; Lee, J.; Childers, M.K. Aquatic versus land-based exercises as early functional
rehabilitation for elite athletes with acute lower extremity ligament injury: A pilot study. PM&R 2010, 2,
703–712.
8.
Irandoust, K.; Taheri, M. The eﬀects of aquatic exercise on body composition and nonspeciﬁc low back pain
in elderly males. J. Phys. Ther. Sci. 2015, 27, 433–435. [CrossRef]
9.
Dowzer, C.N.; Reilly, T.; Cable, N.T.; Neville, A. Maximal physiological responses to deep and shallow water
running. Ergonomics 1999, 42, 275–281. [CrossRef]
10.
Killgore, G.L.; Wilcox, A.R.; Caster, B.L.; Wood, T.M. A lower-extremities kinematic comparison of deep-water
running styles and treadmill running. J. Strength Cond. Res. 2006, 20, 919–927. [CrossRef]
11.
Waller, B.; Lambeck, J.; Daly, D. Therapeutic aquatic exercise in the treatment of low back pain: A systematic
review. Clin. Rehabil. 2009, 23, 3–14. [CrossRef] [PubMed]
12.
Silvers, W.M.; Rutledge, E.R.; Dolny, D.G. Peak cardiorespiratory responses during aquatic and land treadmill
exercise. Med. Sci. Sports Exerc. 2007, 39, 969–975. [CrossRef] [PubMed]
13.
Phillips, V.K.; Legge, M.; Jones, L.M. Maximal physiological responses between aquatic and land exercise in
overweight women. Med. Sci. Sports Exerc. 2008, 40, 959–964. [CrossRef] [PubMed]
14.
Miyoshi, T.; Shirota, T.; Yamamoto, S.; Nakazawa, K.; Akai, M. Eﬀect of the walking speed to the lower limb
joint angular displacements, joint moments and ground reaction forces during walking in water. Disabil.
Rehabil. 2004, 26, 724–732. [CrossRef] [PubMed]
15.
Bergamin, M.; Ermolao, A.; Matten, S.; Sieverdes, J.C.; Zaccaria, M. Metabolic and cardiovascular responses
during aquatic exercise in water at diﬀerent temperatures in older adults. Res. Q. Exerc. Sport 2015, 86,
163–171. [CrossRef] [PubMed]
16.
Sramek, P.; Simeckova, M.; Jansky, L.; Savlikova, J.; Vybiral, S. Human physiological responses to immersion
into water of diﬀerent temperatures. Eur. J. Appl. Physiol. 2000, 81, 436–442. [CrossRef] [PubMed]
17.
Town, G.P.; Bradley, S.S. Maximal metabolic responses of deep and shallow water running in trained runners.
Med. Sci. Sports Exerc. 1991, 23, 238–241. [CrossRef]
18.
Wilder, R.P.; Brennan, D.; Schotte, D.E. A standard measure for exercise prescription for aqua running. Am. J.
Sports Med. 1993, 21, 45–48. [CrossRef]
19.
Borg, E.; Kaijser, L. A comparison between three rating scales for perceived exertion and two diﬀerent work
tests. Scand. J. Med. Sci. Sports 2006, 16, 57–69. [CrossRef]
20.
Thompson, W.R.; Gordon, N.F.; Pescatello, L.S. ACSM’s Guidelines for Exercise Testing and Prescription;
Lippincott Williams & Wilkins: Philadelphia, PA, USA, 2010.
21.
Wilcock, I.M.; Cronin, J.B.; Hing, W.A. Physiological response to water immersion. Sports Med. 2006, 36,
747–765. [CrossRef]
22.
Bartels, E.M.; Juhl, C.B.; Christensen, R.; Hagen, K.B.; Danneskiold-Samsøe, B.; Dagﬁnrud, H.; Lund, H.
Aquatic exercise for the treatment of knee and hip osteoarthritis. Cochrane Database Syst. Rev. 2016, 23.
[CrossRef] [PubMed]
23.
Adsett, J.A.; Mudge, A.M.; Morris, N.; Kuys, S.; Paratz, J.D. Aquatic exercise training and stable heart failure:
A systematic review and meta-analysis. Int. J. Cardiol. 2015, 86, 22–28. [CrossRef] [PubMed]
24.
Bonde-Petersen, F.; Schultz-Pedersen, L.; Dragsted, N. Peripheral and central blood ﬂow in man during cold,
thermoneutral, and hot water immersion. Aviat. Space Environ. Med. 1992, 63, 346–350. [PubMed]
25.
Barbosa, T.M.; Garrido, M.F.; Bragada, J. Physiological adaptations to head-out aquatic exercises with
diﬀerent levels of body immersion. J. Strength Cond. Res. 2007, 21, 1255–1259. [PubMed]
Medicina 2020, 56, 151
9 of 9
26.
Fujisawa, H.; Suenaga, N.; Minami, A. Electromyographic study during isometric exercise of the shoulder in
head-out water immersion. J. Shoulder Elb. Surg. 1998, 7, 491–494. [CrossRef]
27.
Reilly, T.; Dowzer, C.N.; Cable, N.T. The physiology of deep-water running. J. Sports Sci. 2003, 21, 959–972.
[CrossRef]
© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).

Comparison of Subjective Workout Intensities between Aquatic and Land-based Running in Healthy Young Males: A Pilot Study.
Lee, Chang-Hyung,Choi, Jun Hwan,Kim, Soo-Yeon