Intermittent exercise in the heat
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JEPonline
Journal of Exercise Physiologyonline
Official Journal of The American
Society of Exercise Physiologists (ASEP)
ISSN 1097-9751
An International Electronic Journal
Volume 4 Number 3 August 2001
Environmental Exercise Physiology
EVALUATION OF A PHYSIOLOGICAL STRAIN INDEX FOR USE DURING
INTERMITTENT EXERCISE IN THE HEAT
R.W. GOTSHALL, D.J. DAHL AND N.J. MARCUS.
Health and Exercise Science, Colorado State University, Fort Collins CO 80523.
ABSTRACT
R.W. GOTSHALL, D.J. DAHL AND N.J. MARCUS. Evaluation Of A Physiological Strain Index For Use
During Intermittent Exercise In The Heat. JEPonline. 2001;4(3):22-29. A physiological heat strain index
that includes a combination of heart rate and core temperature has been proposed as an index of heat strain for
use to monitor heat strain during continuous, steady-state exercise. The current study evaluates this
physiological heat strain index during work-rest, intermittent, exercise in the heat. Ten subjects each
participated in either a 60-minute steady-state or 60-minute intermittent (15 min exercise, 5 min rest; repeated 3
times) exercise protocol in normal (20oC, 50% RH), hot-dry (40oC, 35% RH), and hot-humid (35oC, 70% RH)
conditions. Exercise was walking on the treadmill at 1.34 m/sec (3 mph), 5% grade. Heart rate was
determined with a heart rate monitor and core temperature was determined by rectal temperature probe. The
physiological heat strain index was calculated from heart rate and core temperature. The physiological heat
strain index distinguished between normal and heat conditions for both protocols. However, only the steadstate exercise protocol resulted in physiological heat strain differences between hot-dry and hot-humid
conditions. Heat strain was less for the intermittent protocol. The results confirm previous investigations using
the physiological heat strain index during steady-state exercise in the heat, and extend the usefulness of this
index to intermittent, work-rest, exercise in the heat.
Keywords: Stress, Work, Environment, Humidity, Thermoregulation, Core Temperature, Heart Rate
INTRODUCTION
While indices of heat stress have become widely used in occupational settings (1), attempts at developing
adequate indices of heat strain have been less successful (5-7). The onset of heat-related injury is insidious.
Athletes, military personnel, firefighters, and other industrial workers often continue to perform unaware that
they are, or are rapidly becoming, victims of hyperthermia (3). From 1979 through 1996, the Centers for
Disease Control reported 6864 deaths attributed to excessive heat exposure, though not all of these occurred in
those at work or exercising (3). The increased need for workers to wear heavy, impermeable clothing while
Intermittent exercise in the heat
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working in hot environments has the potential to increase heat-related injuries. In 1994, the US Air Force
reported 19 individuals hospitalized due to excessive heat exposure (4). That same year, the US Army reported
196 individuals hospitalized for heat illness (4).
To ameliorate risk for workers and athletes who perform in the heat, both environmental heat stress and
physiological heat strain are important to consider. In order to effectively monitor the physiological heat strain,
physiological variables must be included. However, these variables should be combined to produce an effective
heat strain index for use across environments. Therefore, a simple, easily calculated physiological strain index
for use in hot environments that would provide rapid and accurate assessment of heat strain potentially could
reduce the risk of heat exposure.
Moran et al. (11) have developed a physiological heat strain index (PSI) that places the heat strain on a scale of
0 through 10. This potentially standardizes the interpretation of the heat strain across individuals and
environments. Moran and colleagues (8-11) have evaluated the PSI under a variety of hot environmental
conditions, primarily using database information on subjects who had exercised continuously in the heat. Their
data are convincing in that the PSI discriminated among the various hot, hot-humid, and normal conditions. In
one prospective study whose primary purpose was to investigate the potential influence of gender on the PSI,
the PSI again was able to discriminate among the environmental conditions (10).
Most work environments and protocols do not involve continuous, steady-state exercise, but rather, work-rest
cycles. Monitoring of heat strain during intermittent exercise is especially important for those individuals
wearing protective clothing while working in the heat, in which rest cycles are mandatory. The PSI is an index
of the physiological strain and should track the changes in heat strain occurring as rest cycles are employed.
The concern lies in the use of Tr in the setting of work-rest cycling. Tr is usually greater and responds more
slowly than the Tesophogeal (12-14) to changes in work intensity. Tr responds to changes in blood temperature
with a time constant of ~12 min, while Tesophogeal, for comparison, responds with a time constant of about ~1 min
(14). Thus, there is the concern that the PSI might not reflect the heat strain associated with work-rest cycles
during an intermittent exercise protocol due to a failure of Tr to change. However, because the PSI includes HR
as well as Tr, it was hypothesized in this study that the changes in HR associated with work-rest cycles might
obviate this concern. It was reasoned that HR would respond to work-rest cycles reflecting the changes in
metabolic heat production, and more appropriately correspond to heat strain. Therefore, the purpose of this
study was to evaluate the PSI under varying hot-humid conditions in subjects performing continuous, steadystate exercise or performing work-rest cycles. We hypothesized that the PSI would effectively discriminate
among the environmental conditions during both continuous and intermittent exercise.
METHODS
Prior to the onset of this study, approval was obtained from the Colorado State University Human Research
Committee and all procedures were in accordance with the ethical standards of the Helsinki Declaration of
1975. Twelve men and eight women volunteered for this study. All subjects were apparently healthy as
ascertained by health questionnaire. Subjects were regularly exercising a minimum of 3 days per week, 1 hour
per day, for at least the last 6 months. Subjects with a history of heat-related illness were not eligible for the
study. Subjects refrained from exercise, alcohol, and over-the-counter medication for 24 hours and from
caffeine for 12 hours prior to each test period.
Prior to the environmental exposures, each subject performed a maximal exercise stress test using a modified
Balke protocol on a Quinton Q65 treadmill (Quinton, Seattle, WA). Exercise ECG was recorded for heart rate
and rhythm determination. Expired gases were collected and analyzed using a TrueMax 2400 metabolic cart
(ParvoMedics, Sandy, UT).
Intermittent exercise in the heat
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Subjects were divided into two groups of 6 men and 4 women for the two different exercise protocols,
continuous, steady-state exercise (SS) and intermittent exercise (INT). Each subject was exposed in an
environmental chamber to each of three environmental conditions in random order, normal (N: 20oC, 50% RH),
hot-dry (HD: 40oC, 35% RH), and hot-humid (HH: 35oC, 70% RH). Following 10 min of seated rest data
collection, the SS subjects walked on the treadmill at 1.34 m/s (3 mph), 5% grade for 60 min. The INT group
walked on the treadmill under the same conditions, but walked for a 15-min interval followed by 5-min interval
of seated rest repeated up to the 60 min of total time.
Prior to entering the environmental chamber, each subject was weighed in the nude; dressed in t-shirt, shorts,
running shoes and socks; and instrumented for physiological measurements. A rectal temperature probe (YSI,
Yellow Springs, OH) was self-inserted 10 cm passed the anal sphincter. A Polar heart rate monitor (Polar,
USA) was placed around the chest. Core (rectal) temperature (Tr) and heart rate (HR) were recorded
continuously via AcqKnowledge v3.5.3 acquisition software (Biopac, Santa Barbara, CA) and stored on
computer. Core temperature of 39oC or HR in excess of 180 b/min would have terminated the heat exposure
trial. No trial had to be terminated. Tr and HR were averaged for the last 90 sec of rest, and for 60 sec at 5, 30
and 60 min of exercise for the SS protocol. Tr and HR were averaged for the last 90 sec of rest, and for the last
60 sec of each of the exercise and subsequent rest periods for the INT protocol.
Each subject was permitted water ad libitum. A weighed filled water bottle was provided that was again
weighed after the exposure to determine amount consumed. Subjects immediately exited the chamber upon
completion of the protocol, stripped, towel-dried, and obtained a nude body weight. Sweat loss and rate were
calculated by subtraction of the post-bodyweight from the pre-body weight corrected for water consumption.
The physiological strain index (PSI) was calculated according to Moran et al. (11). The equation used was:
PSI = (5 * (Tri - Tr0)/ ((39.5 - Tr0)) + (5 * (HRi - HRo) * (180-HR0))
Where Tri and HRi are simultaneous measurements taken at any time during the heat exposure; and Tr0 and HR0
are the seated resting values in the chamber prior to beginning the exercise protocol. The PSI yields values
between 1 and 10 when values for HR and Tr fall between 60-180 bpm and 36.5o-39.5oC, respectively.
A two-way (time, climate), repeated measures ANOVA was used for each of the two groups to analyze
variables for the three climatic exposures. Where differences were found, post hoc analysis was performed with
Fisher's LSD Multiple Comparison Test. For comparisons between the two groups, a two-way (time, group)
ANOVA was used for each climatic exposure, with the Fisher's LSD Multiple Comparison Test used for post
hoc analysis. All statistics were accomplished with NCSS2000 software (NCSS Kaysville, UT). Significance
was set at p<0.05.
RESULTS
Table 1 presents the subject's characteristics. There were no significant differences between the two groups.
Table 2 shows the volume of sweat lost for each climatic condition for each group. While there were significant
increases in sweat loss for both HD and HH compared to N for both groups, there were no differences between
HD and HH for either group. Sweat loss was the same for both SS and INT groups for each climatic condition.
Intermittent exercise in the heat
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Table 1. Subject Characteristics.
SS, n=10
INT, n=10
Age (years)
251.0
241.0
Height (cm)
1773.2
17.6
Weight (kg)
69.34.4
74.55.4
VO2MAX (mL/kg/min)
50.72.2
49.22.4
Values are mean SEM. SS, continuous, steady-state exercise
protocol; INT, intermittent exercise protocol.
Table 2. Sweat loss in each of the climatic conditions (mL).
SS
INT
N
19463
312114
HD
86953*
1125330*
HH
871112*
1075340*
Values are mean SEM. SS, continuous, steady-state
exercise protocol; INT, intermittent exercise
protocol; N, normal climate; HD, hot-dry climate;
HH, hot-humid climate. *p<0.05, compared to N.
Figure 1 presents the HR, Tr, and PSI data for SS exercise. HR was higher while resting in HD and HH
conditions. HR was elevated with the SS exercise, being more elevated during HD and HH, and becoming
highest in HH by 60 min. Core temperature, Tr, was the same at rest for N and HD, but slightly lower for HH.
All Tr values increased with exercise, reaching higher levels with HD and HH than for N. No differences in
exercise Tr values were detected between HD and HH. The PSI rose with exercise, but increased more with HD
and HH. PSI for HH was greater than HD by 60 min of SS exercise.
For intermittent exercise (Figure 2), HR was again slightly higher than N at rest in HD and HH. With exercisesit cycles, HR increased and declined with exercise and sit, respectively, for all conditions. During the exercisesit cycles, HR was highest for HD and HH, but not different between them. Tr was the same for all conditions
and did not change significantly over the 60 min for any condition. PSI increased with exercise and declined
with sitting rest throughout the 60 min. PSI remained higher during HD and HH, but not different between
these two conditions.
Table 3. Comparison of continuous (SS) and intermittent (INT) protocols for heart rate, core
temperature and physiological strain index.
Condition
SS
INT
N
HD
HH
N
HD
HH
Heart Rate (b/min):
Rest
663
795
755
685
755
756
Peak
984
1197
1238
968
1149
12010
Core Temperature (C):
Rest
37.10.1
37.20.1
36.90.1
37.10.1
37.30.1
37.00.2
Peak
37.40.1
37.80.1
37.70.1
37.40.1
37.70.1
37.40.3
Physiological Strain Index:
0
0
0
0
0
0
Rest
Peak
2.00.2
3.40.3
3.80.4
1.90.2
2.80.2*
2.90.4*
Values are mean SEM. *p<0.05, compared to corresponding SS climatic condition.
Intermittent exercise in the heat
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Figure 1. Responses of heart rate, rectal temperature and physiological strain index to continuous, steady-state exercise in
three climatic conditions. Values are means SEM. Significance among climatic conditions is indicated for each time period
by letter, dissimilar letters are significantly different at p<0.05. Same letters are not different from one another.
Table 3 shows the comparison of resting and peak values for HR, Tr, and PSI between the two exercise
protocols, SS and INT. Resting and peak HR were similar between the two protocols, as were Tr values. PSI
was less for INT during HD and HH than for SS under the same conditions.
DISCUSSION
Heat stress is the combination of environmental and physical work factors that constitute the heat load on an
individual while working. Heat strain is the physiological response to the heat stress that includes the direct
effect of the heat load on the body, as well as the compensatory thermoregulatory responses to the heat stress.
Heat stress increases the risk of increased frequency of accidents and therefore the safety of workers.
Additionally, the risk of heat-induced disorders such as heat cramps, heat exhaustion, and heat stroke increases
with additional heat stress. Finally, worker productivity is lowered by heat stress and the work-rest cycles
employed to reduce heat strain (1,2). The goal of any program developed for working in the heat, especially for
Intermittent exercise in the heat
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those requiring protective clothing, is to optimize productivity while maximizing safety and minimizing health
risks of individuals. Thus, effective monitoring of physiological heat strain would permit the determination of
the level of heat strain relative to threshold limit values (TLV) (1), thereby optimizing the worker's work/rest
cycles and minimizing risk.
Figure 2. Responses of heart rate, rectal temperature and physiological strain index to intermittent exercise in three climatic
conditions. Values are means SEM. Significance among climatic conditions is indicated for each time period by letter,
dissimilar letters are significantly different at p<o.05. Same letters are not different from one another.
It was hypothesized that the combining of HR and Tr within the PSI would make the PSI applicable to the
setting of work-rest cycles in the heat. The results of this study indicate that the PSI was effective in
distinguishing between the normal condition and both conditions in the heat. Furthermore, the PSI increased
and decreased accordingly with work and rest, respectively. Tr did not reflect these cycles, but HR did.
Therefore, the PSI appropriately indicated heat strain during both SS and INT protocols.
In the present study, the PSI discriminated between HD and HH conditions by 60 minutes of SS exercise,
confirming previous studies (9). In contrast, the PSI did not discriminate between these two conditions during
Intermittent exercise in the heat
28
the intermittent exercise protocol. The question is whether this an accurate depiction of the differences in heat
strain between the two protocols involving these environmental conditions, or whether the INT protocol
reduced the ability of the PSI to distinguish between these two heat conditions. In evaluating the individual
indicators of heat strain, HR and Tr, there were no differences indicated between INT and SS protocols with
regard to peak values obtained (Table 3). While, HR was different for these two climatic conditions during SS,
and not during INT. As seen in table 2, the overall heat strain was greater for the steady-state exercise than for
the intermittent protocol. Therefore, it is likely that the PSI is appropriately documenting the lesser heat strain
of the INT protocol. Additionally, the difference in heat strain between HD and HH during INT exercise is
small and likely not detected by any of the measures of heat strain. It is not surprising that INT exercise
generates less heat strain than SS exercise, as heat is permitted to dissipate from the individual during the rest
cycle and there is less heat produced metabolically.
In conclusion, this study has confirmed the use of the PSI in representing heat strain during continuous exercise
in the heat. The results also indicate that the PSI appropriately documents heat strain during intermittent
exercise. The ease of calculation and scaling of the PSI make it attractive as a monitor of heat strain under a
variety of exercise and work conditions.
Address for Correspondence: Robert Gotshall, Ph.D., Professor, Health and Exercise Science, Colorado State
University, Fort Collins, CO 80523-1582; Phone: (970) 491-6374 ; FAX: (970) 491-0445 ; email:
gotshall@cahs.colostate.edu
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