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Copyright © 2018 American College of Occupational and Environmental Medicine. Unauthorized reproduction of this article is prohibited
Journal of Occupational and Environmental Medicine, Publish Ahead of Print
DOI : 10.1097/JOM.0000000000001436
Effects of Wrist Cooling on Recovery from Exercise-Induced Heat Stress with
Firefighting Personal Protective Equipment
Emily Schlicht B.S., Ronald Caruso B.S., Kelsey Denby B.S., Alexs Matias B.S., Monique
Dudar, Stephen J. Ives Ph.D.
Department of Health and Human Physiological Sciences, Skidmore College, Saratoga Springs,
Email addresses of coauthors: email@example.com, firstname.lastname@example.org,
email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org
Stephen J. Ives, Ph.D.
Associate Chair and Assistant Professor
Health and Human Physiological Sciences
815 N. Broadway
Saratoga Springs, NY 12866
Copyright © 2018 American College of Occupational and Environmental Medicine. Unauthorized reproduction of this article is prohibited
Conflicts of Interest: None declared
Sources of Support: DhamaUSA provided funding and equipment (to SJI); though the sponsors
had no role in the design of the study; collection, analyses, or interpretation of data; in the
writing of the manuscript, and in the decision to publish the results.
Copyright © 2018 American College of Occupational and Environmental Medicine. Unauthorized reproduction of this article is prohibited
Objective: To determine the effects of wrist cooling on recovery from exercise-induced heat
stress (EIHS) from wearing firefighting personal protective equipment (PPE) and self-contained
breathing apparatus. Methods: Using a single-blind, counterbalanced, crossover-design, in 11
healthy males, we measured heart rate (HR), HR variability (HRV), core temperature (TCore),
thermal strain (TS) and fatigue at rest, during 30-min of exercise in PPE+SCBA, and during
recovery while wearing a wrist cooling band (control[off] vs. cool[on]). Results: No differences
were observed between trials at baseline or during exercise, in HR, TCore, TS, or fatigue. Time to
50% and recovery were not different between trials. Upon recovery, TCore was lower, while HR,
fatigue, HRV, and TS were relatively indifferent with cooling. Conclusion: Wrist cooling after
EIHS only modestly enhanced recovery, questioning its implementation during on-scene
rehabilitation of firefighters.
Keywords: Heart rate recovery, active cooling, fatigue, thermal strain, heart rate variability
Firefighting is an incredibly dangerous occupation that leads to a variety of physiological
stresses [1-3] and can result in loss of life [4-6]. While trauma and burns are expectedly
causative in firefighter mortality , cardiovascular-related death is also highly prevalent [4-6].
Specifically, extreme heat, dehydration, and physical exertion-induced metabolic heat
production, coupled with impaired heat dissipation associated with the use of personal protective
equipment (PPE), act in concert to dramatically increase cardiovascular demand [1, 3, 7-9],
which may contribute to the elevated cardiovascular-related death in firefighters [4-6].
Additionally, the general nature of the occupation requires irregular bouts of activity and shift
work [2, 10], which further threatens those with an already heightened risk of heart disease [11,
12]. This cardiovascular strain associated with firefighting and thermoregulation, combined with
underlying cardiovascular disease risk  or presence , are likely contributing to the fact
that ~50% of on-duty deaths are cardiac related , highlighting the need for effective
Currently, firefighters manage heat stress through on-scene rehabilitation, which is
recommended and common practice as per the National Fire Protection Association (NFPA) [13,
14]. On-scene rehabilitation offers firefighters time to rest, recover, cool down, and rehydrate, as
well as time for their vital signs to be monitored [13, 14]. However, previous work suggests that
in the line of duty firefighters often experience relatively rapid increases in core temperature of
>1.9 °C, which are slow to return to baseline, compounding core temperature during subsequent
bouts . Such heat exposure reduces time to fatigue during subsequent bouts (1 hr later), and
likely even the following day . Thus, the ability to recover core temperature from bouts of
firefighting is critical for readiness in subsequent bouts during the same call, but also mission
readiness for future calls in the same shift. Moreover, this sustained cardiovascular strain
associated with thermoregulation may be contributory to increased incidence of acute
cardiovascular events after firefighting . Collectively, enhancing recovery is critical for the
health and effective functioning of firefighters.
To this end, multiple strategies have been devised and assessed, such as partial cold water
immersion, cooling shirts, intravenous infusion of cold saline, or cool air exposure to explore the
effects of cooling on heart rate and temperature recovery from firefighting [16-20]. Cooling
shirts or vests have been demonstrated to be similar to passive cooling via cool air exposure, but
may be less effective than forearm immersion [16, 18], the efficacy of which may depend upon
the temperature of water, ambient environment, and/or the depth of hand/forearm immersion [17-
19]. However, there is also evidence suggesting there may be no difference in cooling modalities
(cooling fan, hand cooling, hand + forearm cooling, infusion of cold saline, cooling vest, or
passive cooling in temperate room) after simulated firefighting exercise in PPE . Given the
importance of cooling and reducing core body temperature during rehabilitation from firefighting
coupled with uncertainty about the efficacy and practicality of cooling methods, it has been
suggested that more work is needed to understand optimal and practical cooling strategies in
Recently, a novel wearable wrist cooling device has been developed (dhamaSPORTtm,
DhamaUSA) and could be a viable cooling intervention; however, there is a paucity of
independent peer-reviewed studies of its effectiveness. To this end, Edmonds et al. studied the
impact of wrist cooling in firefighters during recovery from live firefighting training scenarios
. Specifically, during rehabilitation firefighters were randomly assigned to wear a wrist
cooling band, which was either “on”, providing cooling, or “off”, providing no cooling. Despite
trends for improved thermal comfort and improved recovery of heart rate variability, core
temperature was not measured . While this study suggests that wrist cooling during recovery
might offer some relief from heat stress firefighters are exposed to, and thus potentially
beneficial for on-site rehabilitation, whether wrist cooling during recovery might improve the
return of core temperature remains unanswered.
Accordingly, the current study sought to determine the efficacy of the dhamaSPORTtm
cooling band on recovery from exercise-induced heat stress while wearing PPE in a workload-
matched and controlled environmental setting. It was hypothesized that dhamaSPORTtm cooling
band would improve recovery of heart rate and core temperature after physical work in
Subjects and General Procedures
Eleven healthy, physically active, male participants aged 18-34 years old were recruited
by public advertisement and word of mouth from XXX and the greater XXX region in New
York. To be considered healthy, participants could not have been current or recent (<6 months)
smokers, and those with any history of cardiovascular, renal, musculoskeletal, or metabolic
diseases were excluded. Health history was collected using questionnaires (American College of
Sports Medicine Pre-Participation Screening) to assess for eligibility. To be included, and ensure
completion of the exercise protocol, participants must have been physically active, which was
defined, in accordance with ACSM, as regularly engaging in a minimum of 30 minutes of
moderate intensity aerobic activity at least 3 days a week . Further, as core temperature was
monitored using a telemetry pill, participants also completed a screening form, and any
participant with contraindications to telemetry pill use were excluded from the study.
Participants were asked to arrive rested without recent strenuous exercise (within 24 hours) and
without alcohol or caffeine for 12 hours prior. Participants were instructed to arrive >4 hours
post prandial and hydrated, to achieve hydration they were suggested to ingest 500 ml 2-3 hours
prior and 250 ml 15 minutes prior to arrival. Participants were instructed to maintain a diet
similar in macronutrient composition and caloric content for the duration of the study, and to
avoid all dietary supplements (e,g. multivitamins, etc.). All participants provided written
informed consent prior to participation. Approval for this study was granted by the Human
Subjects Institutional Review Board (IRB#1612-569) of XXX and is in accordance with the most
recent revisions of the Declaration of Helsinki. All studies were conducted in a thermoneutral
(21 ± 1°C, 29 ± 12% relative humidity), and normobaric (~750 mmHg) environment.
This study utilized a single-blind, counterbalanced, within-subjects crossover design, to
assess the recovery from exercise-induced heat stress (Experimental Overview can be seen in
Figure 1). Specifically, all subjects were randomly assigned to complete two trials, one
experimental trial and one control trial. On both days the DhamaSPORTtm cooling band was
worn during recovery, but on one day the band was not active and the other day the band was
activated (control vs. cool). In the cooling condition when the band was activated (cool
condition) it was set to the coldest setting (7°C). While we attempted to reduce possible
anticipatory responses through single blinding and not making the participants’ aware of which
condition they were receiving, when the band was active participants’ were able to detect the
cooling, but when the band was off they were unsure. The heat transfer rate (q) for this device
ranges from 0.2-200 watts, with typical values of 0.5-50 watts, depending upon ambient
conditions. The cooling was activated prior to participants putting the band on, but is nearly
instantaneous upon activation. Study visits were scheduled to be matched for time of day on an
individual basis (range from 10am-2pm), minimally 48 hours apart, but were constrained to be
completed within 2 weeks. Participants were instructed to ingest a telemetry pill (CorTemp®
Ingestible Core Body Temperature Sensor, HQInc.) 8 hours prior to the scheduled trial time [23,
24]. To avoid potential confound of fluid intake on gastrointestinal core temperature, participants
were not allowed to consume fluids at any point during the study visit. Upon arrival, participants
height (cm), and weight (kg) were measured, and body mass index (BMI) was calculated.
Participants were then instrumented with a heart rate monitor (H7, Polar, Lake Success, NY),
presence of the telemetry pill was confirmed, and were rested for 10 minutes prior to obtaining
baseline measurements. Resting heart rate (HR), heart rate variability (HRV), core temperature
(TCore), thermal sensation (TS, 0-8 scale), rating of perceived exertion (RPE, 1-10), and visual
analog scale (VAS) for fatigue [25, 26] were measured. Over a 1 minute breathing paced period,
time domain based indices of HRV, including: root mean square of successive differences
(RMSSD), natural log transformed RMSSD (lnRMSSD), standard deviation of N-N (normal R-
R) intervals (SDNN), the number of pairs of successive N-N intervals that differ by more than 50
ms (NN50), and percent of NN50 of total NN intervals (PNN50), were measured using the Elite
HRV Application [27, 28].
After baseline measures participants donned structural personal protective firefighting
clothing (turnout coat, pants, thermal hood, helmet, gloves) and self-contained breathing
apparatus (SCBA) weighing approximately 20 kg. Based upon prior work in our lab, participants
walked on a treadmill (Trackmaster TMX428CP) for 30 minutes at 4.83 kilometers per hour and
5% incline, which has been sufficient to induce heat stress of ~1°C in young healthy controls
. During exercise HR and TCore were recorded every minute. RPE and TS were taken every 5
minutes during exercise. Upon completion, the PPE were doffed as quickly as possible, similar
to an occupational setting, the participant was then instrumented with the DhamaSPORTtm
cooling band, set to its coldest setting (~7°C), and escorted to a chair, upon sitting a timer was
started to document the time to recovery. This transition period was minimal and well-matched
between conditions HR and TCore were recorded every 30 seconds until HR recovered to within
15% of resting HR (recovery = HR<15% of HRREST). As individual differences in recovery time
were likely, upon reaching HR recovery, RPE, TS, TCore, HR, HRV, and VAS were again
Statistical evaluations were completed using SPSS (SPSS v. 22.0 IBM Inc., Armonk, NY,
USA). Time to 50% recovery was calculated to determine if wrist cooling might alter the kinetics
of recovery. Time to recovery and time to 50% recovery between conditions were compared
using paired samples t-tests. To ensure participants arrived to the lab in a similar state, paired
samples t-tests were run to determine if any significant differences existed at baseline. To
quantify if the heat stress and cardiovascular strain prior to cooling was similar during exercise
between conditions, two-way repeated measures analysis of variance (condition x time) were
used to compare the HR, RPE, TS, TCore exercise responses. Further, HR, RPE, TS, TCore during
the final minute of exercise, just prior to cooling, were compared using paired samples t-tests.
The individual nadir for HR and TCore during recovery were assessed using paired samples t-
tests. Given, individual differences in time to recovery, HR, HRV indices (RMSSD, lnRMSSD,
SDNN, NN50, and PNN50), TS, Fatigue, RPE, and TCore were compared between trials, at the
point of achieving HR recovery, using paired samples t-tests. Further, the kinetics recovery of
HR and TCore were interrogated with regression analysis. Alpha values of 0.05 were used for all
comparisons. Data are presented as means ± standard deviation.
Participants and Baseline
Participants were 11 young healthy males (23 ± 5 years, 176 ± 3 cm, 84 ± 12 kg, BMI 27
± 3 kg/m2). Baseline measures of core temperature, thermal sensation, fatigue (RPE and VAS),
HR, and HRV, were not different at the start of each trial (Table 1).
Exercise-Induced Heat Stress
No significant interaction (condition x time) or condition effects were observed in HR or
TCore throughout exercise (Figure 2, p > 0.05), though expectedly there was a main effect for time
where HR and TCore increased over time (p < 0.05). During the final minute of exercise just prior
to cooling, HR (148 ± 22 vs. 145 ±19 bpm) and TCore (37.9 ± 0.3 vs. 37.8 ± 0.3°C) were also not
different (control vs. cooling, p > 0.05). RPE and TS were also not different between trials
throughout exercise (data not shown), as no interaction (condition x time) or main effects of
condition were observed (p > 0.05), though again, expectedly there was a time effect where RPE
and TS increased over the duration of exercise (p < 0.05). During the final minute of exercise,
just prior to cooling, RPE (5 ± 1 vs. 5 ± 1) and TS (6.4 ± 0.8 vs. 6.5 ± 0.4) were not different
between trials (both, control vs. cooling, p > 0.05).
Impact of Wrist Cooling on Recovery
Time to 50% recovery (46 ± 41 vs. 43 ± 41 sec) and time to recovery (519 ± 275 vs. 624
± 289 sec) were not significantly different with the band active (p > 0.05, control vs. cooling).
Figure 3 illustrates the TCore and HR responses throughout recovery measured every 30 seconds.
The average individual nadir HR during recovery was significantly lower with the band active
(79 ± 12 vs. 72 ± 9 bpm, p < 0.05, control vs. cooling), while the average nadir for TCore during
recovery (37.75 ± 0.3 vs. 37.63 ± 0.3 °C) only approached statistical significance (p=0.08,
control vs. cooling). Despite similar intercepts, the slope for average TCore was -0.009 vs. -
0.033°C/minute for control (off) and cooling (on), respectively, or about a 279% increase in rate
of cooling. For HR, again starting from similar intercepts, the rate of HR recovery assessed by
the slope was not different with the band activated, -4 vs. -5 bpm/minute, for control (off) and
cooling (on), respectively.
Given the individual differences in HR recovery kinetics, at the individual point of HR
recovery, with the band actively cooling, TCore was significantly lower (p < 0.05) by 0.2 °C
(Table 2). Heart rate tended to be lower as well, by 4 bpm (p = 0.06) (Table 2). However, indices
of HR variability (RMSSD, lnRMSSD, SDNN, NN50, or PNN50) were unaffected (all p > 0.05,
Table 2). Thermal sensations were unaffected, but perceptions of fatigue, RPE and VAS,
approached statistical significance with the band active (p = 0.06 and p = 0.08, for RPE and
VAS, respectively, Table 2).
The purpose of this study was to evaluate the effects of wrist cooling on recovery from
exercise-induced heat stress (EIHS) while wearing PPE in a controlled environment with
matched workload and physiological demand. It was hypothesized that recovery time to resting
heart rate and core temperature would be shorter when the cooling band was activated. The main
findings from the current study are that use of the dhamaSPORTtm cooling band: did not impact
time to recovery, but at time of recovery; lowered core temperature (TCore) to a greater extent, but
only tended to lower heart rate, and lessen perceptions of fatigue. Wrist cooling via the
dhamaSPORTtm cooling band provided modest cooling, possibly beneficial in aiding recovery
from exercise-induced heat stress, particularly in recovery from situations where heat dissipation
is impeded by encapsulating PPE such as in firefighting scenarios.
Impact of Wrist Cooling on Physiological Recovery
Given the thermal stress firefighters face, through combination of environment, physical
work, and impaired heat dissipation core temperature can reach values 39°C or higher , and
the cardiovascular strain associated with the attempt to thermoregulate could be contributory in
the prevalence of acute cardiovascular events after firefighting . To mitigate this
physiological stress, multiple interventions have been devised and assessed in terms of their
efficacy in cooling and recovering heart rate after firefighting [16-20]. However, as some of
these methods are invasive (i.e. cold saline IV infusion), and some are impractical (e.g. cold
water immersion) or minimally effective, the search for cooling alternatives remains resolute
In the present study, starting from similar baselines (Table 1) coupled with matching of
workload, the physiological responses to exercise (i.e. HR and TCore) were expectedly similar
between conditions (Figure 2). Thus, any differences observed during the recovery period were
not due to differences in baseline, exercise stimulus, or response between days as trials were
counterbalanced. In these contexts, in contrast to our hypothesis, the kinetics of HR recovery
were not meaningfully different, despite trends for a lower HR during recovery (Figure 3) and
lower nadir HR during recovery period. In contrast to the study of Edmonds et al. , we did
not find an impact of wrist cooling via the dhamaSPORTtm cooling band on any measure of heart
rate variability, but did find that HR was ~5bpm lower at recovery and approached statistical
significance (p=0.06). While the mechanisms responsible this disparity are not readily apparent,
in the prior study, HR during recovery, on average, never fell below 100 bpm during the 15
minutes of rehabilitation, likely due to higher ambient temperatures, and HR variability was only
obtained on a subset (n=10) , perhaps complicating such comparison. In comparison to other
work, Barr et al. found that HR was lower during recovery with forearm immersion or combined
with a cooling vest, but not cooling vest alone or control . However, a comparison of
multiple active cooling strategies from exercise-induced heat stress found that arm or hand
immersion, fan, cold IV saline, and cooling vest were not different from passive cooling via
room temperature in terms of heart rate recovery . Although, both aforementioned studies
were lab based exercises, but in agreement in live fire training settings forearm immersion HR
recovery was also found to be not different . Thus, the wrist cooling method employed in the
current study might not affect the rate of HR recovery, per se, but might result in lower HR at
recovery perhaps better preparing individuals for subsequent bouts of activity.
Similar to HR, TCore was also trending for greater resolution with wrist cooling towards
baseline, as evidenced in the steeper slope (Figure 3) and trend for lower nadir observed during
recovery. However, TCore was significantly lower at the point of recovery (Table 2), and as time
to recovery was not different, these paired observations are supportive of a greater rate of
recovery. As the Edmonds et al. study did not measure core temperature , thus the current
study is the first to document the potential cooling effects of the dhamaSPORTtm cooling band.
Though, the exercise-induced heat stress elicited by a single moderate duration bout, while
significantly higher than resting, was relatively modest as compared to other studies with
multiple bouts ranging from 20-40 minutes [16-18]. Thus, the relatively lower starting point of
core temperature might explain the on average lower rate of cooling elicited by the band (-
0.033°C/min) when compared to previous studies (0.04-0.06 °C/min) . The multiple
differences (e.g. exercise intensity, environment, number and length of bouts, and thus degree of
EIHS) makes direct comparisons between studies problematic. In the context of the current
study, the dhamaSPORTtm cooling band did tend to increase the rate of cooling and thus lowered
the core temperature observed at recovery.
Relatedly, Edmonds et al.  reported that thermal comfort and sensation were lower
after 5 and 10 minutes with the dhamaSPORTtm cooling band on, respectively. The sustained HR
and greater improvement in thermal comfort might be due to the on average warmer and more
humid ambient conditions in which the Edmonds et al. study was conducted . In the current
study, we extend these findings by including measures of fatigue, and report that both categorical
(RPE) and continuous (VAS) reports of fatigue were lower (p=0.06-0.08) with the cooling band
activated. The current findings complement the previous work by Edmonds et al.  who
demonstrated that use of the dhamaSPORT cooling band enhanced recovery and thermal comfort
in a live-fire scenario. The improved thermal comfort with wrist cooling may be the result of
relatively dense population of sensory afferent neurons at the wrist and specifically the transient
receptor potential melastatin 8 (TRPM8), the canonical cold and menthol sensitive receptor,
activation of which has been shown to reduce thermal sensation during exercise in the heat .
This portable, lightweight, wrist cooling device modestly impacted recovery of heart rate,
core temperature, and lessened sensations of fatigue after exercise-induced heat stress. Given
cost of implementation, coupled with forearm ice water immersion being at least equally
effective, and perhaps superior, the cost-benefit ratio for on-scene rehabilitation may favor ice
water immersion. Though, wrist cooling devices might provide continued additional cooling in
transfer back to the station, when ice water immersion is perhaps impractical. Populations other
than firefighters may also benefit from this wrist cooling device, such as clinical populations
who may be heat sensitive, for example patients with multiple sclerosis [32, 33] or spinal cord
injury ; however controlled work is needed to determine if this device is beneficial in these
populations. Future studies might explore if such devices may aide in carrying out activities of
daily living in such populations.
In summary, the results of the current study demonstrate for the first time the efficacy of
the novel wrist cooling device in reducing core temperature, heart rate, and fatigue in an
occupationally relevant model of exercise-induced heat stress. These findings suggest wrist
cooling only provides modest effects on recovery from exercise-induced heat stress in young
ACKNOWLEDGEMENTS: The authors would like to thank those who graciously volunteered
for the study and the Faculty Student Summer Research Program for supporting student work
(XXX and XXX).
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Table 1. Comparison of baseline core temperature, cardiovascular, and perceptual measures
Variable OFF ON Significance
Core Temperature (C) 37.3 ± 0.3 37.3 ± 0.4 N.S.
Heart Rate (bpm) 67 ± 11 62 ± 8 N.S.
Thermal Sensation 3.5 ± 0.8 3.6 ± 0.9 N.S.
Rating of Perceived Exertion 1.2 ± 0.4 1.3 ± 0.5 N.S.
Fatigue (VAS) 0.6 ± 0.8 0.6 ± 0.9 N.S.
RMSSD (ms) 97 ± 68 119 ± 61 N.S.
LnRMSSD 4.5 ± 0.8 4.7 ± 0.6 N.S.
SDNN (ms) 143 ± 72 153 ± 78 N.S.
NN50 28 ± 13 32 ± 14 N.S.
PNN50 (%) 42± 21 50 ± 20 N.S.
Mean ± standard deviation
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Table 2. Comparison of post-recovery core temperature, cardiovascular, and perceptual
Variable OFF ON Significance
Core Temperature (C) 37.8 ± 0.3 37.6 ± 0.3 p=0.03
Heart Rate (bpm) 75 ± 12 71 ± 10 p=0.06
Time to Recovery (s) 519 ± 46 624 ± 59 N.S.
Time to 50% Recovery (s) 46 ± 41 43 ± 41 N.S.
Thermal Sensation 3.7 ± 0.8 3.8 ± 0.9 N.S.
Rating of Perceived Exertion 2.7 ± 1.4 2.3 ± 1.5 p=0.06
Fatigue (VAS) 2.9 ± 2 2.5 ± 2 p=0.08
RMSSD (ms) 84 ± 59 78 ± 44 N.S.
LnRMSSD 4.1 ± 0.9 4.2 ± 0.6 N.S.
SDNN (ms) 113 ± 61 109 ± 49 N.S.
NN50 21 ± 16 23 ± 12 N.S.
PNN50 (%) 31± 25 33 ± 20 N.S.
Mean ± Standard Deviation