Continuous palm cooling’s effect on heat transfer and physiology

Abstract

BACKGROUND:

Excess heat accrual is perhaps the body’s most dangerous exercise-induced stressor. While palm cooling uses conduction to reduce body temperatures, to date the volume of heat transferred by this treatment resulting from exercise is unknown.

OBJECTIVE:

Asses continuous palm cooling’s impact on heat transfer and physiology.

METHODS:

Thirty-one subjects did two workouts; one with, and one without, palm cooling. Workouts entailed consecutive stages of submaximal pedaling against prescribed workloads. Gloves were worn at workouts; for palm cooling 10.6^∘C gel packs were inserted into gloves at the workout’s start and conclusion. Heart rate, auditory canal and palm skin temperatures, and heat transfer across the palm were collected. Data were obtained pre-exercise, at the end of a warm-up, and at multiple times during the 25 minutes of pedaling and 30 minutes of recovery.

RESULTS:

Auditory canal temperatures had a significant treatment effect (palm cooling $<$ non-palm cooling). Palm skin temperature had an interaction, with higher non-palm cooling values at multiple times. Conversely, heat transfer also produced an interaction, but palm cooling had significantly higher values at multiple times. Heat transfer was 32% higher for the palm cooling workout.

CONCLUSIONS:

Continuous palm cooling produced significantly higher heat transfer from submaximal exercise.

Keywords

Heart rate auditory canal temperature palm skin temperature

1. Introduction

Palm cooling (PC) reduces excess body heat accrual, perhaps the body’s most ominous and dangerous exercise-induced stressor [1, 2]. Excess body heat accrual evokes sustained evaporative heat transfer that may cause dehydration [1, 2, 3, 4, 5]. Sustained fluid losses from dehydration may impair thermoregulation and cardiovascular function [2, 6, 7]. Hyperthermia from dehydration and excess heat accrual could prove fatal [4, 5, 6, 7]. PC aids evaporation’s efforts at heat removal with greater conductive heat transfer to lower internal body temperatures and abate physiological decrements created by exercise or exposure to warm and/or humid ambient conditions. Research on the ergogenic and physiological benefits provided by PC are well documented [1, 8, 9, 10, 11].

The palm’s glaborous skin covers anastomoses, which are vascular structures that connect arterial and venous circulation [9, 12, 13]. Anastomoses may also be used to aid heat loss by placing a cold object or substance against the hand’s volar surface to induce PC and conductive heat transfer [8, 9]. Most PC studies noted positive ergogenic and physiological benefits [1, 8, 9, 10, 11]. Sometimes PC’s ergogenic benefits were attributed to cold-induced vasodilation (CIVD) [1]. While cold application against the skin normally increases sympathetic vascular tone that causes cutaneous vasoconstriction [11, 14, 15], if it occurs concurrent to 1): vast amounts of heat accrual, and 2): norepinephrine binding to anastomoses’ adrenergic receptors, constriction may be overridden and evoke CIVD [1, 14, 15]. Such a scenario sees a large temperature gradient, created by body heat accrual and cold applied to the palm, elicit CIVD and induces rapid rates of heat transfer [1].

Examined in a variety of subjects and exercise modes, PC had more benefit if applied intermittently during and after bouts of physical activity that produced substantial body heat accrual [1, 11, 16]. This is not surprising given the vast increase in body heat production as persons go from rest to supramaximal exercise [2, 3]. In turn, heat accrual has profound effects on the human body, for instance a 1^∘C rise in body core temperature raises metabolic rates by 10–13% [5, 17]. Since body heat removal rates are slower than those for production, higher core temperatures are inevitable as people exercise, which can evoke hyperthermia and other health-related ailments [5, 18, 19]. Body heat transfer, such as from PC, is encouraged at such times. Yet the magnitude of cold applied against the palm should not exceed the degree of body heat increase [9]. If it does CIVD may still occur, albeit at slower rates [9, 20]. To facilitate intermittent PC, gloves were created to combat excess body heat accrual [21]. Images of the PC gloves appear in Fig. 1.

Figure 1.

Palm cooling glove. Panels A and B show the glove’s front, with a pouch applied against (A) and removed from (B), the volar surface. Panels C-E show the glove’s back. Panel C displays the pouch and a compartment comprised of insulating material. Panel D shows the pouch folded within the glove’s dorsal compartment. It is completely enclosed by Velcro within the insulating material, in Panel E.

The Fig. 1 glove had its palmar surface removed. Sewn along its hypothenar border is a mesh flap that includes a pouch which can hold a 7.6 cm diameter gel pack. The flap may be moved to the glove’s front or black and secured by Velcro to induce intermittent PC. Figure 1’s panel A shows the flap in a position to cover the hand’s palm. If the pouch held a gel pack it would cause PC and heat transfer. Other panels show the flap in various positions; they include gel pack storage in an insulated compartment on the glove’s back.

PC studies often take multiple temperature measurements to assess the treatment’s merits [1, 14, 20]. They include internal body heat measurements, as well as thermistors placed at several sites on the skin’s surface to record temperatures [14, 22]. Yet skin temperatures are, at best, an indirect and transitory index of the body’s heat removal efforts as they do not quantify the absolute volume of heat transferred. Yet thermal flux sensors, which record heat transfer, provide this information. When combined with thermistors, thermal flux sensors should yield a more comprehensive understanding of thermoregulation. To date, thermistors have not been used with flux sensors for data collection in a PC study.

The absence of flux sensor data delays understanding of PC’s true merits. Given this background and the importance of excess body heat accrual, the current study measured heat transfer before, during and after exercise. Heat transfer and other physiological variables were compared from identical workouts done either with or without PC. Since this is the first PC study to assess heat transfer and offer basic knowledge of thermal flux values, workouts entailed low-to-moderate intensity stationary cycling. The current study examined continuous, rather than intermittent, PC to quantify what a lower magnitude of heat transfer may be expected to produce from this treatment. It is hypothesized PC will yield better heat transfer and physiological results than workouts without the treatment.

2. Methods

2.1 Subjects and study design

The University of Louisville’s IRB granted approval (# 22.0425) for the current study to collect human data, which conforms to The Declaration of Helsinki guidelines. Thirty-one (mean $\pm$ sem; age 21.5 $\pm$ 0.3 years, mass 70.0 $\pm$ 2.6 kg, resting heart rate 74.4 $\pm$ 1.9 bpm) subjects completed three visits to fulfill their study involvement. They first provided informed written consent and completed a health history questionnaire. The study physician reviewed each of those documents before subjects were allowed to participate. Visits were spaced 3–7 days apart and did not exceed 21 days per subject. This was a randomized control trial, with two workouts done during each subject’s final two visits. One workout had no palm cooling (No PC), which served as the control trial. First visits were used to estimate aerobic capacity with an Astrand-Rhyming cycling test.

2.2 First visits: Astrand-rhyming test

The Astrand-Rhyming uses heart rate (HR) measurements to estimate aerobic capacity. It was used to prescribe exercise intensities for the last two visits based on each subject’s performance capabilities. The test is safer than those done to exhaustion and is well tolerated by those familiar with aerobic exercise [23]. To begin first visits, subjects wore a heart rate monitor and watch (Polar; Bethpage, NY) and sat quietly for ten minutes to measure their resting HR. Subjects then did the Astrand-Rhyming test on a stationary cycle ergometer (Monark Model 828E; Stockholm, Sweden). The test began with a three-minute warmup as subjects pedaled against a 0 kg load at 50 rpm. After the warmup subjects continued to pedal for six minutes against a predictive load whose intent was to raise HRs into a 125–170 bpm range. HR was recorded every minute of the test. If a HR at minutes five and six were not within five beats of the 125–170 bpm range, they pedaled an extra minute. Per subject, the load that elicited the 125–170 bpm HR response was used to prescribe subject’s exercise intensities for their last two laboratory 0visits.

2.3 Second and third visits: Workouts

Subjects were told to arrive for workouts well rested and hydrated. Workouts occurred in a thermoneutral (21–23^∘C, 40–50% humidity) laboratory. Subjects were not allowed to ingest fluids once data collection began. Randomization to determine workout order (No PC, PC) was decided by a coin flip. Upon arrival subjects were seated and prepped for data collection. A thermistor probe’s tip (Bio-Medical Instruments; Warren, MI) was taped to their left palm’s skin to record temperature. A thermal flux sensor (FluxTeq Model PHFS-01; Blacksburg, VA) was taped against their right hand’s palm to record the quantity heat transferred from that part of the body. Subjects then wore the Fig. 1 gloves, which did not impede palm temperature or heat transfer measurements. The gloves are easily used by a single person, yet for the current study they stayed in the panel A position, and researchers inserted and removed gel packs per study guidelines.

Figure 2.

HR results (mean $\pm$ sem). Letter superscripts (a $<$ b $<$ c) denote significant inter-time differences. Pre-ex.: pre-exercise; 5 min. ex.: 5^th minute of exercise; 10 min. ex.: 10^th minute of exercise; 15 min. ex.: 15^th minute of exercise; 22 min. ex.: 22^nd minute of exercise; Post-ex. 5: 5^th minute post-exercise; Post-ex. 10: 10^th minute post-exercise; Post-ex. 20: 20^th minute post-exercise; Post-ex. 30: 30^th minute post-exercise.

After ten minutes of sitting at the start of visits, pre-exercise data were measured. With the same monitor and watch they wore at first visits; subject’s pre-exercise HR was recorded. Auditory canal temperature (ACT), an indirect estimate of body core and cerebral temperatures, was measured with a hand-held device (Braun; Winamac, IN) [24, 25]. As an estimate of core temperatures, ACT was deemed superior to oesophageal measurements and have been used in both sedentary [25, 26] and exercise-based studies [22, 27]. Palm temperature and heat transfer data were also collected from subjects at this time. For the current study, skin temperatures were only collected from the palm. Skin temperature data from glaborous and non-glaborous sites collected before, during and after workouts showed non-glaborous skin only had non-significant inter-treatment differences [1]. Thus, current study skin temperature measurements were confined to the palm. Heat transfer values were collected at 50 Hz and averaged over 60 seconds. Per measurement, half the data were obtained both before and after a given time point; for instance, pre-exercise data, which reflects heat transfer after ten minutes of sitting, were collected from 9:30–10:30 as they sat quietly, and then averaged. Once pre-exercise measurements were collected, subjects moved to the ergometer and did a warmup.

Figure 3.

ACT results (mean $\pm$ sem). Letter superscripts (a $<$ b $<$ c) denote significant inter-time differences. A hashtag (#) represents the significant inter-treatment (No PC $>$ PC) difference. #: No PC $>$ PC; Pre-ex.: pre-exercise; 5 min. ex.: 5^th minute of exercise; 10 min. ex.: 10^th minute of exercise; 15 min. ex.: 15^th minute of exercise; 22 min. ex.: 22^nd minute of exercise; Post-ex. 5: 5^th minute post-exercise; Post-ex. 10: 10^th minute post-exercise; Post-ex. 20: 20^th minute post-exercise; Post-ex. 30: 30^th minute post-exercise.

The warmup was identical to that performed at their first visit. Immediately as the warmup concluded subjects pedaled for 15 minutes against resistance equal to 75% of their final Astrand-Rhyming load at 50 rpm. After the 15-minute period, they continued to pedal against 55% of their final Astrand-Rhyming load for seven minutes at 50 rpm, which was followed by 30 minutes of sitting. For PC workouts, new 10.6^∘C gel packs were inserted into the gloves to begin 1): warmups and 2): the 30-minute recovery period. PC workouts entailed continuous cold application, which inevitably slows heat transfer rates as temperature gradients, between palm and gel pack, diminish over time [10, 21]. Gel packs were not used for No PC workouts. Dependent variables (HR, ACT, palm temperature, heat transfer) were measured as the 10-minute pre-exercise period ended, as the warmup ended, at the 5^th, 10^th, 15^th and 22^nd minute of cycling exercise, and the 5^th, 10^th, 20^th, and 30^th minute of post-exercise recovery.

Figure 4.

Palm skin temperature results (mean $\pm$ sem). Asterisks (^*) denote data points for significant ( $p<$ 0.05) two-way interactions. Pre-ex.: pre-exercise; 5 min. ex.: 5^th minute of exercise; 10 min. ex.: 10^th minute of exercise; 15 min. ex.: 15^th minute of exercise; 22 min. ex.: 22^nd minute of exercise; Post-ex. 5: 5^th minute post-exercise; Post-ex. 10: 10^th minute post-exercise; Post-ex. 20: 20^th minute post-exercise; Post-ex. 30: 30^th minute post-exercise.

Figure 5.

Palm heat transfer rate results (mean $\pm$ sem). Asterisks (*) denote data points for significant ( $p<$ 0.05) two-way interactions. Pre-ex.: pre-exercise; 5 min. ex.: 5^th minute of exercise; 10 min. ex.: 10^th minute of exercise; 15 min. ex.: 15^th minute of exercise; 22 min. ex.: 22^nd minute of exercise; Post-ex. 5: 5^th minute post-exercise; Post-ex. 10: 10^th minute post-exercise; Post-ex. 20: 20^th minute post-exercise; Post-ex. 30: 30^th minute post-exercise.

2.4 Statistical analyses

Dependent variables were initially screened for outliers with Z-scores. Data with Z-scores $\geqslant$ $\pm$ 1.96 were excluded from further analyses. Next, data were assessed for compliance to ANOVA assumptions (normality, independence, equal variances). To address the study’s hypothesis, dependent variable data were each compared with 2 (treatment) $\times$ 10 (time) ANOVAs, with repeated measures per independent variable. Paired $t$ -tests were used as a post-hoc. An $\alpha=$ 0.05 denoted significance per ANOVA and post-hoc test. In addition, effect sizes (ES) were calculated for statistically significant differences. Arbitrary ES interpretations are as follows: $<$ 0.2 (small), 0.5 (medium), and $>$ 0.8 (large) [28].

3. Results

Each subject completed three laboratory visits. None were injured from their project participation. Anecdotal evidence from subjects affirmed the current workouts as low, or of moderate, intensity. Most stated first visits, whereby aerobic capacity was estimated, were the most difficult. There were no data no outliers, and all ANOVA assumptions were met. HR values appear in Fig. 2. ANOVA results included a significant time effect (ES $=$ 0.85). The post-hoc revealed the following significant differences: 5^th, 10^th, 15^th, 22^nd minute of exercise $>$ warmup $>$ pre-exercise, post-exercise minutes 5, 10, 20, and 30. Greater HR differences may have resulted from a higher exercise intensity.

ACT values appear in Fig. 3. ANOVA results show significant inter-treatment (No PC $>$ PC; ES $=$ 0.12) and inter-time (warmup, 5^th, 10^th, 15^th, 22^nd minute of exercise, post-exercise minutes 5 and 10 $>$ post-exercise minutes 20 and 30, pre-exercise; ES $=$ 0.45) effects. Temporal differences were due to higher amounts of body heat accrued at specific times. Yet the small but significant ACT inter-treatment difference demonstrates PC’s merits, as seen by its comparatively lower values. This denotes PC’s ability to remove heat, even when applied continuously during and after low to moderate intensity exercise.

Figure 4 displays palm temperature values. ANOVA results include a significant treatment by time interaction. Post-hoc analysis shows No PC workouts elicited significantly higher palm temperatures than the PC treatment at the warmup (ES $=$ 0.68), 5^th minute of exercise (ES $=$ 0.57), and the 5^th (ES $=$ 0.63) and 10^th (ES $=$ 0.50) minute post-exercise. Presumably, these treatment by time interactions coincided with gel pack insertion during laboratory visits, as the times they were inserted, and five minutes thereafter, during PC workouts evoked lower temperatures.

Figure 5 shows palm heat transfer values. ANOVA results include a treatment by time interaction. Post-hoc analysis shows PC workouts had significantly higher heat transfer rates than the No PC treatment at warmups (ES $=$ 0.38), and at the 5^th (ES $=$ 0.35) and 10^th (ES $=$ 0.28) minute post-exercise. Higher Fig. 5 values from PC denote more conductive heat transfer, which was most apparent at times closest to gel packs insertion and caused greater body heat removal. Summed differences for the paired inter-treatment mean values, across the ten Fig. 5 time points, show PC led to $\sim$ 24 joules greater heat loss. When expressed as a percentage, PC caused 32% greater heat transfer than the No PC treatment, despite the low to moderate exercise intensity, and continual cold application, of the current study.

4. Discussion

If left unabated, body heat accrual inevitably leads to hyperthermia and heat-related fatalities [2, 3, 4, 5, 6, 7]. The body’s reliance on evaporation to remove excess heat during exercise is very high; for instance, while at rest 65% of a person’s heat is lost through radiation, up to 85% of excess body heat is removed by evaporation as they engage in vigorous physical activity. Evaporative heat loss promotes to a paradox that sees it lower the risk of hyperthermia, but also dehydrate the body. By assisting evaporation in the body’s heat loss efforts, greater conductive heat transfer provided by PC has value at such times.

While skin temperature measurements were helped detect inter-treatment differences in PC studies with high-intensity exercise or exposure to warm environmental conditions [1, 14, 22], PC’s efficacy has been likely underreported since heat transfer values were not provided. With a randomized within-subjects design, the current study was the first exercise-based investigation to measure heat transfer and other physiological variables. Time-based data were compared from workouts done with, and without, PC. Current results support the hypothesis, as PC caused significantly more heat transfer, and lower ACT values, than workouts without the treatment. Prior studies assessed palm temperatures as an index of heat transfer, yet it is at best a transitory and indirect measure [1, 11, 20, 21]. Some assume a palm’s temperature and its heat transfer are interchangeable terms. Yet current heat transfer results had different inter-treatment outcomes than those for palm skin temperature. Thus, heat transfer, when combined with palm temperatures, provide a better understanding of thermoregulation during and after exercise. Thus, it is important to measure both, and to compare current results to those of similar studies.

Current workouts entailed low- to moderate-intensity exercise. Figures 3–5 show No PC workouts had minor palm temperature and heat transfer changes over the times examined. In contrast, lower palm temperatures from PC workouts imply CIVD, at best, was limited. Yet the PC workout evoked more heat transfer despite low intensity exercise and continual gel pack application. Despite contact with cold objects, CIVD facilitates more body heat transfer through higher palm temperatures [1]. However, the investigators examined intermittent PC and high-intensity exercise, which likely made CIVD a more likely occurrence [1]. If CIVD occurs under such conditions, conductive heat transfer is rapidly facilitated and creates an oscillatory pattern of palm temperatures that increase heat removal [1]. If CIVD does not occur with cold application it is because the 1): exercise intensity was too low, 2): temperatures against the palm were too cold, or 3): cold application was continuous rather than intermittent. One or more of these led to the lower palm temperatures seen with the current PC treatment [1, 11].

Caruso et al. saw CIVD occur in response to high-intensity leg press workouts [1]. That study, which also saw higher palm temperatures and a significant ergogenic effect from intermittent PC, administered the treatment with a 15^∘C water bath [1]. Yet multi-stage rowing ergometry workouts, which compared intermittent PC to a non-experimental control condition, did not see significant inter-treatment palm temperature differences [11]. The lack of significance was attributed to a too cold temperature (8.1^∘C) to induce PC [11]. With a similar rowing protocol, a follow-up study assessed three (10.6, 12.6, 14.9^∘C) PC temperatures for their ergogenic and physiological effects [21]. Workouts with the 10.6^∘C treatment elicited significantly higher palm temperatures than the 12.6 and 14.9^∘C conditions after the fourth stage of rowing, and at multiple times post-exercise [21]. Palm temperature differences between the current and Soltysiak studies, which both used 10.6^∘C gel packs, were likely due disparities in exercise intensity and the current study’s continual cold application procedure [21].

Unlike O’Brien et al. who intermittently applied 8.1^∘C gel packs, application of the current 10.6^∘C gel packs elicited a significant inter-treatment heat transfer effect [11]. Yet current skin temperature and heat transfer results appear to contradict each other, which is possible given the study’s lower exercise intensity and continual gel pack application. Yet the current study’s gel pack application initiated temperature gradients that evoked significant differences in heat transfer across the palm. Stark differences in palm temperature and heat transfer outcomes suggest future research should measure both to better understand the acute thermoregulatory adaptations to PC and exercise.

Summation of the current study’s inter-treatment heat transfer differences across the ten time points examined was 24 joules, which is a realistic value given the exercise intensity. Yet when expressed as a percentage difference between the two treatments, PC workouts transferred 32% more heat. Given the current study’s exercise intensity whereby body heat accrual was not thought to be a major concern, this is a surprising large difference and speaks to the merits of PC at body heat removal. Presumably, future workouts with a higher intensity and intermittent PC would transfer greater amounts of heat. Such improvements from PC may only prove more valuable at higher exercise intensities, as rates of body heat production rise [3, 8, 9].

More heat transfer from the current PC workout also explains its ACT values. PC led to significantly lower ACT values, albeit with a small ES, that nevertheless infer greater heat loss. Thermoregulation is governed by the hypothalamic preoptic nuclei. ACT estimates cerebral temperature, which is impacted by blood flow and influences hypothalamic function [29]. ACT data are deemed suspect if 1): obtained from overly cold or warm conditions, or 2): protective equipment or clothing is worn; neither of which occurred in the current study [24, 30]. Laboratory- and field-based core temperature estimates improve with ACT correction factors [31, 32, 33, 34]. In the current study, if ACT values underestimated core temperatures in a thermoneutral laboratory, it was consistent across workouts and unlikely impacted inter-treatment and/or -time differences [24, 31, 32, 34].

Other investigations that measured ACT also had results like those of the current study. ACT data were collected from subjects inside a 40^∘C chamber as they wore firefighter garments to induce heat strain [22]. In a randomized order, they received four treatments as heat loss was monitored; they included PC in 10, 20 or 30^∘C water, while a fourth was a non-cooling control condition [22]. ACT data showed PC reduced heat strain within ten minutes and was most beneficial at colder temperatures [22]. After 20 minutes 10^∘C, a temperature like the current study’s gel packs, evoked the most heat transfer as shown by ACT values [22]. Like the current study, a cooling headband investigation yielded an ACT two-way interaction [20]. Yet with 8.1^∘C gel packs inserted in its pouches; the headband predictably led to lower ACTs than a non-headband trial [20].

The current study’s significantly lower ACT values during exercise and recovery from PC are an indication of greater heat loss and support the heat transfer results for that treatment. In conclusion, current PC procedures produced a significantly higher volume of heat transfer. Yet current PC procedures did not elicit CIVD because the 1): exercise intensity was too low, 2): temperatures applied to the palm were too cold, or 3): cold application was continuous rather than intermittent. The lack of CIVD led to lower palm temperatures produced by the current PC procedure. The 32% higher heat transfer rate shows promise for continued use of the PC gloves. Heat transfer should be included in future studies that seek to address the problem of excess body heat accrual.

Author contributions

CONCEPTION: JC, PQ, BS.

MANUSCRIPT PREPARATION: JC.

WORK PERFORMANCE: NP, JW, JC.

INTELLECTUAL CONTENT: PQ, JD, BS.

DATA ANALYSIS: JC.

SUPERVISION: JC, JD.

Funding

The material is based upon work supported by NASA Kentucky Space Grant Consortium under NASA award 80NSSC20M0047.

Footnotes

Acknowledgments

We thank the subjects for their participation.

Conflict of interest

The authors report there are no interests (financial or otherwise) to declare. JF Caruso is an Editorial Board member of Isokinetics and Exercise Science but had no involvement in the peer review process of this article.

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