Palm Cooling and Heating Delays Fatigue During Resistance Exercise in Women

Journal of Strength and Conditioning Research Publish Ahead of PrintDOI: 10.1519/JSC.0b013e31829cef4e

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PALM COOLING AND HEATING DELAYS FATIGUE DURING RESISTANCE EXERCISE IN WOMEN

Young sub Kwon1, Robert A. Robergs2, Christine M. Mermier3, Suzanne M. Schneider3, Alfred B. Gurney4

Department of Kinesiology, Washburn University, Kansas1; School of Human Movement Studies, Charles Sturt University, Bathurst, Australia; Department of Health, Exercise & Sports Sciences, University of New Mexico, New Mexico3; Department of Orthopaedics and Rehabilitation, University of New Mexico School of Medicine, New Mexico4.

Running title: Palm Temperature and Fatigue in Women

Address for Correspondence:

Young Sub Kwon Washburn University Department of Kinesiology Petro 201 G 1700 SW College Ave. Topeka, KS 66621 Phone: (785) 670-1965 Fax: (785) 670-1059 Email: [email protected]

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ABSTRACT

We previously reported that cold application to the palms between sets of high intensity bench

press exercise produces an ergogenic effect in men. In this study we hypothesized that palm

cooling or heating during rest intervals between high intensity weight training sets will increase

total repetitions and exercise volume-load (kg) in resistance trained female subjects in a thermo-

neutral environment. Eight female subjects (mean±SD, age = 25±6 yr, height = 160±6 cm, body

mass = 56±7 kg, 1RM = 52±6 kg, weight training experience = 6±2 yr) completed 4 sets of 85%

1RM bench press exercise to failure, with 3 min rest intervals. Exercise trials were performed in

counterbalanced order on 3 days, separated by at least 3 days in ThermoNeutral (TN), Palm

Heating (PH), and Palm Cooling (PC) conditions. Heating and cooling were applied by placing

both hands in a hand cooling device with the hand plate set to 45 °C for heating and 10 °C for

cooling. Data were analyzed using a 2-factor repeated measures ANOVA and Tukey’s post hoc

tests. PC repetitions were significantly higher than TN during the 2nd set and PH repetitions

were significantly higher than TN during the 4th set. Total exercise volume-load (kg) for both

PC (1387±358) and PH (1349±267) were significantly higher than TN (1187±262). In women,

both heating and cooling of the palms between sets of resistance exercise increased the total

exercise volume-load performed. This ergogenic response to a peripheral sensory input is

consistent with the central governor theory of muscular fatigue.

KEY WORDS: palm temperature, EMG, central governor theory, sex differences in muscular

endurance

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Paragraph 1

INTRODUCTION

Cryotherapy is a time honored treatment in rehabilitation to reduce inflammation and

control pain. In addition, cryotherapy has been used after exercise to reduce post exercise

discomfort and inflammation (14). Recently, the role of cooling has been expanded to include an

ergogenic effect in athletes in short and intense resistive exercise (4, 15, 32). Verducci (32)

reported cooling of the skin over the exercising muscle during rest periods between sets as a way

to decrease muscular fatigue and increase weight lifted during weight training. Other research on

the effects of local cooling on muscle function during exercise has been inconsistent.

Discrepancies are likely due to different cooling modalities such as cold or ice water baths, gel

packs, or plastic bags with ice cubes, different periods of cooling ranging from 10 to 45 minutes

over the muscle of interest, as well as differences in body regions (5, 11, 24).

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In contrast to Verducci (32), who cooled the skin directly over the exercising muscle,

Hopkins et al. (10) and Palmiei-Smith et al. (21) applied cooling to a joint close to the exercising

muscle. They found increased local muscle reflexes, muscle excitability, and short-term release

of neurotransmitters from the central nervous system. These findings suggest that local cooling

of the periphery may enhance motor neuron output of the active muscles even when the cooling

does not involve the active muscle directly. These findings also suggest a central nervous system

processing of peripheral afferent stimulation or direct central effects of cooling on exercise

performance.

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Less is known about the effects of heat on muscular strength and endurance. There is

some information on the application of deep heat (i.e. ultrasound and shortwave or microwave

diathermy) directly to the muscle (1, 3), but a direct, deep heat such as those used in these studies

could affect mechano-elastic and/or other properties of the muscle. Studies that used superficial

heat directly over the muscle have shown mixed results with regard to strength. Long and

Hopkins (16) showed that a superficial hot pack application did not affect the motoneuron-pool

recruitment or peak torque of the soleus. Thornley et al. (30) showed that superficial application

of heat did not increase knee extensor peak isometric torque. Eyigör et al. (8) applied superficial

heat to patients with osteoarthritis of the knee and found increases in isokinetic muscle strength.

Since all of the above studies either used deep heat (1, 3), heat directly over the muscle tested

(16, 30), or subjects with pathology (8), they differed from our study enough to preclude

generalizability.

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It is reported that there are differences between men and woman with regards to fatigue,

with the majority of the studies showing women have a greater resistance to fatigue compared to

men (7, 12, 13, 25). There also are reported differences in thermal perception between men and

women (6, 17, 19, 26). We previously reported that cold application to the palms during high

intensity bench press exercise produces an ergogenic effect in men (15). No study has examined

this response in women.

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This investigation was undertaken to determine if there are significant differences in

muscular fatigue when trained female subjects perform sets of high intensity dynamic exercise

(bench press) with cooling or heating applied to the hands during rest intervals between sets. In

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addition, by comparing these female results to our previous study using male subjects with

identical protocol (15), we wanted to determine if cooling and heating to the hands affects

exercise performance differently in men and women. We hypothesized that total volume-load

during multiple sets of high intensity bench press exercise will be increased by local hand

cooling or heating compared to the thermoneutral condition in females. We also hypothesized

that because women have heightened thermal and pain perception, women will demonstrate

greater increases in volume-load in response to palm cooling and heating, as compared to our

previous findings in men.

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Documenting improved capacity for resistance exercise through sensory neural

manipulation would add to the growing body of evidence for the importance of central

processing of neurological input signals in influencing fatigue (20, 31). These results also may

have practical application for athletes or patients performing resistance training.

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METHODS

Subjects. Eight healthy female subjects volunteered for this study. The subjects had participated

in regular, intense weight training for a minimum of five years, and their ratio of weight pressed

to body weight during bench press was more than 80% of age-based upper body strength. The

protocol for this study was approved by the University of New Mexico Human Research Review

Committee and all subjects provided informed, written consent prior to participation. Subjects

were screened for cardiovascular and musculoskeletal disease using a medical history

questionnaire, an activity questionnaire, and the Physical Activity Readiness Questionnaire

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(PAR-Q). Subjects were also screened for body composition to improve signal detection from

surface EMG. After the initial screening, body density was determined using the sum of three

skin fold sites and the Jackson & Pollock equations; ethnic and sex specific equations were used

to calculate percentage of body fat from body density. Subjects were excluded from the study if

they had more than two positive cardiovascular risk factors as outlined by the American College

of Sports Medicine, they were taking ergogenic supplements that could affect exercise

performance, or had body fat > 25%. Subjects were instructed to not exercise the day prior to a

trial, to refrain from caffeine ingestion (coffee, tea) the morning of each trial, to otherwise follow

their normal diet, and to eat a light meal two hours before coming to the laboratory.

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Overall protocol. There were a total of four experimental days, with at least 3 days between

each trial. During the first testing day, subjects were familiarized with the testing protocol. They

performed a 1RM supine bench press test, and after 5 min of rest they completed one endurance

set to fatigue at 85% of 1RM. Before the first 1RM test, each subject gained familiarity with the

barbell bench press exercise by performing it under the guidance of the primary investigator.

Subjects were trained to perform each concentric and eccentric phase of the bench press through

a fixed range of motion and at a rate of two seconds up and two seconds down in time. During

the other three days, they performed the same 1RM protocol and one set of 85% 1RM as

performed during the first day, but then they also performed three additional endurance sets at

85% of the 1RM determined on the first testing day. On each testing day, during the rest periods

between sets 1-2, 2-3, and 3-4 the hand was exposed to one of the three temperature conditions,

either 1) thermoneutral (TN) with negative pressure, 2) local palm cooling (PC) with negative

pressure, or 3) local palm heating (PH) with negative pressure (Figure 1). The same temperature

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condition was used during the three rest periods on a given testing day. On the first testing day,

the subjects were randomly assigned to a counterbalanced design for temperature conditions to

minimize any learning or order effects. All tests were done at the same time of day for each

subject. All testing took place at an altitude of 1572 m (PB=635mmHg) in the northern

hemisphere in the months of April through May. Female subjects were tested during days 3-14 of

their menstrual cycle (day 1 is first day of bleeding).

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Treatment conditions. Hand cooling and heating with negative pressure were induced by using

two Rapid Thermal Exchanger (RTX) hand cooling devices, one on each hand (RTX

Heating/Cooling Model # 200962-006B, AVAcore Inc., Palo Alto, CA). The RTX consists of a

metallic cone heat-exchanger surface on which the palm of the hand is placed and a plastic

chamber that encloses the hand. A seal above the wrist maintains a vacuum around the hand. Air

is pumped from the device and negative pressure can be controlled and maintained. The

circulating water temperature was maintained at 10°C during the cooling trials and 45°C during

the heating trials, and negative pressure was maintained at 45 mmHg. The 10°C temperature

was chosen based on prior research of hand immersion showing that this temperature is optimal

and causes limited vasoconstriction in the hand. For cooling and heating trials, the treatment was

implemented for 2.5 min during each rest period between sets 1 and 2, 2 and 3, and 3 and 4. The

control trials consisted of placing the hand in the RTX with negative pressure but without

application of cold or hot water.

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Day 1: 1RM and 1 85% of 1RM endurance test On a testing day, participants were asked if

they had any soreness or injury to their shoulders, triceps, and chest and if they had refrained

from caffeine and vigorous exercise in the previous 24 hours. If subjects complied with these

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requirements, they did their usual warm-up and then positioned themselves under the bar with

their usual grip. The positions of the minimus fingers of each hand were marked on the bar to

insure the same grip distance on the bar during all tests. The type of grip used (closed or open)

was self-chosen. Supine bench press strength was assessed by measuring the 1 repetition max

(1RM), and after 5 minutes an 85% of 1RM endurance test was performed. Subjects were

required to perform a warm-up of 10 repetitions at 50% of (predicted) 1RM, 5 repetitions at 70%

of 1RM, 3 repetitions at 80% of 1RM, and 1 repetition at 90% of 1RM, followed by 3 attempts to

determine the subject’s actual 1RM. All subjects were given 3 minutes of rest between sets.

After the 1RM test, subjects had a 5-minute rest period and then tried to lift as many repetitions

as possible using 85% of 1RM.

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Days 2, 3, and 4: 1 RM and four 85 % of 1RM endurance tests

The participants were required to perform a warm-up of 10 repetitions at 50% of 1RM, 5

repetitions at 70% of 1RM, 3 repetitions at 80%, and 1 repetition at 90% of 1RM, stretching

chest, shoulder and triceps between sets. After 5 minutes they lifted their 1RM. This 1RM test

was performed before each fatigue test to normalize the EMG signals and was not used to

determine the weight to lift. Because the EMG signal from the muscle surface varies from one

location to another, the absolute EMG signal cannot be compared between separate days. After

five min of rest, four sets with weights of 85% of the first day 1RM were performed until fatigue.

Between each set they rested for three minutes. The 3-min rest periods between sets 2, 3, and 4

consisted of a 15 second transition from the exercise to the treatments, 2 minutes 30 seconds of

PC (10°C), PH (45 °C), or a TN trial with negative pressure, and another 15 second transition

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from the rest period to the next set. EMG measurements were obtained throughout the 1 RM and

four endurance tests. (Figure 1).

Insert Figure 1 here

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Specific Measurements:

Palm Temperature. Uncovered skin thermistors (Grant Instruments Ltd, Cambridge,

UK) were attached to the right palm with elastic straps during the TN condition. During PC and

PH conditions, palm temperature was measured by the RTX (by thermocouples embedded in the

hand cone). In each condition, palm temperatures (Tpa) were recorded during 2.5 min of the rest

periods. The thermistor was connected to a data logger (Squirrel™, Grant Instruments Ltd,

Cambridge, UK), which recorded palm temperature during the TN condition every five seconds.

Palm temperature during PC and PH conditions were obtained from the RTX and recorded

manually every 15s.

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EMG. Gel electrodes were placed on the belly of the following muscles on the left side

of the body aligned parallel to the muscle fibers; the sternal head of the pectoralis major (PM),

the anterior deltoid (AD), the long head of the triceps brachii (LT), the lateral head of the triceps

(LTT) brachii, and on the styloid process of the ulna as a ground. In all cases the line between

two electrodes was parallel to the muscle’s line of pull. The electrode sites were prepared by

abrasion with sandpaper, and swabbing with alcohol pads to lower skin resistance. Each

electrode was secured by adhesive tape. The electrode sites were marked using an anatomical

pen on the skin to insure the same site was used on different days. Muscle EMG voltage signals

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were acquired at a rate of 1500 Hz (MyoSystem 1200, Noraxon Inc., Scottsdale, Arizona),

diverted to an analog signal acquisition system through a 68 pin junction box (CA1000 unit,

National Instruments, Austin, TX), connected in series to a data acquisition card (National

Instruments, Austin, TX) and collected using custom written software (LabVIEW, National

Instruments, Austin, TX). To aid in post-collection signal processing, an electronic goniometer

(Biopac Systems, Santa Barbara, CA) was attached to the elbow to provide a signal at 1500 Hz

to monitor changes in elbow flexion and extension. No filters were applied to raw EMG data.

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Raw EMG signal processing was completed post collection using custom written

software which provided objective automated analyses that did not require independent

interpretation by the researchers (LabVIEW, National Instruments, Austin, TX). The program

involved isolation of EMG signals from the PM, AD, LT and LTT during every muscle

contraction based on the goniometer signal. The concentric and eccentric movement from each

contraction was also differentiated based on the goniometric signal. The root mean square (RMS)

and spectrum analysis for frequency domain, mean frequency (MF) and median frequency

(MDF) for each contraction were then calculated based on a signal iteration procedure where the

time difference between each signal spike was computed, converted to frequency, and the total

iterative frequencies summed and computed as mean and median frequencies. The electrical

manifestations of muscle activity and muscle fatigue were investigated by tracking the variation

of the instantaneous RMS and MF and MDF of the surface electromyographic signals during the

1 RM and the first contraction of the 1st set of 85% 1RM and the last contraction of the 4th set of

85% of 1RM.

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Power Analysis. The number of subjects was based on a power analysis using data from

Kwon et al. (15). Without cooling, the average total volume-load lifted was 1972±632 kg

(average±SD) while with cooling the average total volume-load was 2480±636 kg. Volume-load

refers to weight lifted (kg) x total repetitions over the 4 sets. Using the standard deviations from

Kwon’s data, approximately 8 subjects would be sufficient to detect a significant difference in

average total volume-load between hand cooling and no cooling (α=0.05 and a power of 0.7).

Therefore, we recruited eight healthy, resistance-trained female subjects for this study.

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Statistical analyses. All statistical computations were performed using STATISTICA

version 7.1 software (StatSoft, Inc., Tulsa, OK). A two-factor, repeated-measures ANOVA test

was used to compare the differences between conditions (TN, PC, PH) and the 4 sets for each

variable; including heart rate and Tpa. Exercise volume-loads (kg, sets × repetitions × weight)

among conditions were compared using a one-factor repeated-measures ANOVA. When a

significant F-ratio was obtained, a Tukey’s Honestly Significant Difference test was performed.

Statistical significance was accepted at p<0.05. All data are presented as the mean ± SD.

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RESULTS

Subject Characteristics and Environmental Variables. The descriptive characteristics of the

eight healthy female subjects who participated in the study are as follows; age = 25 ± 6 years;

height = 160 ± 6 cm; weight = 56 ± 7; body fat = 20.3 ± 2.6; 1 RM = 52 ± 6 kg; the ratio of

weight pressed to body weight = 0.9 ± 0.1; weight-training experience = 6 ± 2 years. There were

no differences in ambient temperature (TN, 23.9 ± 1.0°C, PH, 24.0 ± 0.8°C, PC, 23.8 ± 0.9°C,

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p=0.99), relative humidity (TN, 13.3 ± 8.0%, PH, 13.2 ± 8.2%, PC, 12.9 ± 6.8%, p=0.98) or

barometric pressure (TN, 630 ± 2mmHg, PH, 631 ± 2mmHg, PC, 629 ± 2mmHg, p=0.55) during

the three conditions.

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Mean palm skin temperature during rest periods among the three conditions. There were

significant differences in palm skin temperature during rest periods between conditions (p<0.01),

Figure 2. There were no significant differences within a condition between the three rest periods.


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Repetitions. The number of repetitions to exhaustion per set decreased (p<0.01), and varied

among the conditions (p<0.01), Figure 3. The PC (7.8±2.2) condition had significantly higher

mean repetitions than TN (6.7±1.8) (p<0.05), and the PH (7.6±1.8) conditions had significantly

higher mean repetitions than TN (p<0.05). The condition × set interaction effect was significant

(p<0.01). In the PC condition, the 2nd set had significantly higher (p<0.01) repetitions (9.4±1.5)

than the TN (7.3±1.0). In the PH condition the 4th set had higher repetitions (6.6±1.6) than the 4th

set in TN (4.9±1.41) (p<0.05).


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Exercise volume. There were significant differences in exercise volume-load between the TN,

PH, and PC conditions. Mean exercise volume-load (kg) of PC (1387±358) was significantly

higher than TN (1187±262), p<0.01. PH (1349±267) was significantly higher than TN, p<0.05.

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There was no significant difference between PC and PH in exercise volume-load (p=0.79),

Figure 4.

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Heart rate. Changes in HR were not significantly different among the conditions, p=0.10.

However, the changes in HR were significantly different between pre, rest, exercise and recovery

periods, p<0.01. There also were significant differences in absolute heart rate between pre, rest,

exercise and recovery periods, (p<0.05), Figure 5.


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EMG. There were significant differences in RMS values during both eccentric and concentric

movements in the lateral head of triceps, the anterior deltoid, and long head of triceps, Table 1.

The RMS % difference between the first rep of the 1st set and the last rep of the 4th set were

significantly different among the TN, PH, and PC conditions. During eccentric movement in the

anterior deltoid, the RMS difference (%) during PC (44±6) and PH (43±10) were significantly

higher than TN (33±10), p<.05. During concentric movement in the anterior deltoid, the RMS

difference (%) during PH (50±4) was significantly higher than TN (41±3), p<.05. During

eccentric movement in the lateral head of triceps, the RMS difference (%) during PC (47±8) and

PH (40±8) were significantly higher than TN (38±7), p<.01. During concentric movement in the

lateral head of triceps, the RMS difference (%) during PC (52±15) and PH (52±10) were

significantly higher than TN (44±5), p<.05. During eccentric movement in the long head of

triceps, the RMS difference (%) during PC (41±8) was significantly higher than both PH

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(31±10), p<.05 and TN (29±5), p<.01. During concentric movement in the long head of triceps,

the RMS difference (%) during PH (37±9) was significantly higher than TN (21±21), p<.05.

During concentric movement in the pectoralis major, the MF difference (%) during PH (78+11)

was significantly higher than TN (40±44), p<.05. During concentric movement in the anterior

deltoid, the MF difference (%) during both PC (28±44) and PH (30±14) were significantly

higher than TN (17±10), p<.01. During concentric movement in the lateral head of triceps, the

MF difference (%) during both PC (84±39) and PH (107±27) were significantly higher than TN

(39±2), p<.01. During eccentric movement in the lateral head of triceps, the MDF difference (%)

during PH (58±20) was significantly higher than TN (20±28), p<.05. During concentric

movement in the lateral head of triceps, the MDF difference (%) during both PC (101±34) and

PH (110±23) were significantly higher than TN (20±28), p<.01. During eccentric movement in

the anterior deltoid, the MDF difference (%) during PH (36±3) was significantly higher than TN

(19±12), p<.05. During concentric movement in the anterior deltoid, the MDF difference (%)

during both PC (31±8) and PH (38±8) were significantly higher than TN (22±9), p<.01.

Insert Table 1 here.

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DISCUSSION

This is the first study to assess the impact of mild palm cooling and heating on fatigue

during high intensity, multi-set bench press exercise in females. In this study, women

experienced a significant increase in exercise volume-load with heat as well as with cold

application to the palms between sets of resistive exercise. The cooling results agree with our

previous findings using an identical protocol with men, who had a 30% increase in exercise

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volume-load with cooling. However, the heating results differ from our previous results in men,

who did not have a significant increase in exercise volume-load (15). The effect of a mild

peripheral thermal input to alter muscular endurance may provide insight into a neuromotor

contribution to muscle fatigue, thus supporting the “central governor model” as described by

Noakes (20). It also has practical implication for enhancing strength training by athletes or by

patients exercising to preserve or improve their strength and muscle mass.

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Improvements of this magnitude in muscle endurance have been found during

simultaneously applied artificial electrical stimulation or cerebral magnetic stimulation (29), yet

our intervention was solely based on voluntary muscle contraction. In our earlier study in men,

since the applications of heat and cold were not sufficient to alter core body temperature, we

concluded that this temperature effect was most likely due to a central effect, supporting the

central governor theory described by Noakes (20). The central governor theory is predicated on

the role of the central nervous system modifying fatigue. According to this theory, subconscious

psychic factors related to the expected duration or intensity of the exercise, as well as peripheral

sensory input related to peripheral physiological factors as arterial or cerebral oxygenation,

muscle glycogen stores or the rate of heat accumulation, are integrated by the central governor,

which then alters force output by decreasing neural drive to active muscles. This reduced drive

results in a smaller number of motor units activated during exercise, thereby inducing fatigue

even though only 35 to 50% of the active muscle mass has been recruited. The overall purpose of

this mechanism is to prevent muscle injury. Our results suggest that in addition to afferent

sensors in the exercising muscles, other sensory receptors located in non-active regions of the

body, in our case thermal receptors in the palm of the hand during arm exercise, are also

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integrated by the central governor to alter muscular endurance. This central governor theory

however, is not universally accepted (27).

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Presumably, the peripheral thermal input we induced in the palms between sets of high

intensity resistance exercise is perceived by the central governor, resulting in a slower rise in the

awareness of effort during PC and PH trials, but interestingly not in the heart rate response. The

central governor then adjusts the motor output to the contracting muscles, allowing less

inhibition of the number of activated motor units, as suggested by the higher RMS values from

the triceps and anterior deltoid muscles with cold or heat application. This release of inhibition of

motor units results in a greater number of contractions until fatigue. The unique finding in the

present study is that in women, not only cooling but also heat application to the palms was

associated with increased overall neural activity and an increased number of reps until fatigue.

Paragraph 26

There are several possible explanations why heat had a greater effect on exercise volume

and EMG output in females compared to our previous research in males. Explanations for the

sex difference in exercise performance may include that there are sex differences in pain

perception (2, 9, 23), in thermal perception (17, 19, 26), or even differences in the mechanism of

fatigue (12, 25).

Paragraph 27

Since pain perception could be a determining factor in exercise tolerance and fatigue

during high intensity upper body resistive exercise, we reviewed the differences between men

and women in regards to pain perception. There has been considerable research showing sex

differences in pain perception, with the majority of articles showing that woman have a lower

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pain threshold and tolerance compared to men, including heat pain threshold (2, 9, 23).

Differences in pain perception could be a result of differences in the number and/or sensitivity of

pain receptors. It has been demonstrated that there are a greater density of nerve fibers in the

skin of women as compared to men. In a study done by Mowlavi et al (19), female cadaver skin

specimens (n = 10) demonstrated increased nerve fiber density (34 ± 19 fibers/cm skin) when

compared with male specimens (n = 10; 17 ± 8 fibers/cm skin; p = 0.038). This is important,

because if cooling/heating in any way affects pain via a counterstimulus effect, such as the gate

control theory (18), it is logical to imply that controlling the pain/discomfort of resistive exercise

could help explain the sex difference between our two studies. Verducci (32) and Burke et al.

(4) discussed the beneficial effects of cooling on pain reduction as the cause of their positive

exercise results; it is possible that heating uses similar mechanisms. Although the gate control

theory has undergone many modifications, including by Melzack himself, who has proposed a

neuromatrix theory to expand on it, the basic premise of a central modulation system in the

dorsal horn of the spinal cord, has remained intact (18).

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There is less information in the literature regarding sex differences in thermal perception.

Sarlani et al. (26) found sex differences in thermal perception outside the nociceptive system.

Matos et al. (17) found significant sex differences with lower threshold (higher sensitivity) in

women for both cold detection threshold and heat detection threshold in the trigeminal region.

Additionally, Denegar et al. (6) found that women with osteoarthritis of the knee were more

likely to report clinically meaningful improvement in pain and symptoms on the Knee Injury and

Osteoarthritis Outcome Score with the use of heat, cold, and a heating pad. The work by

Mowlavi et al. (19) found two times as many subcutaneous nerve fibers in women compared to

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men and this could help explain why women have a lower sensitivity to heat than men.

Although the authors used their findings to help explain why women have a lower pain threshold

compared to men, it is likely that some of the additional neurons were capable of transmitting

heat signals. The role of superficial heat in pain control is well established. The exact

mechanism of pain control is unknown, but an influence of heat receptors as a counterstimulus

has been suggested (33). Therefore, if women do have a greater number of heat receptors, they

might have an enhanced ability to gate the pain associated with muscle fatigue, which could help

explain the sex differences in exercise performance with heat that we found in our study.

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There are a number of studies addressing sex differences in fatigue. Although somewhat

equivocal, where some studies showed greater fatigue in woman than men (2, 28), others showed

no difference (22), while the majority of studies showed that women have greater muscular

endurance than men (7, 12, 13, 22, 25). Of these studies, some tried to differentiate between a

peripheral and a central fatigue effect. Of these, some studies showed a central effect (7, 25),

while other studies cited a peripheral effect (12, 13). This distinction is important, because the

effects of peripheral fatigue include pain in the working muscles due to a buildup of metabolic

waste products. Pain gating is thought to be dermatome specific. The dermatome innervation for

the pectoral region is approximately C4-T4, and the hand is C6-T1, therefore, since there is

dermatomal overlap, hand cooling/heating could have a gating effect on the region of the

working pectoral muscles. Therefore, if fatigue differences between genders are more peripheral,

the cooling/heating of the body would need to be dermatome specific. If gender differences in

fatigue are due more to a central effect, cooling of the hands should affect fatigue in any muscle

of the body. An interesting future study that could help differentiate central and peripheral

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fatigue effect of body cooling or heating would be to test exercise volume-load and EMG

changes in a muscle group that does not share the same dermatomes as the hand to see if exercise

volume-load is still positively affected.

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In conclusion, this is the first study to document the ergogenic effect of hand cooling or

heating in women between sets of high intensity resistive exercise. We propose that this effect

of heating or cooling may be due to an effect of peripheral thermal stimuli modifying the output

of efferent motor neurons, a manifestation of the central fatigue process. Women may be more

sensitive than men to the ergogenic effects of heat application.

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PRACTICAL APPLICATIONS

It is generally accepted that high training volumes using both 6 to 12 RM intensity and 3

to 6 sets per exercise are associated with an increase in muscle mass and, subsequently, enhanced

training adaptions and performance. Our study, which used 85% of 1RM intensity, four sets, and

3-min between-set intervals, represents a commonly prescribed exercise strength and

hypertrophy regimen. Application of hot or cold between sets might therefore be a means to

increase the effectiveness of resistive exercise to improve muscle mass in athletes, to reverse

muscle atropy in patients, or to prevent sarcopenia in the elderly. In rehabilitation populations,

especially patients with compromised exercise tolerance such as cancer patients and patients with

multiple sclerosis and myasthenia gravis, palm heating or cooling might help improve exercise

tolerance and therefore functional gains. In addition, our study could have specific applications

to fatigue and exercise intolerance, which are known barriers to resistance training for women.

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Applying either cooling or heating to hand regions in women during rest intervals between sets

could decrease perception of fatigue and improve outcomes from resistance training.

Acknowledgments AVAcore Inc. lent the RTX devices used in this study; however, this study

was performed as part of the first author’s dissertation project without supplemental funding. The

authors do not have a professional relationship with companies or manufacturers who may

benefit from the results of this present study. None of the authors received funding for research

related to this article. The authors have no conflicts of interest to declare.

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Figure Legends

FIGURE 1-Protocols for Day 1: for Warm-up, 1RM test, and one 85% 1RM exercise bout; and

for Days 2-4 for the thermal condition trials: 1RM and 4 sets at 85% of the day 1 bench press

1RM in thermoneutral (TN), palm cooling (PC), and palm heating (PH) conditions. After the

1RM, the exercise in the 85% 1 RM set sets continued until the subjects failed to complete a lift.

FIGURE 2-Mean palm skin temperature during the four 85% of 1RM sets of bench press

exercise. Data were obtained during rest periods of thermoneutral (TN), palm cooling (PC), and

palm heating (PH) conditions. Each value represents the mean palm skin temperature (n=8).

Error bars indicate SD. Asterisk (*) indicates PC vs. PH conditions (*p < 0.05, **p < 0.01).

Symbol (†) indicates PC vs. TN conditions (†p <0.05, ††p <0.01). Symbol (ψ) indicates PH vs.

TN conditions (ψ p <0.05, ψ ψ p <0.01).

FIGURE 3- Repetitions completed before fatigue during the four 85% of 1RM sets of bench

press test during thermoneutral (TN), palm cooling (PC), and palm heating (PH) conditions.

Each value represents the mean repetitions (n=8). Error bars indicate SD. Asterisk (*) indicates

PC vs. PH conditions (*p < 0.05, **p < 0.01). Symbol (†) indicates PC vs. TN conditions (†p

<0.05, ††p <0.01). Symbol (ψ) indicates PH vs. TN conditions (ψ p <0.05, ψ ψ p <0.01).

FIGURE 4-Total exercise volume-load of the four 85% of 1RM sets of bench during the four

temperature conditions: during room temperature (TN), palm cooling (PC), and palm heating

(PH) conditions. Each value represents the mean value (n=8). Error bars indicate SD. Asterisk

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(*) indicates PC vs. PH conditions (*p < 0.05, **p < 0.01). Symbol (†) indicates PC vs. TN

conditions (†p <0.05, ††p <0.01). Symbol (ψ) indicates PH vs. TN conditions (ψ p <0.05, ψ ψ p

<0.01).

FIGURE 5-Average heart rate during the four 85% of 1RM sets of bench press test during

thermoneutral (TN), palm cooling (PC), and palm heating (PH) conditions. Each value represents

the mean heart rate (n=8). Heart rate increased significantly during each exercise bout but there

were no significant differences among the 3 thermal conditions.

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TABLE 1. The mean difference (%) in the EMG root mean square (RMS), mean frequency

(MF), and median frequency ( MDF) during eccentric and concentric contractions during the

four 85% of 1RM sets of bench press test during thermoneutral (TN), palm cooling (PC), and

palm heating (PH) conditions, (n=8). Values shown are the mean±SD.

Change (%)

Muscle EMG Variable (%) Contraction TN PH PC P Value

ECC 35±8 29±6 35±5 0.13 RMS

CON 35±16 46±3 45±3 0.06

ECC 34±10 35±21 44±8 0.30 MF

CON 40±44 78±11ψ 43±11 0.02

ECC 61±70 61±22 74±29 0.85

The Pectoralis Major

MDF

CON 66±51 80±23 74±17 0.55

ECC 38±7 40±8ψ 47±8†† 0.01 RMS

CON 44±5 52±10ψ 52±15† 0.03

ECC 28±12 36±16 29±12 0.54 MF

CON 39±2 107±27ψ ψ 84±39†† 0.01

ECC 20±28 58±20ψ 42±12 0.02

The Lateral

Head of Triceps

MDF

CON 73±5 110±23ψ ψ 101±34†† 0.01

ECC 33±10 43±10ψ 44±6† 0.02 RMS

CON 41±3 50±4ψ 44±12 0.02

ECC 16±8 17±16 14±3 0.86 MF

CON 17±10 30±14ψ 28±4† 0.01

ECC 19±12 36±3ψ 29±13 0.03

The Anterior

Deltoid

MDF

CON 22±9 31±8 38±8†† 0.01

The Long RMS ECC 29±5 31±10 41±8*†† 0.01

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CON 21±21 37±9ψ 33±16 0.03

ECC 11±15 16±17 13±28 0.61 MF

CON 22±8 24±24 30±30 0.68

ECC 11±27 27±49 20±38 0.32 MDF

CON 31±24 43±30 47±36 0.60

Asterisk (*) indicates PC vs. PH conditions (*p < 0.05, **p < 0.01). Symbol (†) indicates PC vs.

TN conditions (†p <0.05, ††p <0.01). Symbol (ψ) indicates PH vs. TN conditions (ψ p <0.05, ψ

ψ p <0.01).


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Palm Cooling and Heating Delays Fatigue During Resistance Exercise in Women

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