The Influence of Capsaicin Gel During Exercise Within the Heat

The purpose of this study is to examine the human thermoregulatory impact of applying a commercially available capsaicin gel to the skin prior to moderate intensity walking under heated conditions. Experimental Visits will consist of 30 min of treadmill walking at a moderate pace (3.5 mph, 5% grade) under hot conditions (38°C, 60%RH) and will be randomized and counterbalanced for capsaicin gel or a hypoallergenic gel (control) application.

Gels will be applied to areas commonly exposed during outdoor activity in warm conditions (shoulder to wrist, mid thigh to ankle). Accordingly, participants will wear shorts and a tank top shirt during exercise. Core temperature, skin temperature, galvanic skin response, laser doppler blood flow, and heart rate will be continuously recorded throughout the exercise bout via an integrated analog to digital converter. Sweat will be collected during exercise using commercially available absorbent patches. Thermal sensation will be assessed throughout exercise via the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) thermal sensation likert scale (cold to hot).

Lastly, nude body weight will be recorded pre and post exercise for sweat rate determination. The capsaicin and control trials will be separated by a 7-14 day washout period

3 experimental visits to the laboratory will be conducted. All visits will take place at the University of Nebraska at Omaha, Exercise Physiology Laboratory. Visit 1 will consist of the physical activity readiness questionnaire (PARQ), adverse reaction to capsaicin, and body composition (hydrostatic weigh, bioelectrical impedance). Visits 2 and 3 will be separated by 7-14 days, randomized, and counterbalanced for experimental (capsaicin) and control (hypoallergenic cream) trials. Both the experimental and control visits will follow the same protocol.

Participants will arrive following an overnight fast state while also refraining from strenuous activity, alcohol consumption, tobacco use, and recreational drugs for the previous 24 hour period. During these visit researchers will apply capsaicin using a gloved hand from shoulder to wrist and mid thigh to ankle (areas outside of the clothing). A hypoallergenic cream will be applied during the control visit.

Exercise will consist of 30 min of moderate intensity treadmill walking (3.5 mph at 5% grade) within a heated temperature/humidity controlled chamber (38°C, 60% relative humidity). Participants will be weighed before, and after completion of the exercise calculate sweat rate. Prior to the exercise session, participants will sit for 5 min in an ambient room temperature environment as a baseline. During baseline and during the exercise core temperature, skin temperature, laser doppler blood flow, galvanic skin response, and heart rate will be collected continuously. Sweat patches will be adhered to the forehead during exercise at minute 10 for collection of sweat composition over the next 10-20 minutes. Thermal perception using the ASHRAE scale (cold to hot) will be assessed pre and during exercise.

EXPERIMENTAL VISIT 1 Visit 1 will take approximately 1 hour. Participants will then complete the physical activity readiness questionnaire (PARQ) to be cleared for physical activity. Participants will have a small amount of the capsaicin cream applied to a small area of the forearm (\~2 x 2 in) to test for any adverse reaction over a period of 15 minutes. Those without a reaction will have height, weight, and body fat composition measured. Height and weight will be measured using a medical scale and stadiometer, respectively. Body fat will be assessed with hydrostatic weighing using an electronic load cell based system (Exertech, Dresbach, MN) correcting for residual lung volume or via a bio-electrical impedance analyzer (InBodyUSA, Cerritos, CA).

EXPERIMENTAL VISITS 2 AND 3 Visits 2 and 3 will take approximately 2 hours each. Upon arrival at the lab, a nude body weight will be collected with the subject in a private room. While still in the room, subjects will self-insert a rectal thermistor 12-15 cm beyond the anal sphincter and don a chest strap heart rate monitor. After dressing in their exercise clothing, subjects will exit the private room so that chest, forearm, and calf thermistors can be adhered to the skin with tape. Laser doppler flow (forearm and or/finger) will be adhered with tape and galvanic skin response (fingers) will be adhered using Velcro straps to the skin. Measuring instruments will be recorded using an integrated digital to analog converter (ADInstruments, Colorado Springs, CO).

Following instrumentation, subjects will sit for 5 minutes as a baseline in a temperate environment. Immediately after, a researcher with gloved hands will topically apply capsaicin or the control hypoallergenic cream from the shoulder to wrist and mid-thigh to ankle. Subjects will then enter the heat chamber (38°C, 60% relative humidity) and immediately begin a 30-minute walk on a treadmill (3.5 mph, 5% grade). Subjects will be intermittently asked to hold their arm steady for laser doppler flow collection (10, 20, 30 minutes). Subjects will also briefly pause at minute 10 in order to adhere an absorbent patch to their forehead for sweat collection over the following 10-20 minutes. Thermal perception using the ASHRAE scale (cold to hot) will be assessed pre and during exercise. After 30 minutes of walking, subjects will exit the chamber for removal of all skin and finger sensors. Subjects will then re-enter the private room, remove their own rectal thermistor with gloved hands, and then a final nude body weight will be recorded after toweling off.

The rectal thermistor will measure core temperature. The heart rate monitor will measure the beats of the heart. Skin thermistors will measure the temperature of the skin surfaces. Laser doppler flow measure the relative units of blood flow velocity. Galvanic skin response measures skin conductivity changes as individuals begin to sweat. Sweat is collected for assessment of sweat composition. Thermal perception is assessed for individual perception surrounding the temperature of the ambient environment. Nude body weight is measured to calculate sweat rate.

Eligibility Criteria:
Inclusion Criteria:

* 19-45 years of age
* cleared for physical activity via the 2022 physical activity readiness questionnaire (PARQ)
* no self-reported adverse reactions to capsaicin

Exclusion Criteria:

* outside 19-45 years of age
* not cleared for physical activity via the 2022 physical activity readiness questionnaire (PARQ)
* self-report or signs of sensitivity to capsaicin
* pregnant
* breast feeding