Prior to each trial, you will be provided with urine jugs and food/fluid logs to record food and drink intake for 24hr. You will be asked to log food/fluid intake 24hr leading into trials. We ask that you match your diet for both trials, as much as possible. Also, we ask that you match caffeine consumption prior to trials. Lastly, we ask that you refrain from strenuous exercise and alcohol consumption for at least 12hrs leading into trials. Either of these exclude your participation in the study for safety reasons. You will be asked to provide a small urine sample upon arrival to the lab before trials. The purpose of this is to verify hydration prior to exercise in the heat. If you are not hydrated upon arrival, you will be provided and asked to drink 16oz (a bottle) of water prior to the trial. Just prior to, and following each long trial, you will be asked to provide a small saliva sample. To do this, you will be asked to actively produce saliva and guide it into a small container. This process takes <5min. Next, you will have your nude body mass measured and you will be instrumented with a labbased sweat collection patch (large band-aid on the forearm), and the Hdrop sweat detection device on your left upper arm. The Hdrop will be synced with electronic applications (phone apps) to assure data collection. You will then self-insert a rectal thermistor ~15 cm past the anal sphincter to monitor core temperature throughout exercise (for safety purposes). You will also be fitted with a heart rate strap that goes around your chest. You will then be asked to enter the environmental chamber and exercise for 30min. This will serve as the short exercise trial aiming to assure comfort with the 120min exercise trials. Data will be collected during the short trial and comparisons will be made for the longer, 120min trials. The environmental chamber will maintain a consistent temperature (80-100 degrees F) and humidity (40 to 60 % relative humidity). You will be monitored for safety throughout exercise in all 3 trials, and you will be allowed to cycle (stationary bike) or run or walk (treadmill) at your own pace/intensity. Pace and intensity will be recorded during the first trial and attempts will be made to match exercise intensity for subsequent trials. Fluid consumption will be allowed during trials and will be matched between trials. In both trials, you will be allowed to drink for a 5min period at each 30min interval (30-35min, 60-65min and 90-95min). We will provide a sport drink of your flavor choice during exercise.

If you agree to be in this study, you will be asked to report to our exercise science research center for three total visits (1 short trial and 2 separate trials separated by at least 4 days), and the total time commitment for this study will be approximately 7 hours. Prior to each trial, you will be provided with urine jugs and food/fluid logs to record food and drink intake for 24hr. You will be asked to log food/fluid intake 24hr leading into trials. We ask that you match your diet for both trials, as much as possible. Also, we ask that you match caffeine consumption prior to trials. Lastly, we ask that you refrain from strenuous exercise and alcohol consumption for at least 12hrs leading into trials. Either of these excludes your participation in the study for safety reasons. You will be asked to provide a small urine sample upon arrival to the lab before trials. The purpose of this is to verify hydration prior to exercise in the heat. If you are not hydrated upon arrival, you will be provided and asked to drink 16oz (a bottle) of water prior to the trial. Next, you will have your nude body mass measured, and you will be instrumented with a lab-based sweat collection patch (large band-aid on the forearm), and the Hdrop sweat detection device on your left upper arm. The Hdrop will be synced with electronic applications (phone apps) to assure data collection. You will then self-insert a rectal thermistor ~15 cm past the anal sphincter to monitor core temperature during exercise (for safety). You will also be fitted with a heart rate strap that goes around your chest. The first trial will involve only 30min of exercise. You will be asked to enter the environmental chamber and exercise for 30 minutes for the first trial. The participant will self-select the pace of the exercise (walk/run/cycle). The pace they will select is one they think they can maintain for 2 hours of exercising. If they need to adjust their pace, they will. The environmental chamber will maintain a consistent temperature (80-100 degrees F) and humidity (40-60% relative humidity). The second and third trials will be self-selected in terms of exercise intensity - participant chooses the intensity. Participants will be monitored for safety throughout exercise, and you will be allowed to cycle (stationary bike) or run or walk (treadmill) at your own pace/intensity. Pace and intensity will be recorded during the first trial. Subsequent trials will attempt to be matched for exercise intensity. The first trial is to familiarize participants with the protocol and environment to be sure they will be able to complete trials. Fluid consumption will be allowed during trials. In both long trials researchers will replace 80% of sweat rate losses with a sport drink during the trial. The other trial will be matched for fluid consumption. Prior to, and following, the last 2 (longer trials - 120min exercise), participants will be a asked to provide a saliva sample. This normally takes about 5min and is done by thinking of food and attempting to "drool" into a small container. This is for a hydration test to compare to urine samples provided at the same 2 time points (before and after long trials). Exercise trials will be stopped if you experience signs/symptoms of heat illness, get exhausted, or no longer want to continue. Following the exercise, sweat collection devices will be collected and analyzed for sweat electrolyte concentration and/or sweat rate detection. Upon completion of exercise, you will towel dry and have nude body mass measured again. All devices, heart rate strap and rectal thermistor will be returned. You will be provided fluids and snacks following exercise and be monitored for 15min. Following this, you will be allowed to leave, and you will be scheduled for your second trial. Following your second trial, you will be compensated for participation.

Inclusion Criteria:

• Regular exercisers
• Otherwise healthy individuals
• Capable of running/jogging or cycling for 120min straight

Exclusion Criteria:

• Heat illness in the past 3 years
• Taking medications that alter fluid balance or Na retention
• Chronic conditions that contraindicate long duration exercise in the heat

All de-identified data will be available through clinicaltrials.gov and reasonable requests made to corresponding authors once manuscripts have been submitted for publication.