The goal of this repeated measures study is to evaluate acute work rate adjustments during high-intensity interval training (HIIT) in a hot and temperate environment in healthy and active adults when using target heart rate to prescribe the exercise intensity.

The main questions it aims to answer are:

• Will HIIT based on target heart rate in a hot environment result in increased cardiovascular (i.e., elevated recovery HR) and thermal strain compared to HIIT in a temperate environment?
• Will work rate be lowered to a greater extent during HIIT in a hot environment compared to a temperate environment in order to maintain target heart rate?
• Will maximal aerobic capacity (V̇O2max) decrease to a greater extent after HIIT in the heat compared to a temperate environment?

Procedures:

Participants will complete 5 trials on a cycle ergometer. Trial 1 (Control Visit): Participants (n = 7) will have their maximal heart rate and V̇O2max measured in a temperate environment (~22 °C, 40% RH).

Experimental Trials 2-5: The order of the trials will be counterbalanced and randomly assigned to participants.

• 15TEMP: Participants will cycle at 70% of their maximum heart rate for 8 minutes as a warm-up, followed by one round of HIIT. The HIIT protocol consists of 4 minutes at 90% of maximum heart rate followed by 3 minutes at 70% of maximum heart rate, totaling 15 minutes, in a temperate environment (~22 °C, 40% RH). After the HIIT session, a graded exercise test will measure V̇O2max.
• 15HEAT: Similar to 15TEMP, but participants will perform the HIIT protocol in a hot environment (~35 °C, 40% RH).
• 43TEMP: Participants will cycle at 70% of their maximum heart rate for 8 minutes as a warm-up, followed by the HIIT protocol repeated four times (4 minutes at 90% of maximum heart rate and 3 minutes at 70% of maximum heart rate), totaling 43 minutes, in a temperate environment (~22 °C, 40% RH).
• 43HEAT: Similar to 43TEMP, but participants will perform the HIIT protocol in a hot environment (~35 °C, 40% RH).

Inclusion Criteria:

• Participants were healthy males and females who are 18 to 39 years of age, recreationally active as defined by the American College of Sports Medicine, performing moderate-intensity aerobic exercise 30 min or more per session for at least 3 sessions per week for at least the past 3 months, and free of metabolic, cardiovascular, and renal disease

Exclusion Criteria:

• Individuals classified as needing medical clearance according to American College of Sports Medicine (ACSM) Guidelines were not allowed to participate. Needing medical clearance is defined by ACSM as 1) not being physically active and having known cardiovascular, metabolic, or renal disease and asymptomatic; 2) not being physically active and having any signs or symptoms suggestive of cardiovascular, metabolic, or renal disease; 3) participating in regular exercise and having known cardiovascular, metabolic, or renal disease and asymptomatic; or 4) participating in regular exercise and having any signs or symptoms suggestive of cardiovascular, metabolic, or renal disease .