Exercise in hot environments : HEST kinesiology professor researches effects of heat during exercise

Southern New Mexico is known for its hot climate, which can detrimentally affect people with physically demanding jobs, such as migrant farm workers and construction crews.

Exercise in the heat challenges the cardiovascular system more than exercise in a cooler environment. NMSU kinesiology assistant professor Hillary Yoder is looking into how the body responds to continuous and interval exercise in hot environments thanks to an Emerging Scholars grant from the College of HEST.

Yoder joined the college in 2022 after graduating with her Ph.D. from the University of Alabama.

“With increasing frequency and intensity of heat waves, understanding how the body responds to exercise in the heat is of growing importance,” Yoder says. “One reason I became interested in this area of research is the possible practical implications for people who live in hot environments.”

The challenges of exercise in the heat can be observed through a common measurement – heart rate. During exercise in hot environments at a constant intensity, heart rate increases, and the amount of blood getting pumped out of the heart each beat decreases. In theory, a given heart rate should represent a given exercise intensity. However, after this “drift” has occurred, reductions in maximum aerobic capacity, also known as VO2 max are also observed.

VO2 max refers to the maximal amount of oxygen the body can transport and use in a minute. The temporary reduction in VO2 max makes the same intensity of exercise feel harder.

“What we do not know is if the magnitude of drift is different during continuous compared to interval exercise,” Yoder says. “We will evaluate if the continuous or interval exercise results in a larger decline in maximal performance.”

Yoder and her team, which consists of two graduate students and three undergraduate students, conduct this research by having participants ride on a stationary bicycle in a portable greenhouse, heated to 95 degrees with 40% relative humidity. Each participant, a healthy active adult 18 to 39 years old, visits the lab for an exercise session five times over a few weeks.

At the visits, the participants exercise while wearing a mouthpiece connected to a metabolic cart. The metabolic cart analyzes breath for volume, oxygen and carbon dioxide, and the equipment measures VO2 max. The research team also analyzes blood and urine samples in several ways. Before the participant begins exercise, a urine sample is provided to ensure they are well hydrated.

The team also calculates how much body mass is lost due to sweat during the exercise sessions by having participants weigh themselves immediately before and after exercise. Another analysis related to hydration is how plasma volume changes during the exercise. The percent change in plasma volume is calculated by analyzing a blood sample for percentage of red blood cells in the blood and hemoglobin, the molecule in blood that carries oxygen, from before and after exercise. 

They also measure blood lactate, which gives them an idea how hard someone was working when they complete a test to measure VO2 max. Blood lactate is a byproduct of exercise and the research team is looking for it to be similar after the separate VO2 max tests following interval and continuous exercise, indicating similar efforts were given across the five visits.

Yoder has observed increases in heart rate and decreases in performance after 43 minutes of interval and continuous exercise; however, more participants are needed before conclusions are made.

Yoder hopes the results from this research can be used to make recommendations for the community. 

“If our hypothesis is correct and interval exercise does increase the amount of cardiovascular drift compared to continuous exercise, then it might be better to complete continuous exercise in hot environments,” Yoder says.

After the study is completed, Yoder plans to publish and present the results at the American College of Sports Medicine conference. For more information about the study, contact Yoder at hyoder@nmsu.edu.