It can be hard to exercise or do physical work in very hot or cold environments. Fatigue can set in, making any movement feel sluggish and difficult.

The mechanics of exactly how this happens inside the body has long been a fascination for Stephen Cheung. In his Environmental Ergonomics Lab, the Brock University Professor of Kinesiology studies how heat, hydration, altitude and cold air and water affect human physiology and performance.

When the body’s core temperature is lower than normal, for example, the priority is to protect core organs such as the heart, lungs and brain. Blood vessels in other tissues, such as muscle and skin, constrict to conserve warmth, thus reducing the body’s supply of oxygen.

This decrease in oxygen availability can also impact exercise performance, Cheung has found.

Cheung and his team conducted an experiment in which research participants rested in a cold chamber until their core temperature dropped by 0.5°C, and then proceeded to pedal 20 kilometres on a stationary bicycle. Participants took far longer to complete the test when their core temperature dipped below normal levels, Cheung found.

The process was then repeated, but with participants breathing in higher-than-normal oxygen levels.

“We found that there was no difference in their performance level compared to when their core temperature was normal,” says Cheung. He and his team published this finding in their 2018 study “Effects of mild whole body hypothermia on self-paced exercise performance.”

In a follow-up study in early 2024, the research team found exercise performance could be negatively impacted by up to 30 per cent by simply cooling the skin — even without a drop in core temperature.

“This supports the need for athletes and workers to wear proper clothing when working in cold weather,” he says.

Cheung also researches how heat affects the body’s ability to do heavy exercise.

As Cheung demonstrated for CBC’s White Coat, Black Art podcast, extreme heat not only decreases physical and exercise performance, it can also impact a person’s overall health.

In an experiment he conducted for the show, he demonstrated how working in hot environments elevates core temperatures and increases fatigue, which can contribute to heart and kidney problems down the road.

“There’s a lot of research about acute exposure to high heat stress, however, we know a lot less about how day after day of high heat stress affects our body,” says Cheung. “As global warming continues, we’ll be seeing a wider range of workers exposed to higher temperatures for more days, and it’s critical to do pro-active research to see how that impacts our health.”

Cheung’s current research is also exploring how mild heat exposure can actually benefit human health.

Exercise typically boosts levels of a protein called brain-derived neurotrophic factor (BDNF), which regulates a variety of neurons and cells that are vital to brain and body health.

Cheung and his lab are investigating how ‘heat therapy’ — such as ‘passive heat exposure’ through hot tub or sauna use — can be used to boost BDNF levels in older adults.

He says this approach could benefit people who may not be able to exercise because of mobility issues or illness.

Cheung is continuing his exploration of how temperature and oxygen levels affect muscle function through research funded by the Natural Sciences and Engineering Research Council of Canada.

His team is looking to determine how temperature and oxygen interact to affect muscle function, and whether repeated heating of muscles is an effective way to maintain or improve muscle function.

“This research will greatly improve our understanding of how temperature can positively or negatively affect the ability of muscle to work,” he says. “In turn, this will improve our ability to design new ways to maintain muscle health even when exercise may not be possible.”