Winter Olympics, Extreme-Temperature Exercise: Safe or Not?
The “U” Shaped Curve with Exercise in Temperature Extremes
When we look at performing a task or stressing our bodies we hope to get an equal return or a linear response. A linear response is one in which the more we work, the more we receive. Unfortunately, our body’s response to stress is often not that straightforward. With exercise and stress in different temperatures it may follow a “U” shaped curve of response. This means that your body will show signs of distress when performing endurance exercise with both very cold and very hot temperatures. This also means that there is a place in between for maximum performance and health. As discussed in a prior column, our body’s response to exercise in general yields a “U” shaped curve in health benefits as well.
Exercise in Hot Environments
First, let’s consider exercise in hot environments. In a prior study of hydrated athletes, cardiac function was compared during exercise in two different temperature environments. Moderate exercise was performed at 43°C (109 °F) and then at 26°C (79 °F). The higher temperature training was extreme, but it was chosen since it would result in a high core body temperature and more blood flow through the skin. The researchers found that despite adequate hydration, during exercise in extreme heat the heart output actually declines. The decline in output was significant, on average 1.0-1.2 L/min, and was largely attributed to less blood available to the heart since it was preferentially circulated to the skin to help dissipate heat. In this study the temperature exposure was harmful and heart did not adequately compensate for the heart stress. In a second study, with prolonged endurance training in heat that raises the core temperature, the fall in cardiac output can be even greater, up to 2.2 L/min.
An interesting finding was noted in another study that included well-trained endurance athletes. In this study, the training allowed the heart to compensate adequately when stressed due to heat exposure. In a study of 8 trained endurance cyclists, researchers found that with exposure up to 35°C (95 °F) compared to 8°C (46°F) resulted in blood flow to the skin to cool the body and core temperature, but in these athletes the cardiac output did not decline. In this last study there were two important characteristics. First, they included only experienced well-trained endurance cyclists. Second, all of these athletes were adequately hydrated for both temperature phases of training.
Exercise in Cold Environments
With a “U” Shaped curve response with exercise across a temperature range there may also be a corresponding strain on the body and heart with exercise in cold weather. In a study of 7 men, the impact of running in cold, wet weather was measured. These men ran for 30 minutes in 5°C (41°F) temperatures with additional exposure to rain. In these conditions, the runners required more oxygen (i.e. their oxygen consumption increased) and their blood lactate levels also increased. Lactate, or more appropriately lactic acid, is found throughout our bodies. When tissue, including muscle or blood cells, breaks down lactate is released into the blood. As such, an elevated lactate is a good marker of tissue injury.
A second study looked at general healthy people and another group with heart failure that exercise at 20°C (68°F) and -8°C (17.6°F). The research looked at endurance capacity by using 2 constant load exercise tests. What they found was in healthy people their exercise time and capacity improved by 20% in the colder weather. Interestingly, the researchers found the opposite in people with heart failure. In these people exercise time and capacity decreased the exercise capacity of people with heart failure at lower temperatures.
There are some people who develop asthma or asthma like symptoms when exposed to cold weather, exercise, or a combination of both. A previous study examined 20 people with a history of asthma or asthma like symptoms to determine if cold weather impacted their exercise capacity. The people exercised by running at 20°C (68°F) and then at -18°C (0°F). The researchers measured oxygen uptake, heart rate, and ventilation. Oxygen consumption or the ability to bring in oxygen and use it decreased by 6.5 percent. Despite similar levels of ventilation and heart rates, people with asthma ran at significantly lower running speeds. After exercise in cold weather, lung function tests that examined their asthma or bronchoconstriction worsened in the cold by 7 percent.
The second two studies in people with heart failure and lung disease share similar insights. We learned from the study of men running in cold weather and rain that this requires a greater stress on the heart and more energy consumption. These men were not necessarily fit or endurance trained. In people with heart failure or lung disease, the heart’s ability to rapidly adapt and respond to stress from cold weather was lessened. Without the capacity to adapt quickly to different temperature environments and stresses, the exercise performance and response to exercise was impaired.
We learned that training and improving fitness helps the heart’s response to exercise in hotter climates improve. Exercise training improves the heart’s ability to adapt and respond to stress. In this regard, we would anticipate very fit athletes, such as those in the Sochi Winter Olympic games, to do very well in cold environments.
In a trial involving 8 highly trained endurance cyclists, the effects of cold weather on heart function were studied. The cyclists performed two 100 mile cycle trials. The first was done at a temperature of 0°C (32°F) and the second at a temperature of 19°C (66°F). Immediately after the 2 time trials in different temperatures the researchers measured heart function and markers of heart injury. After both time trials there was a small elevation in the markers of heart injury in the blood. Temperature did not influence the elevation. The squeezing function was not affected by temperature and remained normal. What the authors found was that after cycling 100 miles in 19°C (66°F) temperature, the heart was stiffer (impaired diastolic filling) compared to that observed at the lower temperature. In this latter study of athletes, exercise in lower temperatures actually offered some benefit.
Finding the Ideal Temperature to Exercise
There are other things to consider when exercising in cold weather. These include exposure to ice, snow, rain, or a combination of these elements. Clearly running or cycling with poor surface conditions is harmful independent of what happens to the heart.
Another aspect to consider is with exercise our body generates heat. We need a certain amount of heat to offset what is lost due to exposure to the cold weather. Also, care must be taken to minimize exposure to areas prone to cold injury or frostbite such as our hands, toes, ears, and nose. When you exercise in hot sunny conditions wear a hat and sunscreen. When you exercise in very cold temperatures cover up and wear a hat, gloves, and consider a facemask.
In a prior study that included a large review of the available literature, the author found that exercise that is prolonged and at a moderate intensity is better tolerated at slightly lower temperatures around 11°C (52°F). As mentioned in the multiple prior studies that were presented, people in general perform well or even better in colder temperatures as long as they do not have heart or lung diseases. However, if the intensity of exercise is lessened or the temperature is less than 11°C, often the heat generated from the exercise is insufficient to compensate for what is lost due to the environment exposure. You can compensate for this exposure heat loss by increasing your exercise intensity, increasing your clothing for thermal retention, or reducing your exposure time to the ambient air or the water.
A word of caution if you exercise in lower temperatures, remember to keep yourself hydrated. Often with exercise in hot temperatures the process of active sweating and the exposure to heart remind us to hydrate with fluids that help replace what is lost. These stimuli are not necessarily present in cold environments, although our bodies continue to lose fluid and need hydration with the heat generated during exercise. Also, remember that when you lose fluids during exercise you also lose electrolytes. These need to be replaced as well to keep your heart and body performing at a high level.
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