Exercise in the Heat
Exercise Physiology | Muscle Contraction | Muscle Fibers | Muscle Adaptations | Exercise Fuels | CHO Metabolism | Fat Metabolism | Oxygen Uptake | Cardiovascular Exercise | Respiratory Responses | VO2 Max | Temperature Regulation | Heat | Fluid Balance | Fatigue | Sprinting | Endurance | Genes | Practical Case Example
Exercise in the Heat
Learn about exercise in the heat considering both temperature and humidity and the effects of these values on neuromuscular function, hyperthermia, increased carbohydrate consumption, fatigue and improved exercise tolerance with heat acclimatization including both physiological and metabolic adaptations. Consider cooling strategies and fluid ingestion as influencers used to control core temperature during exercise in the heat.
In our last lecture, we saw how the body loses heat during exercise and the implications for fluid balance. We also saw that increasing environmental temperature is associated with a decrease in exercise tolerance. In this lecture, we’ll look at what happens when you exercise in a hot environment. I showed you increasing the environmental temperature from 20℃ to 40℃ was associated with reduced exercise tolerance.
Heat & Humidity During Exercise
If we examine what happens during a more realistic exercise model. In this case, a 30 minute time trial where well-trained cyclists were asked to complete as much work as they could at two environmental temperatures. In the open circles, 23℃ with a relative humidity of 60% and the closed circles at 32℃ with the same relative humidity. This was done in an environmental chamber. You can see that during exercise in a hotter environment, the subjects reduced their power output. And although they’re able to increase the power output for the last five minutes or so, the total work output over the 30 minutes was reduced in the hot environment. Interestingly the body core temperatures were similar in the two trials. And this raises a merely interesting question about how the body regulates its metabolic heat production with increases in environmental temperature. And clearly, the central nervous system is involved in some way and will touch upon this in a little more data when we look at fatigue. But it’s an important observation that rates of exercise, intensity efforts during hot environments are often down-regulated so that the metabolic heat production is less to compensate for the increasing environmental temperature.
- Neuromuscular Function & Hyperthermia
- Increased CHO Use During Exercise in the Heat
- Fatigue During Exercise in the Heat
- Improved Exercise Tolerance in the Heat with Acclimatization
- Physiological & Metabolic Adaptations to Heat Acclimatization
- Pre-Cooling Increases Exercise Tolerance
- Fluid Ingestion & Core Temperature
Neuromuscular Function & Hyperthermia
As an example of this, if we look at neuromuscular recruitment or function, during isometric exercise, at close 2/3 or so, of the maximal voluntary contraction and sustained for two minutes. We can measure the neuromuscular drive to the contracting muscles through the integrated EMG. And when this is done in a normothermic or controlled situation compared with a hypothermic situation, you can see a marked reduction in the activation of the contracting muscle in the presence of hypothermia, suggesting that an increase in body temperature is associated with reduced activation of the muscle. And that makes good sense given that one of the major by-products of muscle contraction is heat.
Increased CHO Use During Exercise in the Heat
If we look at metabolism in a hot environment, when we exercise at a higher temperature, we see an increased reliance on muscle glycogen. And this is associated with an increase in lactate production. In this slide, it’s measured the blood lactate. The increase in muscle glycogenolysis appears to be due to both an increase in the muscle temperature and also an increase in circulating adrenaline that occurs during exercise in the heat.
Fatigue During Exercise in the Heat
And so, if we think about what happens during exercise in the heat, and there’s an increase in body temperature or hypothermia, this can have an impact on metabolism and muscle function. Its known from laboratory studies that the temperature of the muscle can affect its performance too low a temperature or too high a temperature can have deleterious effects on muscle force production. There are effects on circulation and fluid balance. We saw that sweating induced losses of body fluids, they’re associated with a lower cardiac output, a lower stroke volume. And that influences the ability of the cardiovascular system to effectively deliver oxygen to contracting muscles. As we saw in the previous slide, an increase in body temperature can affect central motor drive and the recruitment and activation of contracting muscle. And all of these factors probably work together to increase the fatigue that we experience when we exercise under hot conditions.
Improved Exercise Tolerance in the Heat with Acclimatization
In thinking about how athletes might cope with an increase in exercise in the heat, a number of strategies have been employed. One is to acclimatize, an this involves athletes regularly exposing themselves to elevated core temperature. And this can be done in a cool environment by limiting heat loss by wearing extra clothing. Of course, care needs to be taken that you don’t overheat. So, exercise itself by increasing metabolic heat production will expose the athlete to a heat load. If you exercise in a hot environment, then you get an additional heat burden. And sitting quietly in a hot environment will also induce some of those adaptive responses. But most attention is focused on exercising on a regular basis in a hot environment and watching the adaptations, or observing the adaptations, that occur in response to that period of acclimatization. And as you can see from this slide, on successive days of exercise in the heat in one athlete, you can see that the exercised for a longer duration. But interestingly, they terminated exercise at a relatively similar body core temperature. And again, this has been cited as evidence that an increase in core temperature and the associated physiological responses, be they cardiovascular or fluid regulation, associated with that increase in core temperature, is a factor contributing to the fatigue.
Physiological & Metabolic Adaptations to Heat Acclimatization
If we look at the physiological and metabolic adaptations to heat acclimatization, regular exposure to a hot environment and exercising in the heat, we see an increase in blood volume. Even in previously trained subjects, there is an increase in blood volume with heat acclimatization. During exercise, there’s a reduced heart rate. There is a lower core and skin temperature after acclimatization. Interestingly there’s an increase in sweat rate and an earlier onset of sweating. But that sweat is more dilute. The fluid or the sweat that’s lost after acclimatization tends to include few electrolytes than before. And also after acclimatization, there’s reduced reliance on muscle glycogen. The lower the muscle temperature, the lower the core temperature, the lower plasma adrenaline after acclimatization contribute to this reduction in glycogen use.
Pre-Cooling Increases Exercise Tolerance
Another strategy that has been utilized is pre-cooling or cooling. And this forms the basis of some strategies that athletes have adopted, e.g., ice water baths, cooling vests that have cool fluids in pockets on the chest to try and keep the core temperature down. And you can see that the pre-cooling here, which lowered the core temperature, by just over a degree, was associated with a greater capacity to store heat, and therefore, a longer exercise duration before reaching a critically high core temperature and the associated cardiovascular responses.
Fluid Ingestion & Core Temperature
And finally, another strategy that’s being utilized to try and limit the rising core temperature, is to ingest fluids. As I said, one of the major determinants of the rise in body core temperature is metabolic heat production, as determined by the exercise intensity. You can, however, modify this slightly by the environmental temperature and also in this case, by ingesting fluids. And you can see here, that the rise in core temperature during exercise was generally in proportion to the degree of dehydration. And so a small fluid volume slightly blunted the rising core temperature, a larger fluid volume was associated with a larger blunting. One needs to bear in mind that these laboratory-based studies may slightly overestimate the effect of heat stress. It has been argued that in a freely exercising environment the convected airflow may contribute more, and the laboratory studies may have overestimated the benefits of fluid ingestion. That said, there’s still good evidence that some degree of fluid replacement during exercise has benefits both in terms of body core temperature, cardiovascular function, and exercise performance. And we’ll talk more about that in our next lecture..
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