Exercise alone can be hard, but exercising in the heat is a whole lot harder. Put simply, this is due to the balance between how much heat the body generates and how much it is capable of losing.

Human core body temperature typically remains around 36.5°C to 37°C, with small fluctuations across the day. Larger changes above 40°C can be dangerous.

Fortunately, humans are relatively well-adapted to dealing with the heat. One theory is that humans evolved a survival advantage as hunters because they could outlast animals that were less able to manage long periods of exertion under hot conditions.

How do we regulate our temperature?

1. Radiation: There are so many small blood vessels in the skin that, in total, they can receive up to 60% of output from the heart at rest.

As blood flows from the core of the body to the skin, opening of these small blood vessels allows more surface area for heat exchange with the environment.

Radiation of heat occurs when the surrounding environment is less hot than the skin surface. At rest in a cool environment, 60% of heat loss is by radiation.

2. Evaporation: As core temperature rises, sweat glands in the skin are activated, promoting heat loss via the combined processes of convection, conduction and evaporation.

These forms of heat exchange become more important as heat production rises (such as during exercise), and as the environment becomes hotter and radiation less effective.

Why is it so hard to exercise in the heat?

Muscles that are active during exercise demand more energy and, consequently, increased blood supply and delivery of oxygen. This means the heart has to work harder to re-circulate blood to the active muscles.

Exercising muscles also generate heat, as a by-product of chemical reactions inside cells. This increases core body temperature which, if not compensated for, can compromise the ongoing function of the central nervous system and/or muscle cells themselves.

To lose heat, blood needs to be sent to the small arteries under the skin surface, so that heat transfer can take place via radiation. Sweating alone is relatively ineffective if this re-distribution of blood does not occur concurrently.

So when exercise occurs under circumstances where heat loss is challenged (because the gradient between skin and environmental temperatures is narrow), more blood needs to be directed to the skin at the same time as this blood is needed in the muscles to increase workload.

A competition therefore develops between the skin and working muscles for the limited maximal blood flow that the heart can manage.

Different theories explain fatigue (the point at which you cannot maintain exercise duration and/or intensity) as either the inability to sustain oxygen delivery to the muscle in the face of thermoregulatory demand, or an inability to control body and brain temperature during exercise in hot environments.

The causes of fatigue are of great interest to exercise scientists concerned with sport and workplace performance. We know that acclimation (the process of repeated exercise in hot conditions) can enhance an individual’s capacity for heat loss and, therefore, improve exercise performance and delay fatigue.

Several steps in the exercise and thermoregulatory chain are amenable to adaptations that improve performance, including enhanced function and structure of the blood vessels, the function and size of the heart and even blood volume itself.

Who is at risk of heat stroke?

Heat stroke is a serious illness and those most at risk include adults aged over 65 years, babies and young children, pregnant women, people with existing medical conditions and those on particular medications. Most obviously, those who already have heart problems are at particular risk.

Such is the risk of heat illness that 10,000 excess deaths occurred during a heatwave in Europe in 2003.

In the Australian summer of 2009, hot conditions in Victoria and South Australia were linked with higher rates of illness and more than 200 extra deaths than would normally occur, particularly among the elderly and those with heart disease.

Early signs of heat exhaustion can include dizziness, headache and nausea. More severe cases of heat stroke involve hot dry skin with (paradoxically) no sweating, confusion and fainting.

Sports Medicine Australia has produced a helpful guide to performance of sport or exercise in hot weather, which details the risk and symptoms of heat illness.

What’s the best way to deal with the heat?

For professional athletes, exercising in warm conditions can improve performance and is sometimes essential to prepare for different environmental conditions, such as preparing for an Olympic games in a hot country.

Regardless, it’s important to take precautions to reduce the likelihood of heat illness by staying hydrated, wearing appropriate clothing and knowing your limits.

For people who don’t exercise regularly or those who are at increased risk of heat-related illness, avoid exercising in the heat, such as the middle of the day in summer or rooms with poor air circulation.

Instead, opt for a prescribed, gradual and incremental exercise program that improves your cardiovascular and thermoregulatory tolerance. Gradual and repeated heat exposure, even in the absence of exercise, may also be good for your health.

The good news is that even exercise in normal or cool conditions will improve thermoregulatory capacity and resilience under hot conditions. But if it’s out of the ordinary for you, extreme exercise or exercise in extremes, is equally unwise.

This article was originally published on The Conversation.
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By Jack Goodman, University of Toronto

Parts of the northeastern United States are still digging out after a blizzard. And more snow is on the way. You may have seen news stories warning that shoveling snow can raise the risk of a heart attack, or heart problems, at least for some people. So, why is that?

Snow shoveling can be vigorous exercise

Snow shoveling is a unique form of exertion. It can be vigorous and challenging to the cardiovascular system in general, and the heart in particular. When combined with the inherent environmental conditions of winter, snow shoveling during or after a blizzard may be the “perfect storm” for a cardiac event in those with known or “occult” (hidden) coronary disease.

First, a little background. The “risk-paradox” of exercise is that while physical activity performed over the long term is known to be very healthy and significantly lowers the risk for heart disease and other chronic diseases, vigorous exercise acutely increases the immediate risk of a cardiac event above that expected at rest. The risk of having a cardiac event while you are in a rested state is low, but you are more likely to have a cardiac event while performing vigorous exercise.

Despite this, it is widely accepted that the benefits far outweigh the risks and should be encouraged. In fact, the more physically active you are, the less the acute risk is. Being in good shape can give you added cardio-protection.

Challenging the heart

The reason snow shoveling leads to increased risk of a cardiac event is rather simple: at rest the oxygen supply to the heart is not challenged very much, but during exercise the heart’s oxygen demands increase dramatically. Snow shoveling is particularly demanding in this regard.

When you are healthy, your body has the ability to increase oxygen supply as the demand increases. As the heart beats faster and harder, the coronary vessels open up and supply the additional blood flow that is required to feed the heart’s increased demand.

However, when coronary disease is present, blockages in the coronary vessels can limit blood flow, leading to slowed and inadequate oxygen supply. When the heart muscle (the myocardium) does not get sufficient oxygen, it can rapidly fail because it is highly dependent on blood flow (oxygen) to generate normal contractions. If the heart muscle becomes starved for oxygen, it also becomes vulnerable to abnormal electrical activity that can lead to dangerous arrhythmia’s (irregular heartbeats) or a heart attack.

Other less common types of underlying cardiac disease, often undetected, may also make the heart vulnerable to arrhythmia’s during heavy exertion when oxygen supply is outstripped by demand and can lead to similar, catastrophic consequences.

So what is it about snow shoveling?

Very few studies have examined the physiology of snow shoveling because it is technically difficult to do so. We do know that the cardiovascular demands of snow shoveling are very high. In fact it rivals the demands during maximal exercise testing, such as a cardiac stress test on a treadmill or bicycle, which often exceed 80-90% of your maximal exercise tolerance.

There are two aspects of snow shoveling that make it unusual for the heart compared to, for example, walking or jogging.

First and obviously, it is typically performed in cold weather. Cold air inhalation may cause a reflex constriction of blood vessels, including the coronary arteries. Cold air may also increase the blood’s propensity for clotting. If blood clots form and there is a tear in the inside of the artery, a blood clot could form a blockage.

The second issue is the nature of the exercise – and snow shoveling is unique. It is typically done without a warm up, and includes considerable arm work that increases blood pressure drastically. As blood pressure rises, so too does the work of the heart. Meanwhile, your leg muscles are typically performing isometric work (where you produce a lot of tension but your muscles don’t move your joint – like an even tie during an arm-wrestle). This type of muscle activity, especially in the upper body (as you tightly grip the shovel), raises blood pressure more than, say, walking or jogging.

Each time you thrust the snow, the arms rise high, increasing blood pressure further. In addition, you typically perform the Valsalva maneuver: a natural tendency to generate lots of chest pressure without exhaling breathing out (like tennis players do when they hit a ball, or when you push hard against something). This further increases blood pressure.

Ironically, the metabolic cost of snow shoveling (oxygen consumption for the whole body) may not be more than typical sub-maximal exercise like a light jog, but the oxygen cost to the heart is likely to be very high. Failing to warm up may add further demands on the heart. The blood vessels going to the working skeletal muscles may not open up as quickly, so it takes more effort for the heart to pump blood.

The bottom line: the cardiac cost of snow shoveling is disproportionately high, is compounded by cold air inhalation which may increase the likelihood of blood clotting and coronary vessel constriction. The nature of the exercise is challenging to the heart and is not likely to be accompanied by an adequate warm up. I also suspect that the cool down after this exercise is likely minimal if not non-existent. Throwing off the coat and sitting in a state of exhaustion certainly doesn’t count as a proper cool down.

Snow will keep coming, so what can you do?

For most people, snow shoveling is a safe form of exercise, even if it is a little difficult. But you should get a green light from your doctor if you have been diagnosed with, or are considered to be at risk for, coronary heart disease. If you are not accustomed to vigorous exercise, some caution is warranted.

Snow shoveling can be vigorous exercise, so start the right way. Warm up gradually – at least 5-10 minutes. Consider wearing a scarf around the mouth in very cold conditions, as this will warm inhaled air. Start with small loads of snow and keep them manageable – especially when its wet and heavy. Use a smaller shovel so that your lifts are much, much smaller and can be managed for a long time without getting exhausted. Breathe throughout the entire process of gathering, lifting and throwing the snow – don’t hold your breath as you exert effort. As you warm up, some greater effort can be considered, but easy does it. When you’ve won the battle, cool down – walk it off for a few minutes, perhaps inspecting the shoveling efforts of a few neighbors before going inside to a warm house.

Winter doesn’t end (astronomically speaking, at least) until March 21. For those of us living in colder parts of the world, that means there’s likely plenty more snow still to come.

Remember that being physically fit lowers your risk from cardiac disease, and regular exercise is highly recommended for most individuals. That being said, be cautious and get sound advice from your physician if in doubt. For most people, regular exercise (including both aerobic and strength) will enhance your quality of life – and make your future shoveling much easier – and safer too!

This article was originally published on The Conversation.
Read the original article.