TRAINING
Running Hot Steve Bird
T
HE warm weather provides a great time to be out enjoying the environment. And as evidenced by full Summer-fixture lists, many of us will be competing in bush and street orienteering, as well as continuing to train throughout the hottest part of the year. However, whilst participating in regular exercise all year round is strongly advocated for health, as well as fitness, we must be aware that when exercising in hot and sunny conditions we need to take extra precautions, in order to do so safely. Obviously, some of the key concerns with exercising outside during the Summer are the risks associated with sunburn and exposure to UV. These are well known, highly publicized, and relevant whether we are exercising or not. So I won’t dwell on these, as the necessary precautions are equally well publicized. Therefore, the focus of this article will be on the specific issues associated with exercising in the heat, and what to do to reduce the risks of dehydration and hyperthermia: which may not only adversely affect our performance, but also pose a health risk. This is a perennial topic and one that has been given excellent coverage in previous articles by Gillian Woodward1-4 and Adrian Uppill5. However given recent incidents, including those reported in this magazine6 it is one that warrants revisiting.
Temperature control In order for our bodies to function effectively, our temperature needs to be maintained within a specific and narrow range. To be more precise, it is our internal organs and muscles that need to be maintained at around 37oC (normal range 36.1 - 37.8oC). This is our internal or core temperature, and whilst small increases in muscle temperature of around 2 - 3oC are beneficial and enable our muscles to work more effectively, large increases tend to have the converse effect on our performance, and we get into difficulties if it rises above 40oC. If our core temperature rises too high we become hyperthermic, which is a serious and potentially fatal condition. Fortunately our bodies have a number of physiological responses that are very good at preventing this from happening, which means that we can comfortably exercise in quite a range of environmental conditions. But in extreme conditions such as exercising when its very hot (> 30oC), these thermoregulatory responses may struggle to keep our core temperature at the desired level, and we therefore need to be aware of the risks and minimize them through appropriate behaviour.
Maintaining a balance Our ‘normal’ body core temperature of around 37oC is maintained by balancing the heat produced by our body’s metabolism with how much we lose. We lose heat mainly through the skin, with some additional losses via the air we exhale from the lungs. Incidentally, dogs don’t perspire in the way that we do and therefore they have to lose their excess heat via their exhaled air, which is why they pant when hot. For us, we lose heat through the skin via four processes: Radiation, Conduction, Convection and the Evaporation of Sweat. The first three of these mechanisms are dependent upon a substantial temperature difference between our skin and the external environment in order to be effective. Which means that they work quite well on cold days, but on hot days, when the temperature of the environment is quite close to that of our skin, these processes make very little contribution to our heat loss. So in hot 38 THE AUSTRALIAN ORIENTEER DECEMBER 2006
conditions we are almost entirely reliant upon the evaporation of sweat for losing heat. This means that on hot days, even at rest, our thermoregulatory mechanisms are challenged with the task of evaporating off the heat we produce from normal resting metabolism in order to keep us cool. However, when we run strenuously, our metabolism can increase by about 800%, and therefore so does our heat production and we may struggle to lose this extra heat and risk increasing our core temperature.
It’s good to sweat When our core temperature begins to rise, we start to sweat and more blood is sent to the skin, carrying with it the excess heat from our muscles and the core of our body. The evaporation of sweat off the surface of the skin, in which the liquid water is converted to water vapour, requires heat energy that it gains from our skin. The evaporation of 1 litre of sweat removes 580kcal of heat from the body and as it does so it cools the blood, which then returns to our core. So we have blood being heated up as it passes through our exercising muscles and core, it then loses this heat due to the cooling effects of evaporation as it passes through the skin, and then returns back to the core and muscles to once again collect excess heat. It is important to emphasise that it is the process of evaporating our sweat, or any other fluid on our skin, that cools us, not the process of sweating on its own. Sweat that simply drips off you has no cooling effect. To have a maximum cooling effect the sweat should be free to evaporate and not restricted. To aid the process of evaporation on hot days, clothing should be light and whilst also needing to protect from UV, should not restrict evaporation. If we are not able to lose heat in this way, our core temperature will increase and hyperthermia will rapidly ensue.
Conflict in the blood So on hot days there is competition between the muscles that require large volumes of oxygenated blood in order to fuel their aerobic metabolism, and the skin, which also demands large volumes of blood, in order to effect cooling. Consequently the muscles may receive a reduced oxygen supply on a hot day, which is why endurance events, such as orienteering, are affected and times are generally slower on hot days.
Dehydration During strenuous exercise on hot days, sweating rates can be between 1.5 – 3.5 Litres an hour whereas, even with the most assiduous drinking strategies, we cannot absorb fluids into our body at such a high rate. Note that the fluid needs to be absorbed from the gut into the blood before it can be of benefit, so just because you’ve drunk a litre of fluid doesn’t mean that its all in your system. Indeed we can only absorb around 150 ml every 15 minutes whilst exercising, so it will take a while for it to all be absorbed. Which is one reason why we should take lots of small drinks very frequently on hot days: in order to keep our system replenished and constantly absorbing the fluids that we need into the blood stream. (For details of what to drink and when, see the aforementioned articles by Gillian and Adrian). Dehydration is associated with a reduction in blood volume. This has two major implications. Firstly a reduction in the capacity of the blood to deliver oxygen to the muscles, thereby further impeding performance. Studies have shown that losing just 1.5 litres of body fluid will impair your running performance by about 4 – 5%, and that’s not counting any additional navigational errors caused by being less able to concentrate. And secondly, when the body becomes dehydrated, further sweating is impaired, and as a result there is an increased risk of overheating. This will again detract from performance or in more extreme cases may result in hyperthermia, collapse or even permanent damage to organs such as the kidneys.
