Running in the heat can be brutal for the inside of your body as well as the outside.At the physiological level you are forced to run slower than you would at lower temperatures and the effort required to run your usual pace will be beyond your abilities. So personal bests aren’t to be expected.For every 10 degrees celsius in increase of temperature above 12 degree you can expect drop performance or your time by about 1.5% to 3 % for a road marathon (just to give you a comparison, on the trail it could be more).
The runner is impacted from dehydration which is caused from sweating and increased heart rate and the reduced blood flow and therefore oxygen to the muscles needed to run.Sweating is the body’s inbuilt cooling system and we have sweat glands all over our body. The sweat cools the body when air flows over the skin however in humid condition the air is moist and the sweat doesn’t evaporate so easily so we perceive it to be hotter. So races in humid conditions are very hard, harder than even in hot dry climes.
Sweating leads to fluid loss but also electrolyte loss.Dehydration has a huge effect on performance a loss of just 2% of body weight can lead to a drop in performance of 4 to 6%.At around 20 to 23 degrees celcius the heart rate is already up by 2 to 4 beats a min compared to 12 degrees.When it reaches 32 degrees celsius the heart rate is up to 10 beats per min higher. (in humid conditions even more) and to make things worse the bloody volume decreases which means less blood goes to the heart and less oxygen rich blood reachers your working muscles. As it gets even hotter like when I ran through Death valley in the USA with temperatures up to 57degrees celcius it was hard to run at all. It gets even worse as the more heat that needs to be gotten rid of the more blood has to go to the skins surface so you have less energy to run and the heart and lungs have to work harder.
The upshot of all that scientific information means you have to run considerably slower. Training in heat will provide some physical adaptation (not always possible when travelling to an event). Being from New Zealand meant going usually from our winter to the northern hemisphere summer and to compound the issues you had jet lag and dehydration from flying to deal with too.Also the fitter you are the more plasma you carry and so the more you can adapt as compared to an unfit individual. In addition when you train in heat over a period of time (minimum of 10 to 14 days is recommended before a race in the heat, although thats usually an impossible dream so substitute running in extra layers of clothing and visiting the sauna if you can ).During the adaptation phase you will produce more plasma, you will have an increase in your sweat rate, you will sweat quicker and your sweat will have less sodium, all this will help you perform better to a degree.The most important thing to remember is that you need to adjust your time and result expectation and to change your attitude to your results. Its crucial not to expect the same results as you would in cooler weather.Learn to listen closely to your body during a hot race or training session. Your stomach tends to want to shut down digestion quicker too as more blood is forced to the outer skin layer which can lead to a halt also in urine production and often comes vomitting and even the occasional collapse. The stop in digestion can also mean no more nutrition/fuel coming through the stomach so your blood sugar levels can drop making you feel weak, sick and dizzy. Electrolytes can also then suffer. So if you get too hot and your digestion shuts down you can be in for a collapse. Which doesn’t necessarily mean the end of your race but it will take a lot out of you and be very unpleasant if not a downright torturous experience. You will need to take a break (don’t just collapse though in the sun, get to shade, get to a check point or at least let some other runner know you are in trouble so they can get help) If you pass out in the sun this can be fatal and I have seen this happen so caution.However if you can have a break, rest, rehydrate and wait for your kidneys to start producing urine again you can get back out there and carry on albeit slowly. This happened to my partner recently during the Grand to Grand Ultra on day one (again came straight from a NZ winter to USA summer and had jet lag and was dehydrated from flying) he had only three days to acclimatise due to work commitment restraints so wasn’t at all adapted to the heat. Then between check point three and check point four in soaring heat he started to experience a shut down in kidney function, vommitting and an inability to ingest any water or food for a number of hours. But after a 40 minute break he was able to balance himself again and to finish, slow for the day but he made it and the rest of the week, although if you had asked him at the end of day one what he thought his chances were of completing the entire race he would have given you long odds but he didn’t give up and he fought through and got there.Another thing to consider is that racing elevates the stress levels in the body and the cortisol levels which can exacerbate further the reactions to the heat. I advise taking extra electrolytes in the build up days before a hot race and to hydrate well.
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