It’s moving into summer down under and the temperature is starting to edge its way into the uncomfortable realm with a high of 30 degrees on my ride today. Kona has come and gone and with that excitement we got a number of heat based questions via our weekly Foot Traffic facebook Q and A session. So it was quite fortuitous to be at a sports science summit where two presentations in particular focused on rehydration after exercise and the effects of heat on performance.
I’ll start with the effects of heat on performance because that seems to be a logical place to start. We have probably all experienced the energy sapping effects of heat at some stage during our training and racing and alongside this understanding there has been a lot of discussion around core temperature. In science we often measure the core temperature as a more accurate way to assess the heat stress on the body. As a result slushies had a brief feature in the endurance communities collective conciousness but one thing we found was that it’s hard to keep a slushie a slushie when you’re out on the bike in greater than 30 degrees for 4+ hours. I found a fantastic bottle in 2014 when I was on the Big Island that managed to keep the blocks of ice I put in it frozen for almost the entire ride which was bliss because water always tastes nicer cold but the volume of the effect of that internally applied cool fluid compared to the amount of externally applied fluid was pretty minimal.
The talk of interest was an AFL study (because Australians interestingly drop a lot of money into improving the performance of a sport who no one else in the world plays) but the previous work that he talked about was in fact done on cyclists because with cycling its pretty easy to get a similar sort of training scenario in a lab setting and we can thrash cyclists for quite a bit longer than other athletes. The key finding of one particular study where cyclists were allowed to cycle at their self-selected pace, was that when the difference between the core body temperature and the skin got smaller then performance decreased.
Your core temperature is going to heat up when you exercise, because our muscles are quite inefficient in their energy usage, and we use a variety of modalities to cool ourselves down. All modalities require pumping the hot blood from the centre of the body out to the skin to cool down. With sweating, the energy used to evaporate the water off your skin cools the blood at the surface of the skin which can then be pumped back towards the core and hot blood replaces it. Another, less effective method of cooling, but one that is relied on a bit more heavily in post-menopausal females, is conductive cooling, which relies on a heat gradient between the skin and the layer of air in close contact with your body. If the air is cooler then heat energy can be lost to the air from the skin effectively cooling the blood. If the air is moving around us or we are moving through the air then this layer of air is replaced by cool air increasing the efficiency of this process, however if the air is still then a convective process replaces the hot air at the surface of the skin with cooler air. Conduction is not nearly as effective as evaporative cooling, which is why we sweat and dogs pant.
Think back to that studies conclusion, reducing the gradient between the skin temperature and the core temperature results in reduced performance. Which makes sense if the skin is our conduit for heat loss, then the cooler the skin is relative to the core temperature the more heat loss we will be able to achieve
In a humid environment like Kona, or in my case the Gold Coast in summer, the air is already saturated with water and so evaporation doesn’t work so well, once you have 100% saturation no more water will fit in the atmosphere and so evaporation stops happening altogether and you usally get rain.
Which brings me to an observation one of the Foot Traffic athletes made regarding the Pro-race in Kona. It appears there was a greater application of fluid externally than internally. Don’t get me wrong internal fluid application is important to try and offset the dehydration effects of sweating, and in Kona this will be worse because the body will try and sweat more as your body heats up to try get some relief but the evaporation isn’t working. The external application of water can assist in two ways. If it’s humid and evaporative cooling isn’t very effective, applying cool fluid to the body will help to reduce the skin temperature, it’s why ice cold sponges on the run feel so good. If it’s not as humid then the external application of water can replace the sweat, it is evaporated off the skin and results in cooling of the skin. In both cases, the skin is being cooled and that gradient between skin temperature and core temperature is increased.
I’ve been in races where people come across the finish line complaining about the sprinklers and hoses the locals have out on the run because they get blisters. When I see a hose, blisters be damned, I’ll take the cooling any day. It feels good and there is a reason it feels good. Things your body doesn’t want you to repeat feel bad, things that are good for your body feel good. Your body in it’s own way is telling you to get some more of it because it’s beneficial to you.
So how does all of this lead to a performance decrement? Well the leading theory is currently that, the lower the gradient between the skin and the core temperature, the more blood is pumped to the skin, but the skin is less effective at cooling this blood down and so it kind of pools in the skin (we have complex regulatory networks of blood vessels in the skin which allow more blood into the skin in hot circumstances). If there is more blood hanging out at the surface of the body there is effectively less blood going to your lungs for oxygenation and back to your heart for pumping to the muscles. A descrease in stroke volume is observed (for the science geeks). Or a decrease in blood going to the muscles to do work.
So how do we improve performance in the heat. That goes for training and racing, because even in training we are looking at pushing the body to new limits. Keep the skin as cool as possible and then your body will cool itself down.