When we run in warm conditions our body temperature rises and in order to keep from overheating our body starts to take steps to reduce our temperature. The main way our body tries to reduce our temperature is by sweating, which cools the surface of the skin when the sweat evaporates, and then diverting more of our blood flow to the skin surface to cool it. This diversion of more blood to the skin surface means that less blood is available to be used by working muscles and organs. This decrease in blood flow to the muscles means that the same work load will require more effort /energy to accomplish or that less work can be performed using the same amount of effort.
That last part is key to how we train as distance runners in the heat and humidity of summer time. Trying to run at the same pace for the same distance in the warm weather can require significantly more effort/energy than it will in the cool weather, so we must either reduce the speed or the distance we run (or a combination of both).
Since we are “distance” runners and the length of races don’t change, I am do not believe in reducing the distance of the training we do in preparation for a certain race distance. This means that if we are training for a marathon in the summer we still run the distances in training required to properly prepare for a marathon, because on race day we still have to run the race distance. They don’t shorten the race because it was hot when people trained for it.
Having said that, many distance runners choose to lessen the total mileage needed in training during the summer by racing shorter distances, choosing to focus on speed rather than endurance during this time of year. This shorter distance focus in the warm weather months is a very valid approach, but may not fit everyone.
Training / Racing Pace
So if the distance we run in training for a specific race isn't changing, than it becomes mandatory to adjust the pace at which we run that distance in warm weather. The logical question then is: how much should we be slowing our paces? The answer that question depends on several factors including the air temperature, the humidity level and how acclimatized we are to the heat. The higher the temperature the harder the body will have to work to cool itself so the more blood that will diverted away from muscles and to the skin surface. The higher the humidity levels the slower the sweat evaporates which slows the cooling process. And how acclimatized to the weather we are affects how efficient our body is at this cooling process.
Here are a few rules of thumb that will help you get in the right ball park when it comes to weather adjustments.
Adjust your pace approximately 0.1% to 0.15% for each degrees F above 60 F. Early in the season, when you aren't acclimatized, the adjustment will probably be on the 0.15% side but by the end of the season it may be down to 0.1% (or less) when you are use to (more efficient at dealing with) the heat. Then I would adjust the temperature for the humidity as well. So if the humidity level is high I might add 5-10 degrees to the temperature I use to adjust the pace further pace.
This is just a tough rule of thumb to help you get started figuring out an adjustment for you, but you may have to customize this some for you and your specific body make-up and situation.
Example: If I normally run 5 miles at 7:00 pace on a cooler weather day, but the temperature when I go out for this run is 80 degrees, than I would slow the pace down by:
80-60 = 20 degrees over 60F. 20 x 0.15% = 3.0% So 3% slower than 7:00 is 7:13 per mile. I would do this run at 7:13 per mile target.
If it was also fairly humid that day (60-80%) I might adjust that temperature up to 85 degrees or so giving me an adjusted pace of 7:16 per mile. If high humidity I might be running at 7:20 or slower.
Note: reduce your adjustment by roughly half when doing intervals/repeats, as the body gets a chance to cool itself back down during the recovery intervals. There is still an adjustment necessary but it is not as big as on continuous runs.
Since the body’s main way of dealing with the heat revolves around increased sweating, our ability to deal well with the heat will require that we stay well hydrated. This includes making sure we have good hydration habits all through-out the day, in addition to re-hydrating well immediately after our runs. While water will be our primary concern, we also need to be sure that we are replacing the electrolytes we lose in our sweat as well. Our body can take much of these electrolytes from a healthy diet, but during the warmer weather times we may need to also increase our intake of electrolytes through sports drinks or supplements. During longer runs in warmer weather (over 40 minutes) it may also be a good idea to arrange to take in some fluids during the run. This will be vitally important during long runs, where some electrolyte supplementation may also be beneficial.
Positive Effects of Heat Training
So now that we have talked about the downside of training in the heat, let’s talk about the benefits. There seems to be good evidence that training in the heat may have very similar effects to training at altitude. Think about it for a minute. At altitude the level of oxygen delivered by our blood to our muscles and organs is less because oxygen levels are lower at altitude. The body responds to this by increasing our blood supply so we have more blood with which to deliver the oxygen. Similarly in the heat, if a portion of blood supply is being diverted to the skin, that leaves less going to the muscles and organs and the body can respond to that over time by increasing our blood supply. So altitude and heat may be two different ways to achieve the same beneficial adaptation (increased blood supply). Additionally there is evidence that warm weather training increases our body’s efficiency at cooling itself, which has carry over benefits even when we are running/racing in cooler temperatures. The less the body has to work to cool itself, the more blood available to transport oxygen to the muscles.
So instead of dreading training in the warmer weather this summer, embrace it as an opportunity improve yourself as a runner with bout of “poor man’s altitude” training. Sorry Boulder, Albuquerque, and Mammoth Lakes, I am getting my altitude training done here in the Deep South this summer. J