A Pacing Strategy for Running in Heat

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Most running events and races start early in the morning before the heat of the day.  In addition, they are typically scheduled in the spring and fall in areas of the country where it is not a hot time of the year for that particular event.  However, in triathlon, where you have a swim and a bike prior to getting to the run, the weather dynamic is different — you are much more likely to be running in the heat of the day.  Not only that, you will be more fatigued because you have swam and biked prior to running.

In warmer conditions, it is more difficult to dissipate heat and keep your core temperature down.  This years’ Boston marathon was a great example.  There is a great article about it here: http://www.sportsscientists.com/2012/04/boston-strikes-back-boston-2012.html, where the “Sports Scientists” (Ross Tucker, Ph.D. and Jonathan Dugas, Ph.D.) discuss the effects of the uncharacteristic heat on the 2012 elite race.  The key point is that, in cooler weather, an athlete can recover from a surge in effort, but in warmer conditions, the accumulated heat that goes along with the increase in effort cannot be dissipated on the same time scale – you cannot really “recover” from it.  Normally, thermoregulation is not an issue, but in the race in 2012, it reached about ~88F and was hot enough to be a factor and really slowed down the elite race.

In warm conditions, over time, you will see more and more decoupling between pace and HR (i.e. pace may stay the same, but HR will increase).  This effect is amplified by the heat and attempting to maintain your “target” pace in the heat will result in a substantial increase in HR.  The additional stress can cause a number of issues, including digestion problems as your body may stop processing the fuel in your gut.  The only choice at that point is to stop or dramatically slow down, allowing your body to catch up.  You see this effect in every Ironman, no matter what the conditions, but is exacerbated by the heat and happens much more quickly in extreme conditions because of the increased stress caused by the heat and the bodies’ inability to dissipate it at the same rate it is accumulated with increased effort.  You can acclimate to heat by training in it, but this will only slow the effect, it will not prevent it.  You simply cannot go as fast as you could in cooler weather.  In addition, larger athletes are more susceptible to the heat as they generate more power and therefore more heat.

So how should you take the heat into account with respect to pacing your event?  One effective strategy is to use heart rate more strictly, targeting a cap value that you will not go over, at least not until a final push to the finish.  The goal would be to keep your HR as steady as possible, while doing your best to keep yourself as cool as possible (ice, sponges, etc.).  The challenge with this strategy is that any expectations you may have had with respect to pace are no longer valid.  The heat has changed the dynamic of the race and a smart athlete will respond accordingly.  One Vitality Multisport athlete did just this at the 2012 Boston Marathon and made adjustments based on the extreme conditions.  

In the plot above, you can see she got to ~1 hour into the race and her HR had risen close to her threshold heart rate, even though her RPE was low and she was taking it pretty easy on the “downhill” part of the course.  The athlete made a conscious decision to keep her HR in check and threw her time goal out the window.  While she didn’t PR, she did have a successful day, managing to maintain a reasonable pace while hordes of other athletes, who were more aggressive, were victim to the conditions and slowed to a walk.  She still had nearly 10% decoupling between pace and HR but ran strong to the end and only walked short stints at aid stations.

The bottom line: with heat, either be conservative at the beginning, or you can run the risk of losing a lot more time at the end.

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