Heat training and cardiac drift
As we head into the summer months, something you will notice is it will be a lot more difficult to keep your heart rate lower in higher temperatures. You may be asking yourself why an easy effort run that your heart rate is normally in the 130s is now a similar effort but heart rate is in the 140s-150s even. Or maybe you’re noticing that the same exact run and pace on a cool day feels so much harder on a hot day when you have approached the beginning of summer heat.
According to a study in 2015 by J.D. Periard et al, heat acclimation can start developing within 4 days of daily exposure to the heat but can fully take up to 3 weeks. What types of adaptations occur during heat acclimation? Improved thermoregulation (how the body maintains its internal temperature), reduces physiologic strain, reducing the risk of serious heat illness, improved aerobic performance, improved sweat secretion, reducing skin temperature and heat storage.
Although heat acclimation can take 2-3 weeks to fully show improvements in aerobic fitness, if exposure is not maintained, then aerobic capacity in response to heat can decline within 2 weeks. If you are residing in a hot temperate region in the summertime, daily exposure to the heat and short daily exercise can help acclimate and maintain. Other activities to consider are daily exposure to sauna or hot tub if you are in a colder climate.
Cardiac drift is a phenomenon where cardiovascular responses and higher heart rates occur after 10-15 minutes of running in the heat without an increase in workload. Some factors which can influence cardiac drift are an athlete’s training status, hydration status, dehydration, and heat acclimation. In the beginning of heat training, you will notice this cardiac drift phenomena. As you acclimate to the heat, it should improve over time. Pay attention to your hydration and electrolytes if you notice this happening for a lot longer than a month of heat training.
I have my athletes train with heart rate zones for running, but I also use pace and RPE (rate of perceived effort) to prescribe the intensity and zone of the run. For these reasons above, it is important to not get hung up on heart rate zones. In my opinion, for a zone 2 run, it is okay to drift into zone 3 heart rate during heat acclimation as long as the perceived effort RPE is between 2-4/10 throughout. Once the effort starts to increase, then you know you’ve started creeping up into the next zone. Some running coaches in TX have even mentioned throwing out heart rate training in the summer months and focusing on effort and pace alone due to this.
References:
Périard, J. D., et al. “Adaptations and Mechanisms of Human Heat Acclimation: Applications for Competitive Athletes and Sports.” Scandinavian Journal of Medicine & Science in Sports, vol. 25, no. S1, May 2015, pp. 20–38. https://doi.org/10.1111/sms.12408.
Souissi, Amine, et al. “A New Perspective on Cardiovascular Drift During Prolonged Exercise.” Life Sciences, vol. 287, Dec. 2021, p. 120109. https://doi.org/10.1016/j.lfs.2021.120109.
