Summer Training - Heat Effects on Training
Storytime
In August 2020, I was training to break 4 flat in the 1500 after running 4:17 in June. As one does in the summer, I was building my base, and trying to increase the duration of my long runs. So, silly little Bill decided to embark on a 13 mile run starting at 9 AM with a Dewpoint of 70 degrees. Well, this is my heart rate and pace data.
Despite slowing my pace, my heart rate jumped from a starting rate of 128 to a whopping 171 BPM. For context, this is like going from an easy run to a hard tempo whilst slowing down.
What happened?
The Physiologically:
This is textbook cardiac drift, which is the slow upward drift in heart rate at a steady work load due to a progressive loss of water in the blood, called blood plasma. As a result, each beat of the heart delivers less blood (because there is less volume!), so you increase heart rate to sustain oxygen delivery and blood pressure.
Because metabolism is so inefficient, heat dissipation is one of the unexpected necessities to prevent physical fatigue. In fact, a classic study (1993) showed that when cyclists start at 36, 37, or 38 degrees Celsius, they always terminate the exercise at 40 degrees Celsius, which is now referred to as “critical core temperature.”
While we don’t know exactly why this happens, the theory is that proteins begin to denature at 41-42 degrees Celsius. Thus, your brain detects this threat and says, “no, nope, nadda, I’m out”, and terminates exercise.
I know I need to drink water, but how much?
Before you run, you need to drink. Typically, consuming at least 1-2 Liters, 2-3 hours prior to the onset of exercise is important. Moreover, drink it slowly. When you drink rapidly, you increase gastric emptying and the water simply moves through your small intestines without getting taken up.
Moreover, DRINK IT DURING THE RUN. You’re not being a tough person by running without water, and it’s literally only hindering your performance. You’re just hurting yourself. Specifically, you need to drink the following during exercise.
· ~0.7 L/hr in some hot-humid conditions
· ~1.2 L/hr in trained athletes exercising in hot-dry conditions
· 2–3 L/hr in highly trained, heat-acclimatized individuals
Keep in mind, if you’re well trained, you’ll have a higher sweat rate.
For what it’s worth, in ultramarathoners, they plan to drink much more than they do. So if you think you’re drinking a certain amount, you’re probably drinking less than you actually think. One quick note: Hyper-hydration is scary, so make sure you are taking some salt with your water.
But Dr. Evans, I drink water, what else can I do?
In addition to drinking water, you could also try some form of pre cooling. There are 3-4 research supported methods to do so.
1) Drinking an icey slurry, basically water with ice chips
2) Cold water immersion
3) Ice vests
4) A mix of these
What you can see from this plot (called a forest plot) is that pre cooling enhances exercise performance. Each of these methods are slightly different and have different guidelines, so I can’t cover them all, but you’re more than welcome to go read the paper.
Alright, this took longer than I wanted. That concludes my first newsletter. WOOHOO. I hope you learned a thing or two.
The TLDR:
• Expect slower paces and higher heart rates in heat
• Heat, dehydration, and plasma volume loss contribute to cardiac drift
• Drink before you run, but don’t chug huge volumes right before starting
• For longer hot runs, estimate your sweat rate and drink regularly
• Include sodium if you’re drinking large volumes or running long
• Cooling strategies like ice slurry, cold water immersion, or ice vests can improve heat exercise performance
• Next week: heat acclimation
If you found this newsletter insightful, I think you would really appreciate some of the work content we have on the podcast, where I have discussed the science of running with incredible guests, including professionals, olympians, and top NCAA athletes. Additionally, make you follow me on Instagram for more insight. Keeping crushing it!!
