Using RPE to Run in the Heat Over Heart Rate and Pace

June 30, 2024

Mike Ricci


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While the temps are rising across the landscape and as triathletes ramp up their training as they prep for their biggest events of the year, we have the same conversations behind the curtain at D3: Even though we prescribe a lot of Z2 for our athletes, when the rubber hits the road, and the road heats up, it may be time to shift your approach.

Before we dive into how and why we think this shift is important, let’s first discuss why training by heart rate is a far superior way to train for most of us for the better part of the year. When you have ideal conditions indoors on your treadmill or even outside in cool settings you can rely on your heart rate to give you great information as to what’s going on with your body. When we talk about using HR for measuring effort, we talk about using a chest strap and not a wrist strap. Wrist strap technology has gotten better over the years, but it’s still not as accurate as a chest strap in our experience. A chest strap remains the gold standard for measuring heart rate. 

We won’t completely dive into everything as we’ve written enough on the subject already (Zone 1 and Zone 2 Explained link here) - but most of your training is going to be in the Zone 1 and Zone 2 range. This means 70-90% of total training time for most of us. The newer you are to the sport, the more Zone 1 and 2 you’ll need in your program. For example, if you are coming from a non-endurance sport - and you haven’t run a 5k, much less a half marathon, you’ll want to build up your endurance and lean into the Zone 2 work. The more ‘easy’  (Zone 2) work you can do, the bigger the capillary bed you’ll build and the better you’ll be at delivering oxygen to your muscles and carrying lactate away from your muscles as you fatigue. We’ve seen lots of short course PRs come from “long and low” training - (long sessions at a low heart rate). If you want to learn more about Zone 2 training, one of our latest podcasts covers the subject pretty well. Listen to D3 Coaches Mike and Jim discuss the topic here

Building that nice and robust aerobic base will only help you create a bigger engine and allow you to train and race at faster paces over time. There’s nothing like years of aerobic work to build that engine and see how much you can improve year over year. We see this with our athletes time and again and our experience says that patience in building this base is a key skill set. 

Now, many of you may say, “I don’t use heart rate and I’ve always trained by pace” and that’s fine too! Although we have written about that as well here. I still think you will run into trouble when it heats up and you can’t deliver the same amount of oxygen to your muscles because your body is trying to cool itself, and therefore your pace will be hard to maintain at the same effort or you will push too hard to maintain that effort. You can run into some problems if you get dehydrated, such as have heat stroke. So it’s smart to know your limits in the heat. 

And on that note, let’s back up a minute. Your skin is the biggest organism of your body. Therefore, when it gets hot, your skin will send a signal to your brain that your skin needs cooling. Your sweat glands will then be activated and it will send water to the surface of your skin, in the form of sweat. Water then evaporates on the skin and cools the body, but only when you are in a dry climate. When it’s humid, evaporation will slow. This is all in direct conflict with your muscles asking for more oxygen as it's trying to maintain a certain pace. Your heart rate will rise and you may be running ‘easy’ but as your pace drops and your heart rate rises, your body is giving you a different signal: This is getting harder to maintain. If your body temp gets too high and your heart rate rises too much, you can get heat exhaustion, as we mentioned above. 

There is another option to running by heart rate and pace and that’s running by RPE or Rate of Perceived Exertion. As you learn to use all your running metrics: heart rate, pace, and RPE, you learn how they all give you different information but hopefully lead you to the right pace, heart rate, and/or effort depending on the environmental conditions. Using a subjective measure is a skill set you learn over time while using RPE. From here you can figure out how hard you are working physically and if it’s sustainable. We like to use a 1-10 RPE scale to align effort with paces. 

RPE 1-4 - usually very light 

RPE 5 - very moderate or a low Zone 1 

RPE 6 - Easy, all day effort - Zone 2 - can hold a conversation

RPE 7 - Tempo, sustainable for a while and you can still talk in sentences

RPE 8 - Threshold effort, short, one-word answers

RPE 9 - Very hard effort - VO2 - lasting 5-6 minutes

RPE 10 - all out effort only last a few seconds

Here are some thoughts on using RPE in your training and racing: 


1. Be honest with your RPE. It’s easy to say ‘It’s easy’, but you need to think about it and be honest with yourself. Is your breathing really where it should be? Can you hold that effort for the desired time, be it 2 hours for a half marathon or 40 minutes for a 10k? 

2. Make sure you have tested RPE in training. Trying to use RPE only in racing will probably lead to a result that is less than what you want. 

3. Post race - align HR, Pace and RPE and see what trends you can find. Do the paces and HR align with what you are seeing in training at the same temps? 

4. Most important, from D3  Coach Dave Sheanin, “RPE has to reflect how you are feeling dispassionately. It’s not aspirational. If you lie to yourself, you’re sunk.” This goes back to number one above, but it’s so important we had to say it again. 

Above all else, when the temps are high and heart rate seems to be out of range, start working some RPE into your training so on race day, you are dialed in and can have a great race, no matter the weather! 

Coach Mike Ricci is the Founder and Head Coach for D3 Multisport.  His coaching style is ‘process-focused’ vs. ‘results-focused.’ When working with an athlete, their understanding of how and why they are improving is always going to take precedence over any race result. Yes, there is an end goal, but in over 2 decades of coaching, experience has shown him that if you do the right work, and for the right reasons, the results will follow.

Coach Mike is a USAT Level III Elite Certified Coach, Ironman University Certified Coach, and Training Peaks Level II Certified Coach. He was honored as the USAT Coach of the Year.

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