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Recognizing and Managing the Impact of Heart Rate Variations on Zone-Specific Training

BY Pete Stock

Coach Pete Stock breaks down some of the factors that can affect heart rate while training, and how you can adjust your zone expectations accordingly.

A few days ago I got a TrainingPeaks feedback comment from one of my athletes (named Peter), after what should have been a long Zone 2 run workout in winter temperatures. He said he couldn’t keep his heart rate down enough to stay in Zone 2, and ended up at a pace between the aerobic zone effort I’d asked for, and what he felt was right (which turned out to be more like Zone 3). This is a common problem among athletes, since what feels “natural” to most of us is actually just above Zone 2.

Peter is a young lad and only recently got a heart rate monitor, so we are making the shift from using rate of perceived exertion (RPE) to heart rate zones. But there are many reasons why an athlete’s heart rate might vary from session to session, including stress, lack of sleep, alcohol, caffeine, heat, hydration, and, of course, overtraining. As coaches, we can’t instantly cure most of these for our athletes, but we can be aware of them and help the athlete understand their impact so they can deal with the consequences more effectively.


Stress and lack of sleep are common and have many underlying causes. I have one athlete who will complete every session, every time, to 100 percent of the effort, regardless of any external factors. If I don’t see or anticipate any issues, she will knock herself out every time until she gets really sick (a lesson I learned the hard way). Now, I tell her to miss sessions when I think she needs it.

It’s important for us as coaches to help athletes know their own bodies and their own limits, and encourage them to be proactive in keeping their balance right.


Hydration is easy enough to understand, although getting the balance right can be difficult. Being dehydrated can increase HR by five percent and reduce performance. There is also a recent study that suggests just believing you are dehydrated can reduce performance.

Athletes also need to understand that too much water can be bad if you don’t take enough salt and electrolytes, and that they need to hydrate when it isn’t obvious, like in cold weather and when swimming. It’s amazing how many athletes stop drinking when they get in a pool, or when the weather is cooler. We need to help our athletes by educating them and advising them on the correct hydration levels so that they are more capable of managing themselves.


Overtraining is probably the biggest danger we need to be aware of, and a raised resting HR is one of the big signs. Overtraining might also manifest as a lower training HR or an inability to get your body working. If you can’t increase your heart rate, this decreases your ability to exercise at the intensity you desire. Teaching your athletes to take their resting HR in the morning is a good way to help you to step in when needed, and to help the athlete become more self-aware.


Most of us are aware that HR goes up when it’s hot, but what isn’t quite so obvious is that it also goes up when it’s cold. In cold weather, the blood vessels shrink, which reduces the amount of oxygen getting to the heart. The heart then has to work harder in order to pump the blood, and this increases HR. This problem is exacerbated further as the temperature really drops, since the body then starts to shift blood into the core areas and away from the peripheral regions.

The variation is lower the more fit an athlete is, though having lower body fat will increase the variation potential. Dressing appropriately is an easy fix. The formula of dressing for a temperature eight to 10 degrees celsius higher than the thermometer says is a good guide. In other words, if it’s five degrees celsius outside, dress as if it’s 13 to 15 degrees celsius. The working muscles will soon generate heat with their contractions to keep you comfortably warm without overheating.


In Peter’s case, I think it’s most likely that temperature was a big factor in his higher-than-expected heart rate values during his Zone 2 run. I wanted him to run at between 123 to 133 bpm, but he’d drifted up to 135 and above (into his Zone 3) without necessarily feeling the exertion, due to the sub-zero temperatures in which he was running. Although it’s only a few beats, the effect can be crucial when it comes to avoiding fatigue for later training. This Zone 3 effort meant he wasn’t going to be able to perform at the higher intensities when I wanted him to later in the week.

I felt bad about this because I hadn’t anticipated it and told him what to do. I’d simply loaded up one of my standard TrainingPeaks workouts onto his calendar and left TrainingPeaks to calculate his zones using the values on Peter’s profile. It was a simple session, in a relatively simple week, and I didn’t give it enough real thought.

To make matters worse, I ignored two prompts that should have alerted me: another athlete had the same problem a few days earlier, and I had the same issue myself on my own Zone 3 run that weekend: my HR had been up in Zone 5b for a while before dropping to normal values after 30 minutes or so.

While HR is a valuable metric, it’s important to remember it can vary from session to session for a number of reasons. As coaches, we have to be aware of these and make sure our athletes understand what might happen due to these variables. We need to think about more than just the session — we need to think about the athlete, the conditions, the variables, and we must anticipate potential issues in order to reach maximum training potential.

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About Pete Stock

My name is Pete Stock and I work as a triathlon, running and swimming coach near Manchester in the UK. I’ve competed as an age-grouper in races in the UK and Internationally, with highlights including Escape from Alcatraz triathlon, Death Valley half-marathon, Helvellyn triathlon and the recent Age-Group World Triathlon Championships in Gold Coast, Australia. Sport and competition have always been a big part of my life, but now I’m coaching, marshaling and volunteering in order to put a little bit back into the sport that has given me so much over the years.

TrainingPeaks Level 1 coach, British Triathlon Level 1 coach, British Triathlon S&C certified, British Triathlon Open Water certified, L2 British Swimming teacher, L2 Gym Instructor.