# Letters to the Coaches: Alignment of Power Zones and 80/20 Heart Rate

David,

I am pushing through first weeks with Garmin + Stryd to track it. I noticed that my power zone 2 running results in running in HR zone X calculated on your calculator for threshold HR delivered by my Garmin fenix 5. Would it be a clear sign that I overstated my threshold Power?

Is alignment (on rather flat terrain) of 1-2 power zones with 1-2 HR zones sign of properly defined zones?

Regards,

M

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Marcin,

Great question. Because HR is easily influenced by external factors, Power and HR will only ever align under ideal conditions, and is also why Power and Pace are superior measures of intensity.

Using an automobile as an example is helpful in this regard. A car can measure horsepower, speed, and temperature very accurately. Horsepower is an output, speed in an outcome, and temperature is an indicator. Sometimes these measurements line up perfectly and predictably, but often they do not. We can be traveling at 65 miles per hour at 250 horsepower with an engine temperature of 180 degrees Fahrenheit OR traveling at 30 miles per hour at 200 horsepower with an engine temperature of 230 degrees. We can’t really use temperature (an indicator) to accurately judge intensity (an output).

When we train, we have similar options. Power is our output, Pace is an outcome, and HR is an indicator. HR is not actually measuring any kind of result, it is simply indicating how our body is reacting to the stress we are applying. And, all kinds of environmental factors play into how the heart responds to exercise, including:

– Temperature
– Time of day
– Indoors vs. Outdoors
– When you last ate
– Chronic fatigue
– Stress
– Sleep

As a result, we can perform the exact same workout with the exact same power output and get completely different HR results. I once witnessed a test with a runner on a treadmill locked in at 6:00 per mile in a room maintained at 70 degrees Fahrenheit. The average HR was 160. A week later the same runner, same room, same treadmill, same speed (and therefore the same power output), but the temperature was maintained at 95 degrees. Average HR was 185 in the second test.

Pace is obviously influenced by terrain, but that is the only practical limitation, and Power is not influenced by terrain nor the pitfalls of HR.

In summary, the 80/20 HR Zones were created based on ideal conditions. I would say that most runners on a rest week, with good sleep, in the morning, on an empty stomach, outside on a flat surface, with low humidity at 65-70 Fahrenheit would have 80/20 Power, Pace, and HR zones all line up nicely. Because we probably won’t train ideal conditions, the most consistent results will come from Power and Pace.

HR still has it’s place as a secondary measure. For example, if you don’t have a running power meter, HR is very helpful in hilly terrain where Pace becomes invalid.

David

Matt and I periodically publish anonymously your inquiries to us, particularly when the answer may benefit the community. Have a question about 80/20 training or training in general? Feel free to e-mail me. David W.

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