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  Last updated: 1/6/2021

A useful training tool?

The Whoop wristband is just one of many forms of wearable technology that claim to be able to retroactively analyze daily activities and then recommend an appropriate level of exercise to optimize training and performance while simultaneously reducing the risk of overtraining. The Whoop algorithms used are a proprietary secret but online documentation suggests a key parameter is Heart Rate Variability (HRV).

There are a number of apps available to use a smart phone to measure HRV. As with any training tool, diet, or supplement there is a lot of information and numerous personal anecdotes to be found online.


The time between heart contractions (the RR interval on an electrocardiogram tracing) always varies by a small amount. With an average heart rate of 60 beats per minute, as an example, there is an average of 1 second between individual beats. Even though the average RR interval is 1 second/beat, there may actually be 0.9 seconds between the first two beats, then 1.15 seconds between the second and third, and so on.

Measures of HRV quantify the extent of this time variance between consecutive beats. Those interested in the specific metrics used to express this variability can find a detailed explanation here.

Your ultimate heart rate depends on various factors including nervous system input from the sympathetic and parasympathetic autonomic nervous systems. The sympathetic nervous system input (flight or fight) speeds the heart and the parasympathetic slows it.

When you are rested, your body is responsive to both sets of inputs (parasympathetic and sympathetic) and the HRV is high. This indicates your nervous system is balanced, and that your body is very capable of adapting to its environment and performing at its best. When you start your daily training or competition, the sympathetic system is primed and ready to increase the heart rate and cardiac output.

A lower HRV on the other hand indicates an imbalance of nervous input. Fatigue, dehydration, or not completely recovered from a prior bout of training are a few of the possible reasons. The result is less sympathetic stimulation, more parasympathetic tone, and a lower HRV. And less reserve to ramp up for exercise.

To look at it another way, the less one branch is dominating the other, the more room there is for the sympathetic (activating) branch to be able to come in and dominate, which is why high HRV suggests you're fit and ready to go. Greater HRV = it's time to train. Lower HRV = think about taking the day off.

This suggests a metric that would be ideal to fine tune a training program. What does the science say? Is this possible? And how can we apply this information to improving your training program?


Each of us have our own normal range of heart rate variability – which is the end result of training factors, lifestyle factors, and biologic variables.

In addition, I depends on when it is measured. Heart rate variability varies through out the day and from day to day.

This variability makes it a personal training metric, but not precise enough for fine tuning daily training. But the trend over time might provide the clues we are looking for.


Swimming - Day-to-day HRV did not significantly correlate with training volume or sleep duration. But there was a consistent reduction in HRV in response to markedly increased training loads over several consecutive days.

Triathletes - The average trend of HRV over time did reflect overreaching. Daily values did not.

There was a wide day-to-day variability, and they suggested that isolated, once per week HRV recordings may not detect training-induced autonomic modulations in overtraining athletes.

Three articles have suggested that a HRV based program may be slightly superior to a pre defined training program (such as Block Periodization). Its value was in avoiding overtraining and the skiing article specifically noted no change in VO2max or final 10K roller ski times.

But it would seem that if you are “fresher” and more enthusiastic on your training days, you would expect a better outcome at the end of the training cycle.


This article from Outside magazine best summarizes the use of HRV as a training metric. High points:

And perhaps most important: Start with a well-designed training plan. HRV data may suggest occasional deviations from that plan, but it's not a replacement for it.

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