How to Use Heart Rate Variability (HRV) to Guide Your Training

Heart Rate Variability (HRV) is a valuable fitness indicator that can be used to make smarter training decisions. In order to appreciate the power of HRV, it is helpful to have an understanding of the principles associated with training adaptation.

Homeostasis and Allostasis

The human body is constantly working to maintain its internal environment. Your brain takes feedback on the status of your body and drives behaviors to keep you in a state of balance. This process of the body striving for biological equilibrium is called “homeostasis,” and is attributable to competing inputs from the sympathetic and parasympathetic branches of the autonomic nervous system (ANS). “Allostasis” is mediated by the brain and is a term used to describe how the body maintains balance by responding and adapting to external stimuli. Allostatic load is the wear and tear on the body which accumulates in individuals over time. Heart rate variability can be used to better discern if load is functional/optimal (and contributes positively to fitness) or non-functional (and contributes negatively to fitness).

How Heart Rate Variability Fits In

The two branches of the ANS, the parasympathetic and the sympathetic, are at the heart (pun intended, haha) of homeostasis and allostasis. HRV is a measure of the variation in time between each heartbeat and is controlled by the ANS. High variability means your heart is responsive to both inputs from the ANS, and in this state training adaptation is likely ideal--your body is ready to perform. Low variability means your heart is less responsive to inputs from the ANS and will likely yield poorer adaption. Your level of autonomic balance (quantified by HRV) gives us insight into how your body is responding to training. Because of this, HRV is an objective measure of the phenomena of allostasis. This knowledge can help us understand how we can better apply training load and intensity to meet our physiological intent.

HRV Training Alone, or WHOOP Recovery?

It is important to understand that the WHOOP recovery algorithm is based on a lot of existing HRV and exercise research (Kiviniemi, Plews, Buchheit, Flatt, etc). From combining this groundwork into our ecosystem we’ve been able to show that things like basketball shooting percentages, exit bat velocity and fastball speed in baseball, swim times, running times, and much more all correlate to WHOOP recovery. And while heart rate variability is an important input into recovery, research has shown that our proprietary algorithm, which also factors in resting heart rate (RHR), respiratory rate and sleep, is more predictive of next-day capacity than any of those signal markers taken in isolation. So when designing your training, you should feel confident that WHOOP recovery is a good estimator of your capacity, especially in the context of the previous 2-3 days.

Allostatic Load, Recovery & Overtraining Syndrome

The drive for allostatic balance is powerful, which is why if your recovery is low you will likely feel little urge to train--your body wants to use all its resources to get you back into homeostasis. Exercising in these moments pushes all your energy away from recovery and toward meeting the energy demands of the workout. Over time, this practice can lead to “overtraining syndrome” (chronically taking on too much strain relative to capacity). On the other hand, when WHOOP recovery is high, your body is positioned to provide the energy you need to crush your workout.

Training Spectrum

Your daily WHOOP recovery is bucketed into green, yellow, and red, which gives you objective feedback on how the stress/strain you are putting on your body might be contributing positively or negatively to cardiovascular fitness. Recovery and HRV trends over time allow you to understand if your training is leading to an “optimal,” “overreaching” (which can be functional or non-functional, more on that below), or “restorative” state.

OPTIMAL

In this state, individual stress response to each day of training decreases and cumulative stress load is elevated. This occurs when you are applying load and intensity and your body is adapting to that load in a positive way. You should see small incremental improvements over time:

• Some distribution of higher yellow and green recoveries (which means that strain is generally not exceeding capacity)
• HRV trends slightly upward
• RHR trends slightly downward

To maintain this state, you should avoid training volume and intensity that create acute changes (2 or more consecutive days with greater than 20-25% deviation) to your baseline metrics.

FUNCTIONAL OVERREACHING

To accelerate fitness gains, advanced athletes will require a period of time in an “overreaching state” to achieve a new adaptation. This is literally pushing beyond the point at which your body can fully recover from training, without it breaking down. In these cases, as you increase load and intensity expect to see the following:

• Mostly yellow and perhaps 1-2 green recoveries per week (with strain slightly exceeding capacity 3-4 days of the week)
• HRV trends downward (around 10-15 ms)
• RHR trends upward (maybe 2-3 bpm)

You shouldn’t feel a need to create major changes in your baseline metrics--red recoveries are not necessary for fitness gains. WHOOP recovery is a powerful tool to help you monitor the delicate balance between ‘non-functional’ overreaching (which leads to overtraining) and ‘functional’ overreaching (which enables fitness gains). You will need to experiment to find your sweet spot, but generally speaking you should expect a lot of yellow recoveries during these functional overreaching phases of training. It’s also important to monitor other lifestyle choices (sleep, diet, alcohol consumption, etc.) to accurately evaluate the impact of training on your body.

NON-FUNCTIONAL OVERREACHING

A combination of reduced HRV and significantly elevated RHR is an indication that the body is stressed and not adapting to training stimulus in a positive fashion (it can also be a product of acute anxiety or illness). You will see decreases in performance and might lose the motivation to train. In either case, rest is in order. You should prioritize mobility work and only partake in exercise that is enjoyable to you if you notice these signs of overtraining:

• Mostly red and perhaps 1-2 yellow recoveries in a week
• HRV is greatly suppressed relative to baseline (potentially a drop of 25-30 ms)
• RHR is highly elevated (roughly 3-6 bpm)

RESTORATIVE

There are a lot of different ways to think about a “restorative phase,” but generally speaking it means you should take on less strain than you have the capacity for. Early on in this taper period (after a hard functional overreaching block) your metrics will trend downward, but if you continue to keep strain low and focus on recovery they will begin to bounce back. The goal is for your HRV and RHR numbers to end up better than they were before the functional overreaching phase. The taper period is a great way to evaluate if your training yielded physiological benefits, and you will likely see:

• Yellow recoveries to start, then trending upward with ideally green recoveries toward the backend of the taper (7-10 days)
• HRV is higher than pre-functional training baseline
• RHR is lower

Putting it All in Context: Other Factors Affect HRV and Recovery Too

As mentioned previously, it is important to note that training is not the sole influencer of whether or not a workout ends up meeting whatever your intent was. You can train perfectly, but still undo its benefits with other lifestyle factors (work stress, poor sleep, stressful family life, poor nutrition, etc). It is essential to always contextualize your training, and if you are trying to illicit a very specific response you must control for the “other” hours in your day outside of training. That time will either serve to amplify your efforts or detract from them. Continually testing and retesting both performance measures and perceived efforts is a necessary part of the process, as well as assessing them in conjunction with measures of total training loads and other lifestyle factors. This will help determine the cause and appropriate strategies to meet your intent.