Heart Rate Variability (HRV)

HRV is the time difference between each R–R interval; that is, the time difference from heartbeat to heartbeat. HRV tracking shows that the time between each heartbeat is not the same each time; it varies with every beat. Hence, the term heart rate ‘variability’.

HRV has been shown to be a predictor of illness in elite athletes. However, its ability to predict injury is yet to be validated in humans. It wasn’t so long ago that HRV tracking was only possible using expensive medical equipment, such as an electrocardiogram. Now, HRV can be reliably measured using health-tracking technology such as smartphone applications, heart rate monitors and finger-wave pulse sensors. This technology can accurately measure HRV during ultra-short durations (1 minute) in either a supine, seated or standing position.

While many metrics are used to measure HRV, the most common and (and arguably most reliable) is  the root-mean-square difference of successive normal R–R intervals (RMSSD). The RMSSD measure is the most valid and most-researched measure and is therefore practical to apply to sporting settings.

How HRV is used to determine recovery status

It is important to remember that HRV provides critical information about the function of the ANS and that it is also the most reliable measurement of ANS function. An increase in HRV represents a positive adaptation/better recovery status, while a reduction in HRV reflects stress and worse recovery status. Having said that, it is also important to note that higher is not always better, and lower is not always worse (this will be discussed below). But as a golden rule of thumb, the higher the athletes HRV, the fitter/better recovered they are, and vice versa.

The ANS comprises two branches: the sympathetic nervous system (SNS) and the parasympathetic nervous system (PSNS). The easiest way to distinguish the differences between these two branches is to associate fight or flight responses with the SNS and rest and digest responses with the PSNS. Therefore, the SNS increases heart rate and the PSNS slows it down.

How HRV is used to determine improved wellbeing

Numerous studies provide evidence that coherence training consisting of intentional activation of positive emotions, paired with HRV coherence feedback, facilitates significant improvements in wellness.

5 tips for tracking HRV

For more accurate and relevant comparisons, consistent and repeatable measurements should be performed as much as possible. This will minimise confounding factors or misleading results. When using HRV trends to make important health and performance decisions and assessment, athletes must ensure they are comparing ‘apples to apples’ and not ‘apples to oranges’. Factors that affect repeatability and comparability include:

• Time of day – Athletes should attempt to target the same 1-hour window each day, as circadian rhythm has a significant effect on HRV.
• Body position – Athletes should ensure they use the same measuring position (i.e., sitting, standing or laying) every time, as torso angle has been proven to affect HRV.
• Activity before HRV reading – Exercising, conversing, eating and caffeine intake are common factors that affect HRV. For optimum comparability between readings, athletes should perform the same activities between readings (e.g., always take baseline readings soon after waking up).

Chronically low HRV is, as a general rule, not what athletes should be striving for. However, a single or handful of low HRV readings is not always bad. In fact, strategic acute drops in HRV can be favourable, as long as HRV recovers to normal or better levels. Listed below are situations in which a low acute HRV is desirable.

• A slightly lower HRV compared to the athlete’s baseline (associated with increased sympathetic nervous system activity) on a competition day can be favourable, depending on the activity.
• An acute HRV drops after an intense workout or series of workouts that returns back to normal or better within a few days or weeks’ time can indicate good health.
• A sudden drop in HRV after a change in nutrition, training method or lifestyle can indicate that the change is working.

Just as a low HRV reading is not always bad, a high HRV reading is not always good. If a single HRV measurement is abnormally high compared to the athlete’s baseline or norm, then it can mean that something is off. Listed below are examples of when a high reading does not indicate better health.

• When in a state of hyper recovery. This indicates that the body has accumulated too much stress, to the point that it can no longer effectively handle the stress and the body’s resources become depleted. In this situation, the body might force itself into a hyper-recovery mode  to protect itself. This state is not ideal for long-term health or performance.
• When under continuous low-grade stress. These constant stressors can cause HRV to be higher in the short term because the body is constantly trying to recover. If an athlete’s HRV is high but they frequently feel fatigued or drained, then they might be exposed to chronic low-grade stress that is constantly consuming energy and resources.

The body is in a constant state of flux as it responds to stressors and undergoes recovery processes. As such, one short HRV measurement might be skewed and not capture the body’s ‘normal’ state. One measurement could only capture a good day or a bad day and may not reflect the athlete’s norm. For example, a person’s HRV the day after staying up all night drinking alcohol will be very different to their HRV on a night that they go to bed early and don’t drink any alcohol.

When trying to understand HRV, it is important to take several days’ worth of measurements within a week (ideally over several weeks). This ensures that the single day is not an outlier. This also increases confidence in the baseline values that can then be used to more effectively set health and performance goals and to assess progress.

An inaccurate measurement means an inaccurate (and invalid) HRV value. Accuracy is critical when when using HRV as a tool for training or health. Heart rate monitors like the Fitbit Charge or other wrist-based PPG or pulse-oximetry monitors are not compatible with measuring HRV. This is because many of these devices were not designed with the intention of measuring the more detailed heart rate fluctuations needed to calculate true HRV. They either do not record the R-R intervals or they provide smoothed, averaged or altered R-R intervals that remove the variability that comprises HRV.

Although these devices are suitable for measuring basic heart rate, there is no such thing as a ‘quick and dirty’, ‘good enough’ or ‘almost accurate’ HRV measurement. HRV values are a measure of normally imperceptible changes in heartbeat activity that often correlate with activity of the autonomic nervous system (ANS). A quick and dirty measurement from an inaccurate device yields a completely useless HRV value that should not be used for decision making.

There are many new HR devices with bold promises and the lure of convenience. Although wearable technology is progressing quickly, athletes should still be wary of these devices. Due diligence is important for understanding if the device is compatible with HRV calculations.