What exactly is “HRV?”

Most of you are here because you are passionate about fitness. You are passionate about your health, your bodies, and your wellness. In your continual quest for self-improvement, you’ve likely tried various exercise modalities, training programs (maybe even several apps or sensors) and it led you here: to Athlytic.  

Athlytic presents your wellness data in a visually straightforward way that helps you understand both the effect your training has on your body, and the effect of external (or internal) stressors on your body. This data allows us to act on different changes, make adjustments based on the data, improve our recovery, and ultimately, improve our performance. As you are likely aware, the primary metric utilized here to examine recovery is heart rate variability (or HRV). For this first post, I want to take a step back and briefly examine the history of HRV and why this metric matters. 

The history of HRV dates back at least 150 years, when Carl Ludwig identified differences in heart rate associated with respiration, now termed respiratory sinus arrhythmia. But it wasn’t until much later, with the increased use of EKGs (which allowed precise beat to beat measurements in the heart), that HRV became a topic of increased study. Initially, HRV was popularized because it was found to have a strong correlation with increased morbidity and mortality, particularly with cardiovascular disease [1]. The lower the HRV, the higher the risk [2].

For example, in a study of middle-aged and elderly men examining all cause mortality, heart rate variability (measured as standard deviation of duration of normal RR intervals) of < 20 milliseconds (msec) compared with men with heart rate variability of 20-39 msec was associated with a relative risk of 2.1 (meaning the lower HRV cohort had double the risk over the 5 year period) [2].

Numerous studies, including the example above (and those included in the footnotes), have noted that increased cardiovascular fitness and health are correlated with higher HRV [3]. As you exercise—and become more cardiovascularly fit—your HRV increases; in essence, HRV can be a marker of cardiovascular health.

Why would HRV have such a powerful correlation to our health? Put simply, it represents a marker of resilience of the heart. HRV acts as an indicator of the body’s autonomic nervous system. As a refresher, the autonomic nervous system is the unconscious regulator of our body, controlling what goes on “behind the scenes,” like digestion, respiration, heart beats, going to the bathroom, etc. Think of it like an invisible conductor in an orchestra—able to speed things up or slow things down, depending on the situation and its needs. 

How does it know whether to speed up or slow down? The autonomic nervous system is made of up two components: the parasympathetic (rest and relaxation) and sympathetic (fight or flight) systems. It is the balance of these two that’s important, with the sympathetic system ready to activate in case of a stressor or threat, and the parasympathetic calming you down to rest or digest. Now, just like a good tune, the tempo of the heart isn’t just one steady, solitary beat. Even during rest our heart rate has subtle variations from one beat to the next, subtle differences in timing to go from beat one to two, beat two to three, and beat three to four. This is heart rate variability, and it is determined by the balance of our unconscious nervous system, between parasympathetic and sympathetic signals.

While it’s clear that HRV is an important metric to monitor, we still haven’t gotten to why we care to check it so frequently, or even daily. But when we take a step back and realize that we can use HRV as a metric to identify where our body stands in terms of ‘rest or relaxation’ vs ‘fight or flight’, we can see the utility. If our body seems relaxed and well rested, of course it can handle more stress! But if we’re already stressed, fatigued, or otherwise activated, maybe we ought to take it easier. But don’t just take my word for it—what does the science say for HRV as it relates to exercise? 

There are a few interesting things data has shown when HRV is monitored in response to exercise. To some extent, the stressors placed on the body through exercise are evidenced by an immediate decrease in HRV following exertion. Our bodies are fatigued, and the HRV reflects that. Unsurprisingly, this decrease tends to be proportional to the intensity of exertion; the more intense the workout, the greater the effect on HRV. What’s even more interesting is that data shows that higher trained athletes demonstrate their resilience by experiencing a quicker return of HRV to baseline after exercise (within hours), while inactive adults may take days to return to baseline after intense exercise [4].

While that is promising enough, the most convincing argument for monitoring HRV-based recovery is that it’s been proven to work for performance improvement. In studies analyzing HRV and curating HRV-guided training regimens (either based on 7 day rolling averages or just daily HRV measurements), participants were able to make improvements in VO2 max and peak performance compared to more traditional approaches [5-7]. 

 In a 2019 study by Javaloyes et al, experienced cyclists were split into two groups: A pre-determined ‘block periodization’ training program vs training guided by heart rate variability (ie. if HRV was low, the training was altered to become a lower intensity or recovery day). On analysis, the overall training intensity and overall time spent exercising was not significantly different between the athletes... but the results were. While the block periodization training did result in increased power at ventilatory threshold 2 (something we can discuss in a future episode!), the HRV guided cyclists experienced improvements in their 40 minute time trial, peak power output, VO2 max, as well as power at ventilatory threshold 1 AND 2 [5].

While there is so much more to say about HRV we can summarize it as: work smarter, not harder. HRV offers insight into our body’s readiness for exercise and its resilience. It reflects internal and external variables like illness, poor sleep, alcohol use, recent exercise (and more) all incorporated into one variable. The data, as we reviewed above, suggests that HRV can be an incredibly valuable and powerful tool when used for exercise training. And even more, increasing our cardiac fitness (and HRV) can help us to lead longer and healthier lives! So what are you waiting for? Get out there and be active!  

Sources and further reading

1. Frontiers | Hidden Signals—The History and Methods of Heart Rate Variability.  https://www.frontiersin.org/articles/10.3389/fpubh.2017.00265/full

2. Dekker JM, Schouten EG, Klootwijk P, Pool J, Swenne CA, Kromhout D. Heart rate variability from short electrocardiographic recordings predicts mortality from all causes in middle-aged and elderly men. The Zutphen Study. Am J Epidemiol. 1997;145(10):899-908. doi:10.1093/oxfordjournals.aje.a009049

3. Souza HCD, Philbois SV, Veiga AC, Aguilar BA. Heart Rate Variability and Cardiovascular Fitness: What We Know so Far. Vasc Health Risk Manag. 2021;17:701-711. doi:10.2147/VHRM.S279322

4. Stanley J, Peake JM, Buchheit M. Cardiac parasympathetic reactivation following exercise: implications for training prescription. Sports Med Auckl NZ. 2013;43(12):1259-1277. doi:10.1007/s40279-013-0083-4

5. Singh N, Moneghetti KJ, Christle JW, Hadley D, Froelicher V, Plews D. Heart Rate Variability: An Old Metric with New Meaning in the Era of Using mHealth technologies for Health and Exercise Training Guidance. Part Two: Prognosis and Training. Arrhythmia Electrophysiol Rev. 2018;7(4):247-255. doi:10.15420/aer.2018.30.2

6. Javaloyes A, Sarabia JM, Lamberts RP, Moya-Ramon M. Training Prescription Guided by Heart-Rate Variability in Cycling. Int J Sports Physiol Perform. 2019;14(1):23-32. doi:10.1123/ijspp.2018-0122

7. Kiviniemi AM, Hautala AJ, Kinnunen H, Tulppo MP. Endurance training guided individually by daily heart rate variability measurements. Eur J Appl Physiol. 2007;101(6):743-751. doi:10.1007/s00421-007-0552-2