If you use a smart watch or fitness tracker, you have probably come across the term “heart rate variability or HRV.” At first glance, it may seem like just another statistic, but gathering your heart rate variability data can provide a glimpse into your body’s response to stress, recovery and training. 

By tracking trends in your heart rate variability, you can optimize your recovery, avoid overtraining and improve your health and performance.

What is heart rate variability?

Heart rate variability is a non-invasive biomarker of how your body is responding to stress. A lot of things can be stressors– external stressors include training, work, and lifestyle habits, while internal stressors can be attitudes, thoughts, and emotions. Stress, regardless of whether it is training or non-training related, is important to track because it will affect your ability to adapt and recover. Your body views and responds to any type of stress as stress; it can’t tell apart work-related from training from psychological. If you aren’t recovered (lack of sleep, poor nutrition, too hard of a session, etc) or highly stressed (anxious, mentally fatigued, etc), how can you expect yourself to train well or perform at your best? Having this objective data available is like having a personal gauge on your nervous system’s recovery and readiness.

How is heart rate variability intertwined with the nervous system?

Heart rate variability is the variability in time between each heartbeat or more specifically, between each R-R interval on an electrocardiogram (ECG) measured in milliseconds– it can be consistent or inconsistent. Our heart rhythm is regulated by our autonomic nervous system, which consists of sympathetic and parasympathetic pathways. Our sympathetic system puts us in “fight or flight” mode– think adrenaline rush prior to competing or maybe prior to a public speaking event. On the other hand, our parasympathetic system is known for “rest and digest” and countering the sympathetic system– think recovery and slowing our heart rate and blood pressure. When there is an imbalance within the autonomic nervous system, that can lead to an effect on your heart rate variability. 

 

What does the higher and lower heart rate variability value mean for my health, training, and recovery?

Generally, an increase in your heart rate variability means there is more inconsistency between each heartbeat. This reflects a system that is more responsive, more recovered and healthy. A decrease in your heart rate variability means more consistency between each heartbeat, and less readiness or more fatigue. There are some nuances that this blog won’t get into, but a general rule of thumb for most people is: the more stable or higher your heart rate variability value is compared to your baseline, the more ready and recovered you are. 

For example, if you decided to start going on easy runs consistently and on each subsequent session your heart rate variability stabilized or increased, it would mean that your body was able to adapt and recover from the demands of the previous day. Over a long period of time, you may observe a general increase or stability in your heart rate variability value which means an improvement in your health and fitness. The same can be said with positive changes to your diet, stress levels, or other lifestyle factors.

On the other hand, chronic stress, overtraining, reduced sleep, sickness, alcohol, or smoking can negatively affect your nervous system, decreasing your heart rate variability and fitness. When there is more stress and/or higher engagement of the sympathetic system (flight or fight mode) compared to the parasympathetic system, this can lead to a decrease in your heart rate variability. If you notice a significant drop or consistently lower value, that could be a sign that you are under-recovering and struggling to adapt. 

What if my heart rate variability value is lower in comparison to others?

Your heart rate variability value is individual to you and is not a reliable measure of how fit you are in comparison to someone else. There is currently no research-backed “normal” or cut-off value to strive for as a lot of factors contribute to one’s baseline: age, gender, genetics, stress, comorbidities, general health, fitness/training, lifestyle habits, and medications. You may have the same value as an Olympian or a desk worker, but we don’t worry too much about the absolute value. What matters most is tracking it over time and seeing if the trend is stabilized, higher, or lower compared to your baseline to improve your health and performance. 

Additionally, we each have our own limitations and capacities for how much stress we can handle. Individuals can respond differently to the same stress because nothing ever acts in isolation. We always have to consider the physiological, mental, environmental, and emotional factors involved.

How do I track heart rate variability?

Because it is like any data point, you want to make sure the data is reliable and valid. The gold standard is an electrocardiogram (ECG). Wearables (Polar H9 chest strap, Oura ring, Whoop, and Garmin) and smartphone apps (HRV4Training) can be more practical and cost-effective. However, for most wearables, especially watches, there are inconsistencies in their methodology of data collection and therefore, can affect your decision making on training and recovery. We won’t get too much into the weeds of wearables in this blog, but it is important to understand how to properly and accurately collect the data. 

To analyze your heart rate variability, you first need to establish your baseline value and normal ranges over 1-2 months using the same method and wearable. And remember that the main interpretation is based on your relative changes/trends in response to an acute or long-term stressor(s).

It should be measured consistently at the same time of day: immediately in the morning or throughout the night. If measured in the morning, ideally, you’re immediately measuring it at rest (when you’re laying down) and after waking up because anything can potentially influence the nervous system. If measured at night, ideally, it is measured throughout the entire night and not just for brief sporadic amounts of time as you sleep. This is because our sleep stages and circadian rhythm affects our autonomic system and therefore, affects our heart rate variability.

It is unnecessary to measure your heart rate variability throughout the day because that information is not actionable nor impactful. As mentioned, mostly anything can influence your nervous system, from being upset to drinking coffee to light activity, and so fluctuations within the day are normal. However, the cumulative effect of chronic stressors that is captured in the value when measured consistently in the morning or throughout the night provides more effective information on how to proceed.

How do I use heart rate variability to optimize my training routine?

In the research literature, the optimization of training guided by heart rate variability has been mostly studied in endurance athletes (runners, cyclists, cross country skiing). In these studies, the timing and intensity of training was related to whether the athlete’s daily or baseline heart rate variability data increased, stabilized or reduced. If reduced below the standardized amount, the athlete was prescribed a session with reduced training load. If stabilized or increased, the athlete was prescribed a moderate or high intense training session. The basis of heart rate variability-guided training is to improve the timing of training to when the body is most ready, and therefore, able to optimize positive adaptations and improve performance. However, the training protocols (length of training, type of training) and methodology (measurement of HRV, what normal range is) of each study have varied. Most studies have also contained small sample sizes and geared more towards trained endurance athletes rather than the untrained, so it is difficult to generalize results to the general population.

Several meta-analyses and reviews (Manresa-Rocamora et al., 2021; Lundstrom et al., 2022; Addleman et al., 2024) about heart rate variability-guided training compared to predetermined training in endurance athletes have found small but favorable effect sizes for heart rate variability-guided training. Both groups improved their performance measures such as maximum running velocity, VO2 max, and peak power, but the heart rate variability-guided group found more significant improvements in the submaximal exercise threshold rather than peak maximal performance measures. They also noted that despite the reduction in volume of high and moderate intensity workouts for the heart rate variability-guided training, outcomes between both groups were similar. There were more positive responders and fewer negative responders to heart rate variability-guided training. Overall, there are positives to implementing this type of training as it is simple to follow and you do gain performance and fitness benefits. However, it is important to note that more often in the real world, your coach or yourself are probably already adapting your workouts and training according to what your response is (was it too hard/ was it too easy/ what the goal is/ injury). 

In strength training, the effects of heart rate variability-guided training has yet to be studied in this population. It is unknown what variety of resistance training affects heart rate variability the most, and muscle hypertrophy and strength responses have not been studied. What can be extrapolated is that the response to high intensity and overuse is similar; in a study done in Crossfit athletes (Williams et al., 2017), they were better able to handle high intense workouts when their heart rate variability measure was normal or higher compared to when it was lower. 

To look at the big picture and appropriately utilize the data, you want to look at your acute and long-term trends, the context behind the numbers (if you just had a hard training session or a stressful day at work the day before), and subjective reports (like how you’re feeling).

Because of the high sensitivity of heart rate variability to stress, daily fluctuations are generally normal (hence, why it is important to determine your normal range). You want to understand the changes early on to gain confidence in interpreting your data and to determine what are potential causes (lifestyle, emotional, training, health, etc).

When it comes to training, you may observe daily changes as a result of different intensities; for example, comparing your responses to an easy run vs tempo vs highly intense interval workout. For high intensity training sessions, this will stimulate your sympathetic nervous system more and may reduce your heart rate variability value for up to 72 hours. If you have an adaptive and ready system, the value will return to baseline or increase. If your value does not at least return to baseline, then it shows your recovery is incomplete, which may be a result of overload in intensity/volume or lack of recovery time or high life stress outside of training. 

Over the span of a training period and looking at the long-term data, the ideal scenario is to see a generally stable or upward trend of your heart rate variability value despite the daily fluctuations. The best way to track your long-term progress is to take a 7 day average of your heart rate value and compare it to the prior months of your training block. Even if you are in an intense training block, having a stable or increasing value shows that you are still able to cope and respond positively to stress. 

Additionally, it is important to keep in mind how you are feeling. The objective data should not replace your subjective feelings, but be used in conjunction together. Sometimes the value aligns such that it falls within normal and you’re feeling good. However, other times, the value and our feelings aren’t aligned; either we might not be as in tune with our bodies or might not be able to finely detect stressor effects. For example, the data might be capturing an oncoming stressor like sickness or effects of chronic stressors that we don’t feel at the moment. Over time, you want to be able to fine-tune your subjective assessment and build your self-awareness based on your objective responses to different stressors.

A subjective assessment for training is providing your perceived effort of the session (ie. score 0-10 with 0 being no effort and 10 being the hardest effort possible). Perceived effort reflects your individual experience of the session. It can vary per person doing the same session based on their state of recovery, response to the environment (hot, cold, humid), injury, illness, fitness, etc. For example, person A felt the session was a 6/10 effort (moderate) compared to person B feeling an 8-9/10 (high) with the same training session. Person B might expect a more acute reduction of heart rate variability as a result of larger subjective effort expended to the same training load. Afterwards, you want to track whether your heart rate variability measure recovers appropriately. Utilizing subjective measures such as perceived effort with heart rate variability provides a more comprehensive decision making and feedback loop on your training planning and progress.

Can heart rate variability detect negative training effects?

Heart rate variability has been linked to illness, injury risk, and overtraining. 

A meta-analysis (Addleman et al., 2024) determined that heart rate variability is inversely related to inflammatory processes in the body. Heart rate variability can be suppressed from upper respiratory infections, but the predictive ability of illness is unclear. 

In relation to injury risk, there was one study in CrossFit athletes (Williams et al., 2017) that found a correlation between low heart rate variability paired with high workloads was associated with higher risk of overuse injuries. Even though the findings are associative (correlation doesn’t equal causation), it can be surmised that overuse/under preparation has stressful effects on your body, reducing one’s heart rate variability. 

Overtraining can be defined as an imbalance between training/other stressors and recovery over a period of time. Signs might include fatigue, mood changes, worsening sleep/restlessness, increased muscle soreness, and reduced performance and exercise capacity. Overtraining does not occur from one session but multiple workout sessions that the athlete is not prepared or recovered enough for. A series of abnormal/lower heart rate variability values over the span of a week or two can determine whether you are overtraining. Remember one daily value isn’t enough to make a decision, but being able to view the bigger picture is important and more worthwhile information to make training decisions.

Do’s and Don’ts

• • Do track over a period of time (7 day, monthly, training block) to see trends rather than making decisions off acute or daily changes

• • Do consider the other factors outside of training that can affect your heart rate variability

• • Do measure when at rest and either at morning or throughout the night

• • Do not compare your heart rate variability against another person

• • Do not rely solely on heart rate variability to make decisions

Conclusion

Heart rate variability helps us quantify our individual responses to physiological and psychological stress. It reflects a balance of our nervous system (sympathetic/flight or fight vs parasympathetic/rest or digest). Therefore, it requires a holistic approach to address the external and internal stressors, that don’t just include training loads. Using the data establishes a tool that can help us avoid an imbalance between training load and recovery, and give us insight into our whole body’s readiness levels. 

It is important to ensure the accuracy of the data by using the appropriate method and measuring using validated equipment. Additionally, determining normal ranges per individual will assist with interpreting the data. 

Heart rate variability is just one piece of the puzzle to consider. Coaches and athletes should use heart rate variability to help them understand responses to training as well as other life factors and activities (training doesn’t just happen in a bubble; life can impact training). In conjunction with the acute and long-term trends of heart rate variability, context, and subjective reports, it can help in the decision making process during training to progress, maintain, or pull back.

If you have any questions about optimizing your training or enhancing your running performance, at Perfect Stride, our team offers specialized personalized training and run coaching services tailored to your goals. Reach out today to take your health and performance to the next level! Click here for more information!

References

Manresa-Rocamora A, Sarabia JM, Javaloyes A, Flatt AA, Moya-Ramón M. Heart Rate Variability-Guided Training for Enhancing Cardiac-Vagal Modulation, Aerobic Fitness, and Endurance Performance: A Methodological Systematic Review with Meta-Analysis. Int J Environ Res Public Health. 2021;18(19):10299. Published 2021 Sep 29. doi:10.3390/ijerph181910299

Addleman JS, Lackey NS, DeBlauw JA, Hajduczok AG. Heart Rate Variability Applications in Strength and Conditioning: A Narrative Review. J Funct Morphol Kinesiol. 2024;9(2):93. Published 2024 May 27. doi:10.3390/jfmk9020093

Lundstrom CJ, Foreman NA, Biltz G. Practices and Applications of Heart Rate Variability Monitoring in Endurance Athletes. International Journal of Sports Medicine. 2022;44(1). doi:https://doi.org/10.1055/a-1864-9726 

Altini M. The Ultimate Guide to Heart Rate Variability (HRV): Part 2. Medium. Published February 8, 2020. https://medium.com/@altini_marco/the-ultimate-guide-to-heart-rate-variability-hrv-part-2-323a38213fbc

Li K, Cardoso C, Moctezuma-Ramirez A, Elgalad A, Perin E. Heart Rate Variability Measurement through a Smart Wearable Device: Another Breakthrough for Personal Health Monitoring?. Int J Environ Res Public Health. 2023;20(24):7146. Published 2023 Dec 6. doi:10.3390/ijerph20247146

Altini M. HRV4Training everything you need to know: Garmin, WHOOP, and Oura HRV measurements. the5krunner • tri bike run. Published October 19, 2022. https://the5krunner.com/2022/10/19/everything-you-need-to-know-garmin-whoop-hrv/.

Williams S, Booton T, Watson M, Rowland D, Altini M. Heart Rate Variability is a Moderating Factor in the Workload-Injury Relationship of Competitive CrossFit™ Athletes. J Sports Sci Med. 2017;16(4):443-449. Published 2017 Dec 1