Stress is a normal part of life and your body is designed to react to it. According to the American Psychological Association, your body is well-equipped to handle small increases in stress levels, like running late for a meeting. But when stress becomes chronic or too intense, it can have negative effects on all systems of the body—including negatively affecting your running performance.

You have probably experienced physiological changes like increased heart rate, heavy breathing, or sweating before the start of a race or a big presentation. One effect of stress that is not as noticeable is the change to heart rate variability (HRV), or the variation in the time between heartbeats measured in milliseconds. Paying attention to heart rate variability can help runners recognize when stress is impacting performance and when you need to make modifications to training or stress management.

Here’s what to know about stress and heart rate variability, how this measure can be a tool to monitor your training, and ways to improve HRV to help manage stress and boost running performance.

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How Stress Affects Heart Rate Variability

Stress affects your heart via the autonomic nervous system—the direct line between your heart and your brain. The autonomic nervous system consists of two parts: the sympathetic nervous system initiates the “fight or flight” response to stress, which triggers increases in heart rate and blood pressure, while the parasympathetic nervous system controls the relaxation response, lowering heart rate, and blood pressure.

“We view heart rate variability as a reflection of the balance between the two primary arms of the nervous system, the sympathetic nervous system and the parasympathetic nervous system,” Timothy Churchill, M.D., a cardiologist at the Cardiovascular Performance Program at Massachusetts General Hospital and an instructor at Harvard Medical School tells Runner’s World.

According to the Cleveland Clinic, your heart rate’s variability reflects the adaptability of your body. A higher heart rate variability is a good thing and can indicate your body is more resilient, whereas lower heart rate variability may reflect difficulty adapting to changes and may be considered a sign of current or future health problems.

“We would think of [stress] as augmenting the sympathetic side of the nervous system and resulting in reduced heart rate variability, and theoretically manifesting in increasing the resting heart rate,” adds Churchill. While both heart rate variability and resting heart rate are ways to see the effect of stress on the cardiovascular system, much of the research to date focuses on heart rate variability.

For example, a review published in Psychiatry Investigation in 2018 showed HRV variables changed in response to stress. Researchers induced stress in various ways, including simulating a medical emergency, responding to work or school-related stressors, or taking psychological tests in a lab environment. The most common effect of stress across studies was low parasympathetic activity, or an inability to calm down an activated sympathetic nervous system, resulting in low heart rate variability.

Churchill adds that the relationship between stress and heart rate variability applies to different forms of stress: emotional stress such as worrying about an exam or presentation, physiological stress like overtraining or dehydration, or environmental stress including heat or noise. Other factors like diet, sleep, and medications can also affect HRV.

However, stress doesn’t always result in a decreased heart rate variability. Whether you frame your pre-race butterflies as excitement or anxiety can determine if it results in an increase or decrease in HRV. “When we are interpreting stress as bad, as damaging, as anxiety and fear that are somehow detrimental, that is going to decrease heart rate variability. That’s going to decrease our objective performance. That’s going to decrease our feeling of competence. It’s going to further increase anxiety,” says Inna Khazan, Ph.D., clinical health and performance psychologist in Boston and lecturer in the psychiatry department at Harvard Medical School, tells Runner’s World. “If in the same situation, the person interprets this sensation of physical activation as helpful and adaptive, that leads to the opposite—heart rate variability is higher. You get a much more positive response, both physiologically and emotionally.”

The Role Heart Rate Variability Plays in Performance

As an indicator of adaptability and resiliency, runners can use heart rate variability as a training tool to suggest when you might be able to push harder or when you need some time to recover.

Low HRV might reflect being stressed, overworked, or inadequately rested. Training in that state of fatigue might make it feel like you are working harder to achieve a normal pace on the run or like you are not ready to tackle your next workout.

A review in the Journal of Exercise Physiology Online published in 2013 looked at how heart rate variability can be applied to exercise training, including running. The studies reviewed showed HRV can be an effective tool to prescribe exercise intensity, as well as monitor training to prevent and diagnose overtraining. The review added that age, gender, and athletic conditioning are independent factors that can also affect HRV and should be considered when prescribing training or recovery plans for individuals.

For the amateur runner, Churchill sees heart rate variability as a tool to assess if you are allowing your body sufficient time to recover from workouts, where low HRV is an indicator that more light or moderate work should replace harder efforts in your training plan. “Probably the most underutilized element in training programs among amateur or recreational athletes is appropriate rest and recovery,” he says. “I think these metrics are one pathway that people can use to gain insight into that. It’s a lens that people can use primarily to see if they should be modulating down their intensity and/or building more rest into their training programs.”

Tracking Heart Rate Variability

While the most accurate way to measure heart rate variability is in a medical setting with an electrocardiogram (ECG), for the purposes of monitoring stressors and overtraining, many of the devices that runners already have can track your heart’s response to stress.

While HRV can be a valuable metric to monitor your body’s readiness to perform, the data can also be tricky to interpret. According to Khazan, while there are normal ranges for heart rate variability, it’s not as simple as identifying ranges for normal resting heart rate because there are several ways to measure HRV. The most common ones are statistical analyses of the intervals between heartbeats like RMSSD (root mean square of successive differences) and SDNN (standard deviation of normal-to-normal interval).

Devices like those from Garmin, Whoop, and Apple collect heart rate variability data and may use the same measurements, like RMSSD, or create their own scores or variables based on these measures. For example, Garmin notes that it uses HRV in other measurements like the stress score and body battery.

With varying ways to measure and varying characteristics among individuals, it’s hard to prescribe a precise change in score at which to intervene, but using HRV numbers to identify what’s normal for you, and then monitoring changes to those numbers, can still be a valuable tool. One day of lower HRV may not be a big deal, says Khazan, but if the trend continues for several days, you might want to pay attention to factors like stress, sleep, diet, and training load to get you back on track.

How to Improve Heart Rate Variability

If you see a decline in HRV or you’re simply seeking another way to optimize your performance, there are ways to improve heart rate variability. For example, practices like mindfulness can help in stress reduction and athletic performance. A study published in PLoS ONE in 2020 found an increase in HRV both during and following mindfulness practice, indicating the stress reduction benefits can stick around even after you’re done.

Biofeedback training—or the process of measuring your physiological responses, like heart rate and breathing, and displaying it visually on a device, so you can make changes in your body like altering your breathing and see how those responses change—has also shown success in increasing heart rate variability.

The most direct way to increase your heart rate variability is by practicing slow, paced breathing exercises. “As you breathe in, your heart rate goes up; as you breathe out, your heart rate comes down. There is an optimal rate of breathing, that will produce maximum heart rate variability,” Khazan explains. “That breathing is called resonance frequency breathing rate. It’s basically a way to stimulate the heart rate to increase as much as possible on each inhalation and decrease as much as possible in each exhalation.”

If you look at it on a monitor, the goal is to get the ups and downs of your heart rate to look like a sine wave and coincide with your breath, says Tim Herzog, clinical and sport psychologist and certified mental performance coach in Annapolis, Maryland.

When you practice breathing at this resonance frequency rate it’s like a workout for your nervous system, says Herzog. Practicing your paced breathing while not running strengthens the connection between the brain and the heart and can increase heart rate variability that carries through even when you are not breathing at your resonance frequency rate.

The research suggests practicing breathing for twenty minutes twice a day, but Herzog encourages starting with whatever time you have available. “By proactively engaging in our breathing regimen, we can manage to take that baseline level of anxiety down. So that when the stressor comes along, we’re starting from a lower place to begin with,” Herzog says.

Professionals, like Herzog and Khazan, certified in biofeedback can help you identify that resonance frequency breathing rate, usually around six breaths per minute. Although working with a licensed counsellor has its advantages beyond biofeedback training, if you are not interested or able to work with a professional, there are ways to do it yourself.

Khazan helped develop the Optimal HRV app, which she says will determine your resonance frequency, provide biofeedback training, and track heart rate variability. It just requires a Bluetooth-enabled HRV reader, like a Polar chest strap or Biostrap. Khazan adds other apps, like Elite HRV and Heart Math, do slightly different things but are also good options for helping to increase HRV.

HRV Biofeedback Training’s Influence on Health and Performance

While there are limited controlled studies, biofeedback training seems to show promising positive effects on heart rate variability. A 2021 study published in the journal Applied Psychophysiology and Biofeedback looked at HRV, running, recovery and perceived exertion following a treadmill test before and after HRV biofeedback training. The study did not include a control group, but found improvements in HRV, time spent exercising, perceived exertion, and recovery time when participants used HRV biofeedback in recovery.

Research on specific performance improvements are harder to come by. A systematic review also published in the journal Applied Psychophysiology and Biofeedback in 2017 found improvements in psychological and performance outcomes including reduced anxiety, improved concentration, and improvement in skills like dribbling and shooting accuracy in basketball players.

In the one running study included in the review, runners who received biofeedback training showed greater improvements in their 5K times and VO2 max. However, upon further review of the specific study, the improvements in 5K times over a ten-week period were dramatic (18.27 minutes to 15.89 minutes) and call into question if other factors also contributed to the lower times.

Additionally, most of the studies included less than thirty participants, and vary in HRV biofeedback training guidelines, so results can be difficult to generalize across broader populations.

However, a systematic review and meta-analysis from 2020 in the journal Applied Psychophysiology and Biofeedback provides evidence that HRV biofeedback had positive effects on a variety of physical, behavioral, and cognitive conditions, athletic/artistic performance, depression, anxiety, and gastrointestinal problems.

Even if it doesn’t result in a 5K personal best, HRV biofeedback training is safe, accessible, and easy to learn and may be a complementary practice to enhance your training and stress management.

Whether you try biofeedback training or decide to track your heart rate variability as you train for your next race, regardless of what the data says, Churchill reminds runners that the most important tool for maximizing performance is listening to your body: “If your body tells you it needs rest and recovery, then you should be doing that.”

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