[Jason is currently pursuing his masters degree from Marshall University in kinesiology with an emphasis on exercise physiology. He is a high performance tennis coach with experience coaching nationally and internationally ranked players all over the world. Jason is a certified High Performance Tennis Coach with the USTA and hold certifications with the USPTA and the iTPT. He is taking part in the Athletic Lab Coaching Mentorship program.]

Most athletes understand the concept of progressive overload. The body must be stressed with a workout followed by a period of recovery to improve fitness. Traditional periodized training follows a predetermined training schedule, whether linear, block, or any other variety. Training days and rest days are assigned in advance. These training progressions have been proven to be more effective than random training. However, they do not consider how an individual responds to the training. If someone is feeling run down or sick or is going through an exceptionally stressful time at work, their training will be compromised. For this reason, the traditional model may not be the most effective training approach for some people.

There is an alternative to typical periodized training called heart rate variability (HRV) training. HRV training differs from standard periodized training in that training plans are flexible. An HRV measurement in the morning determines the volume/intensity of training. The most difficult workouts are assigned on days when HRV is high. Rest days occur when HRV is below average. HRV training can account for individual training and life stress responses, theoretically creating more optimal training.

What is Heart Rate Variability?

A resting heart rate of 60 beats per minute (bpm) means the heartbeats 60 times in one minute. As impressive as the heart is, it is not a perfect metronome. Those 60 beats are not perfectly spaced within that minute. The time between each beat can fluctuate slightly because the heart speeds up as a person inhales and slows down as a person breathes out. That fluctuation is referred to as heart rate variability.

These fluctuations, measured in milliseconds, can give much information about the body’s stress levels. Therefore, HRV is a way to monitor the body’s internal stress. A high HRV, relative to a person’s average level, is a sign that the body is experiencing a low internal stress level. Conversely, a low HRV demonstrates that the body is experiencing a high internal stress level. Many different factors can cause internal stress. Physical activity, mental stress, illness, poor diet, and drug/alcohol use are all sources of stress on the body. For athletes, HRV can be an estimate of how ready the body is to take on physical strain. Improved fitness, a temporary reduction in training, more sleep, and healthy life habits can all increase HRV.

How does HRV work?

A person’s heart rate is controlled subconsciously by the autonomic nervous system (ANS). The ANS is continually working to increase or decrease heart rate to meet the body’s oxygen demands. The ANS is split into two opposing branches: the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS).

The SNS is the get-up-and-go branch responsible for the human fight or flight response. Evolutionarily speaking, the SNS is designed to rapidly divert the body’s resources to the brain, heart, and working muscles to be as mentally alert and physically ready for action as possible. Actions of the SNS include increasing heart rate, cardiac output, breathing rate, and blood pressure; diverting blood to the brain and muscles and away from other internal organs; and increasing glucose and fatty acid mobilization and metabolism. The SNS is active any time the body is experiencing physical or emotional stress.

The PNS is sometimes referred to as the rest and digest side of the ANS. It guides the body to a state of rest. The PNS can reduce heart rate, breathing rate, and blood pressure. It controls digestion and absorption of nutrients and other functions of the body that were inactive during times of high stress. PNS activity is highest when the body is calm and relaxed.

Heart rate variability changes depending on which branch is more active. As sympathetic activity increases and the body’s demand for resources becomes more urgent, there is more strict control over heart rate. As a result, there is less variability in heart rate, and HRV decreases. Conversely, as the body relaxes and parasympathetic activity becomes more prominent, the SNS has less control over heart rate, and HVR increases. In a sense, there is a push and pull effect; both branches of the ANS are working against each other.

For good health, it is essential to have a balance between the two branches of the ANS. The branch that is in control will naturally fluctuate throughout the day. At night during sleep, the PNS dominates. Therefore, HRV will be higher. During the day, as activity and, potentially, stress increase, HRV will decrease.

Typically, HRV will return to baseline levels within 24-48hrs after being exposed to an acute stressor. However, during periods of exceptionally high stress, it is possible to get stuck in a sympathetic-dominant state. Left for long periods, this can wreak havoc on an individual’s health, affecting the immune system and causing systemic inflammation. For athletes, it prevents the body from achieving the adaptations necessary to improve fitness. Having a daily HRV measurement allows athletes to see how training stress affects the body.

How is HRV Measured?

Understanding HRV and how it relates to internal stress levels is not new. In the past, specific lab equipment was necessary to get a measurement making it impractical for the everyday consumer. The recent increase in wearable technology has made accessing an individual’s HRV much easier. There are two common ways to get an HRV measurement. Both require mobile applications. One method uses a chest strap heart rate monitor, familiar to most athletes, to measure the heart’s electrical signals. The other is by photoplethysmography (PPG) to detect the heart rate optically through the skin. This technology is found in most fitness trackers, the apple watch, Whoop bands, etc. Some brands on the market use PPG in a finger or ear sensor or even the camera on a cell phone.

To get an HRV measurement requires the user to sit or lay calmly for a few minutes and allow the device to monitor heartbeats. Once a reading has been recorded, different algorithms are applied depending on what technology you are using, and an average inter-beat variability in milliseconds is given. Most companies have their own method of presenting the daily reading, green, yellow, red, etc., to make it easier for the user to understand. Average HRV ranges between 30-100ms, but it is highly individual and varies significantly from person to person. It is typically higher in males than females and younger people than older people. More physically fit people tend to have a higher HRV than less fit people.

A single day HRV reading doesn’t provide much insight, no different from taking a heart rate reading once a day. It is a snapshot of the ANS in that small moment in time. The value of HRV as a metric lies in seeing how it trends over time. Fluctuations from an established baseline can give insight into how the body responds to stress (most monitoring systems report HRV relative to baseline as opposed to raw scores). A downward trend is a sign that the body is struggling to recover from its stress. An upward trend generally signals increased recovery or increased physical fitness.

Validity & Reliability

HRV isn’t a perfect metric. The reliability of different systems can vary greatly. The method of measuring and the time-of-day that measurements are taken can drastically affect results. To be accurate, it must be taken every day at the same time. It is essential to be as consistent as possible. HRV should be measured in the morning soon after a person wakes up before they have accumulated any physical or mental stress. Some of the PPG wearable devices do this automatically. However, most HRV monitoring systems require the person to take a reading themselves physically. Though not a perfect system, if the user tries to maintain consistency, any device should be able to provide accurate enough information to make educated decisions.

What is HRV Based Training?

An HRV-based training plan can work in two ways. Using either method requires that an HRV baseline has previously been established. This usually takes a week of recording measurements. The first method dictates daily training. It requires workouts to be split into three categories: normal training workouts, high intensity and/or high-volume workouts, and rest/active recovery days. Training starts with a normal workout. This progresses until a low morning HRV measurement occurs. This is the signal to do a recovery day. Rest/recovery is maintained until HRV returns to baseline, then the regular workouts resume. Whenever a high HRV occurs, it is time to do a high-intensity or high-volume day. When HRV is high, the body is especially adaptive to taking on high levels of physical stress. When high HRV readings consistently follow high-intensity days, it is a sign that fitness has improved, and load/volume should be progressed.

The second method is an HRV training block. Using this method, an extremely demanding block of training is prescribed for a set number of days with the intention of having many days of low HRV in a row, i.e., highly stressing the system. This is followed by a light training/recovery period. This is what pro cycling teams are doing when they have high-altitude training camps, for example. Evidence supports that such high-demand training blocks may promote greater gains than more linear training. Following the high-demand training block, a structured de-load takes place until a stretch of high HRV days occurs. These types of high-demand training blocks only work because they allow for adequate recovery. Using HRV takes the guesswork from knowing that the training block is hard enough and that the athlete has recovered sufficiently.

For athletes that are not getting the results they would like from traditional training plans, HRV-based training could provide a good alternative. HRV-based training allows individuals to account for all aspects of stress, sleep, and recovery. It allows for much more individualized training than traditional periodized plans. For this reason, it might be worth experimenting with for athletes that struggle to follow a traditional plan or find that they are not progressing as they would like.

References

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  • Düking, P., Zinner, C., Trabelsi, K., Reed, J. L., Holmberg, H.-C., Kunz, P., & Sperlich, B. (2021, 2021/11/01/). Monitoring and adapting endurance training on the basis of heart rate variability monitored by wearable technologies: A systematic review with meta-analysis. Journal of Science and Medicine in Sport, 24(11), 1180-1192. https://doi.org/https://doi.org/10.1016/j.jsams.2021.04.012
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