Heart rate variability (HRV) has been known about and measured for quite some time, however, until recently it hasn’t been highly publicized or turned “mainstream.” HRV is now showing up in fitness blogs and training rooms to measure fitness, sports teams and high level athletes are using it to maximize training potential and track progress, and physical therapy practices are starting to use it to track overall health in patients. This blog will cover the WHAT, HOW and the WHY behind HRV.

What is heart rate variability (HRV)?

Simply put, HRV is the measurement of the variability between each of your heart beats. Most people think that when you take your resting pulse and get, let’s say, 60 beats per minute (normal is considered anything under 100 beats per minute) that your heart is beating once every second. However, with research we no know that your heart rate is in fact not beating once every second (in this example) but rather there is variability (typically measured in milliseconds) between each beat. This variability can be accounted for by inputs from your autonomic nervous system which includes your parasympathetic (“rest and digest”) and sympathetic (“fight or flight”) nervous system.

How does the autonomic nervous system impact HRV?

Let’s take a step back to Biology 101 and review what the autonomic nervous system is. The autonomic nervous system (ANS) is an involuntary (meaning you can’t control it like you control your arm movement) nervous system that controls many of the involuntary systems of the body (heart rate, respiratory rate, etc.). The ANS stems from your spinal cord and then branches out from there. It consists of your sympathetic and parasympathetic nervous system. Your sympathetic nervous system is responsible for your stress responses and is often considered the “fight or flight” part of your nervous system. This system will react to stress by increasing adrenaline levels, it causes muscles to contract, your heart to beat faster and your lungs to inhale/exhale more frequently (it prepares you to run or fight). On the other hand your parasympathetic nervous system is responsible for maintaining your body’s homeostasis (keeps everything running smoothly and as it should) at rest. Your parasympathetic nervous system will slow down your breathing and heart rate, relax your muscles and aid in digestion.

Why is HRV important?

The reason why HRV is so important is because it is essentially a way to measure for any imbalance in the ANS. Typically, if someone has chronically elevated activity in their sympathetic nervous system (fight or flight) they will display lower heart rate variability. If someone has more activity in the parasympathetic nervous system (rest and digest) they will display increased heart rate variability. So great, why is that important? This is important because if someone is constantly in “fight or flight” mode they are typically experiencing increased stress levels for longer times, they are less resilient to stress, they are typically in poorer cardiovascular health, they display increased levels of cortisol which, if chronically elevated can cause muscle breakdown, weight gain and fatigue. Think back to a time where you were extremely stressed (finals week, the last month of wedding planning, moving houses, starting a new job, etc.). How did you feel? How did you eat during that time? How much QUALITY sleep were you getting? How much were you able to exercise or meditate? Were you engaging with your friends and family like you normally do? The point is, if the body is just trying to survive a particular stressful situation, all other healthy behaviors typically go down the tank. Short-term this is ok and can be somewhat beneficial, the body needs stress. However, if the sympathetic nervous system is active for long periods of time this can lead to some pretty significant long-term health consequences including: increased risk of injury, increased blood pressure, increased risk for heart attack, depression, anxiety, obesity, increased risk of inflammatory conditions, stroke and other chronic illnesses. Decreased heart rate variability can also be a sign of acute stress (seen with increased alcohol consumption, poor diet, poor sleep, acute sickness/flu, etc.) and should be monitored in that if you wake up and notice your HRV is lower than what it normally is, don’t push yourself as hard as you normally would.

On the other hand, if your body displays appropriate levels of parasympathetic nervous system activity we will typically see increased heart rate variability.  This is because through physical training/conditioning, living a healthy lifestyle, sleeping well, and eating clean foods your body becomes significantly more resilient to stress. What used to be stressful no longer becomes stressful so you end up spending less and less time in a state of “fight or flight.” Your body is able to recover from workouts better (decreasing risk of injury), you are able to think more clearly, it becomes easier to maintain healthy lifestyle behaviors, chemical imbalances associated with depression and anxiety are normalized, cortisol levels decrease and your social interactions improve. Additionally, if you see an acute spike in HRV in the morning you know you can go out and push yourself a little more than you normally would. In that way, it can be used as a tool for training.

How is HRV measured?

As discussed earlier, HRV is not a measurement of just your heart rate. Rather, to capture HRV you need to collect hundreds of data points per second to see what the difference, in milliseconds is between each individual heart beat. Heart rate variability can be measured in a variety of ways, there are numerous monitors (wrist bands) like Whoop, and there are also apps for your iPhone and Android like HRV4Training that allow you to use your phone’s camera to take hundreds of data points in seconds on your phone. They will typically also track sleeping patterns and can be very indicative of overall health and wellness. Whichever system you decide to use this is something that should be tracked every day (with the phone app it is every morning as soon as you wake up).

Bottom line

HRV is an objective tool which measures imbalances in your autonomic nervous system that can be used to help guide rehabilitation, maximize your training, identify risk factors for potential injury risk or chronic illness. It is easy to track, implement and interpret and gives you A LOT more information than other metrics we often track like weight, heart rate, etc.

KEEP MOVING!