What is Heart Rate Variability?
Heart rate variability is literally the variance in time between the beats of your heart. So, if your heart rate is 60 beats per minute, it's not actually beating once every second. Within that minute there may be 0.9 seconds between two beats, for example, and 1.15 seconds between two others. The greater this variability is, the more "ready" your body is to execute at a high level.
A healthy human heart rate is almost never totally regular. A degree of arrhythmia (irregularity) is considered to be normal. The variation in people with a similar resting heart rate will still vary.
These periods of time between successive heartbeats are known as RR intervals (named for the heartbeat's R-phase, the spikes you see on an EKG), measured in milliseconds:
What Causes Heart Rate Variability? Parasympathetic vs. Sympathetic
Although HRV manifests as a function of your heart rate, it actually originates from your nervous system. Your autonomic nervous system, which controls the involuntary aspects of your physiology, has two branches, parasympathetic (deactivating) and sympathetic (activating). The ANS can be thought of as our 'Autopilot'.
The parasympathetic branch (often referred to as "rest and digest") handles inputs from internal organs, like digestion or your fingernails and hair growing. It causes a decrease in heart rate.
The sympathetic branch (often called "fight or flight") reflects responses to things like stress and exercise and increases your heart rate.
Heart rate variability comes from these two competing branches simultaneously sending signals to your heart. If your nervous system is balanced, your heart is constantly being told to beat slower by your parasympathetic system and beat faster by your sympathetic system. This causes a fluctuation in your heart rate or heart rate variability.
The brain is the central conductor and will maintain the homeostasis of the heart and other organs through the autonomic nervous system (ANS). The ANS is linked to the hypothalamus - pituitary - adrenal (HPA) axis, and the endocrine and immune systems.
Why is HRV a Sign of Fitness?
When you have high heart rate variability, it means that your body is responsive to both sets of inputs (parasympathetic and sympathetic). This is a sign that your nervous system is balanced, and that your body is very capable of adapting to its environment and performing at its best.
On the other hand, if you have low heart rate variability, one branch is dominating (usually the sympathetic) and sending stronger signals to your heart than the other. There are times when this is a good thing--like if you're running a race you want your body to focus on allocating resources to your legs (sympathetic) as opposed to digesting food (parasympathetic).
However, if you're not doing something active, low HRV indicates your body is working hard for some other reason (maybe you're fatigued, dehydrated, stressed, or sick and need to recover) which leaves fewer resources available to dedicate towards exercising, competing, giving a presentation at work, etc.
To look at it another way, the less one branch is dominating the other, the more room there is for the sympathetic (activating) branch to be able to come in and dominate, which is why high HRV suggests you're fit and ready to go.
What is a Normal Heart Rate Variability?
Below is an average heart rate variability chart based on age:
You can see that for the most part, HRV decreases abruptly as people get older. The middle 50% of 20-25 year olds usually have an average HRV in the 55-105 range, while 60-65 year olds tend to be between 25-45.
HRV is Highly Individualized
Heart rate variability is an extremely sensitive metric. It fluctuates greatly throughout the day, from one day to the next, and from one person to another. People often wonder "What should my HRV be?" and "How does my HRV compare to others?"
Younger people tend to have higher HRV than older people, and males often have slightly higher HRV than females. Elite athletes usually have greater heart rate variability than the rest of us, and within that subset endurance athletes regularly have higher HRV than strength-based athletes. But, none of this is absolute. There are plenty of extremely fit and healthy people out there whose HRV is regularly in the 40s. What constitutes a healthy heart rate variability differs for everyone.
Better questions to ask are "What is a good heart rate variability trend for me?" and "What can I do to make that happen?"
Heart Rate Variability (HRV) testing not only determines what your stressors are but more importantly, your body's ability to ADAPT to these stressors.
Next the goal is to find to more precisely identify these stressors and remove them so that the body can enter into a 'rest and digest' state over time.
Factors that Affect Heart Rate Variability
There are a great number of things that impact your HRV. The figure below breaks them down into three categories: Training factors, lifestyle factors, and biological factors.
Training factors include the frequency and intensity of your workouts. If you go extra hard today, or for several days in a row, your HRV is likely going to take a hit. There are also many other choices you make each day (lifestyle factors) that significantly affect your heart rate variability, ranging from what you put into your body, to the quality and consistency of your sleep.
And lastly, there are biological factors that are out of your control, like age, gender and genetics--some people are just born to have higher HRV than others.
How to Improve Heart Rate Variability
Methods for how to increase include the following:
Intelligent Training. Don't overdo it and push too hard for too many days without giving your body an opportunity to recover.
Hydration. The better hydrated you are, the easier it is for your blood to circulate and deliver oxygen and nutrients to your body. Aiming to drink close to one ounce of water per pound of body weight each day is a good goal.
Avoid Alcohol. One night of drinking may negatively affect your HRV for up to five days!
Steady Healthy Diet. Poor nutrition has adverse effects on HRV, as does eating at unexpected times.
Quality Sleep. It's not just the amount of sleep you get that matters, but also the quality and consistency of sleep. Going to bed and waking up at similar times each day is beneficial.
Auto-Regulation. In general, trying to get your body on a consistent schedule (in particular with sleep and eating) is helpful. Your body does things more efficiently when it knows what's coming.