The human heart beats at a non-constant rate; heart rate variability (HRV) is a measurement of this irregularity. HRV has been shown in numerous studies to positively correlate with athletic performance and training adaptation, to negatively correlate with the risk of overtraining, and to generally correlate with overall fitness and health. This document provides a high-level overview of HRV with a focus on its utility as a training tool.
Heart rate variability (HRV) is a measure of irregularity in the heart rate. Consider a heart rate of 60 beats per minute; one might take this to imply that the heart is beating consistently once per second, while in actuality, beat-to-beat times could range from 1⁄2 a second to 2 seconds. The time between heartbeats, called an RR-interval, is named for the heartbeat’s R-phase, the most distinguishable phase of the heartbeat.
On an ECG, a series of heart rates might look something like the figure below. Notice that the first RR-interval (between R1 and R2) is significantly shorter than the second (from R2 to R3).HRV is a function of the difference in the lengths of successive beats in a series of RR- intervals, usually between 2 and 5 minutes worth of data. While there have been many different formulas used to calculate HRV from RR-intervals, they all share this common foundation.
Although HRV manifests as a function of heart rate, it originates in the nervous system; HRV therefore provides unique information from the information contained within resting heart rate.
The human autonomic nervous system controls the involuntary aspects of our physiology, and is often subdivided into two branches: parasympathetic (deactivating) and sympathetic (activating). Parasympathetic stimulation reflects inputs from internal organs, like the need to digest after eating a meal, and causes a decrease in heart rate. Sympathetic activation is a response to stress, exercise, and disease, and causes an increase in heart rate.
HRV emerges from the interplay between these two competing branches. Essentially, in a balanced nervous system, our hearts are constantly getting “mixed messages” — commands to increase heart rate from the sympathetic nervous system and commands to decrease heart rate from the parasympathetic nervous system — these mixed messages cause the resulting heart rate to be in a constant state of fluctuation.
HRV positively correlates with whole-body health. As mentioned above, HRV emerges from the competing interests of the autonomic nervous system’s two branches. When HRV is high, it is a sign that the body is highly responsive to both sets of inputs, and therefore highly capable of adapting to changing environmental conditions.
When HRV is low, one branch of the autonomic nervous system is sending stronger inputs to the heart than the other. Sometimes one-sided dominance is a positive thing, when you are running away from an immediate danger — like a hungry tiger — you want your body to listen only to your sympathetic nervous system (which is sending signals to divert resources to your legs) and to ignore signals from your parasympathetic nervous system (which is trying to divert resources away from your legs and towards your digestive system to process your lunch). However, when there is no immediate danger or need to strongly favor one branch’s inputs over the other, being balanced is advantageous.
Many factors besides the immediate threat of a hungry tiger can temporarily tip the sympathetic/parasympathetic balance and reduce HRV. These include pain, psychological stress, illness, fatigue, and hydration.
An individual’s highest attainable HRV is determined by fitness, age, gender, genetics, health, and environmental conditions. Maximum HRV varies considerably among individuals; for example, HRV decreases dramatically with age, such that the maximum attainable HRV of a fit 20-year-old athlete may be more than double that of a fit 50-year-old. Therefore, it is particularly meaningful to consider your HRV today in the context of your own recent history – How does it compare to yesterday’s HRV? Is today’s HRV high or low compared to the past week or month? – and it is less valuable to make a point comparison between your HRV today to your teammate’s HRV today.
HRV was first documented in 1733, when Reverend Stephen Hales noted that human pulse appeared to vary with respiration rate. However, it wasn’t quantified until the advent of the ECG in 1895, and was not popular until the introduction of digital signal processing in the 1960s. In 1965, one study found that fetal distress was preceded by alterations in HRV before alterations in the heart rate itself could be observed. After this discovery, HRV quickly became a popular research subject and widely-used diagnostic tool.
Since the 1960s, HRV has been used as a diagnostic and predictive tool in the medical field. Physicians have used HRV in wide-ranging applications from detection of autonomic neuropathy in diabetic patients, to predicting survival after a heart attack, to predicting the likelihood of post-transplant organ rejection.
It was comparably recently that exercise physiologists discovered the connection between baseline HRV levels prior to training and improved athletic performance. Now, HRV is widely used in athletic training and studied by exercise physiologists.
One key study found that training when HRV levels are at baseline or above results in significantly higher improvements in maximum running velocity than does training according to a pre-planned schedule. In doing so, they were the first to prospectively demonstrate the utility of HRV as a predictor of exercise readiness.
A recent series of studies in elite athletes has shown that HRV can be used to track over-training — a condition defined by persistent decreases in athletic performance despite relative inactivity.
HRV is an easy to measure, information-dense metric with wide-ranging value for increasing the safety and effectiveness of training. For this reason, it has become extremely popular among athletes and their training staff. HRV has been a trendy topic across medical and athletic research fields, and will likely only continue to increase in popularity.