Today’s market inundates us with countless self-quantifying metrics, wearables, and apps that promise to change the way we approach health and fitness. It can be time consuming to sift through the BS to figure out what works best.
Heart rate (HR) and heart rate variability (HRV) have received a great deal of attention recently. Both metrics have many pros and cons that should be considered before using either one, though. In order to determine when to use these metrics correctly, it is important to understand the basic science behind each one.
What’s the difference between HR and HRV?
- Heart rate (HR) is measured in beats per minute. It does not require exact times – just the average of the beats in a given time period. For example, a 60 beats per minute HR could mean 1 beat per second or it could mean an average of 1 beat every 0.5s, 1.5s, 0.5s, 1.5s, etc.
- Heart rate measurement is a simple test that has been in existence for thousands of years due to the low-tech requirements for measurement.
- Generally, a low HR indicates rest, while a high HR corresponds with exercise or exertion.
- While heart rate focuses on the average beats per minute, heart rate variability (HRV) measures the specific changes in time (or variability) between successive heart beats. The time between beats is measured in milliseconds (ms) and is called an “R-R interval” or “inter-beat interval (IBI).”
- Generally, a low HRV (or less variability in the heart beats) indicates that the body is under stress from exercise, psychological events, or other internal or external stressors. Higher HRV (or greater variability between heart beats) usually means that the body has a strong ability to tolerate stress or is strongly recovering from prior accumulated stress.
- At rest, a high HRV is generally favorable and a low HRV is unfavorable. When in an active state, lower relative HRV is generally favorable while a high HRV can be unfavorable.
Autonomic nervous system, in vertebrates, the part of the nervous system that controls and regulates the internal organs without any conscious recognition or effort by the organism. The autonomic nervous system comprises two antagonistic sets of nerves, the sympathetic and parasympathetic nervous systems. The sympathetic nervous system connects the internal organs to the brain by spinal nerves. When stimulated, these nerves prepare the organism for stress by increasing the heart rate, increasing blood flow to the muscles, and decreasing blood flow to the skin. The nerve fibres of the parasympathetic nervous system are the cranial nerves, primarily the vagus nerve, and the lumbar spinal nerves. When stimulated, these nerves increase digestive secretions and reduce the heartbeat.
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.
Your resting heart rate can reflect your current — and future — health
- One of the easiest, and maybe most effective, ways to gauge your health can be done in 30 seconds with two fingers. Measuring your resting heart rate (RHR) — the number of heart beats per minute while you’re at rest — is a real-time snapshot of how your heart muscle is functioning.
- Your resting heart rate, when considered in the context of other markers, such as blood pressure and cholesterol, can help identify potential health problems as well as gauge your current heart health.
- For example, a 2013 study in the journal Heart tracked the cardiovascular health of about 3,000 men for 16 years and found that a high resting heart rate was linked with lower physical fitness and higher blood pressure, body weight, and levels of circulating blood fats. The researchers also discovered that the higher a person’s resting heart rate, the greater the risk of premature death. Specifically, an RHR between 81 and 90 doubled the chance of death, while an RHR higher than 90 tripled it.
- “In certain cases, a lower resting heart rate can mean a higher degree of physical fitness, which is associated with reduced rates of cardiac events like heart attacks,” says Dr. Jason Wasfy, director of quality and analytics at Harvard-affiliated Massachusetts General Hospital Heart Center. “However, a high resting heart rate could be a sign of an increased risk of cardiac risk in some situations, as the more beats your heart has to take eventually takes a toll on its overall function.”
- Check your resting heart rate early and often
- Dr. Wasfy recommends checking your resting heart rate a few times per week and at different times of the day. Keep in mind that the number can be influenced by many factors, including stress and anxiety, circulating hormones, and medications such as antidepressants and blood pressure drugs.
- Talk with your doctor if your resting heart rate is regularly on the high end. There are ways to lower it and keep it within its proper range. One example is keeping your cholesterol levels in check. High levels restrict blood flow through the arteries and damage blood vessels, which can make your heart beat faster than normal to move blood through the body.
- Another reliable way to lower your resting heart rate is to exercise. “Even small amounts of exercise can make a change,” says Dr. Wasfy. However, the intensity of the exercise is key. One study that involved 55-year-old adults found that just one hour per week of high-intensity aerobic training (about 66% of maximum effort) lowered RHR more efficiently than a low-intensity effort (33% of max effort).