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The basic concept of heart rate is simple: it’s the number of times the chambers within the organ squeeze per minute, pushing blood through the rest of your body. That blood delivers the fuel you need to run and move well—the nutrients and oxygen that power your muscles—and also carries waste products away.
Underneath this lub-dub, of course, lies a complex cascade of physiological reactions. An electrical network, called the cardiac conduction system, sends a pulse from the top to the bottom of your heart.
The beat begins in your right upper chamber, or atrium. There, a group of so-called pacemaker cells, in a structure called the sinoatrial node, kick off the charge. The signal prompts your atria to pump blood into the lower chambers, called the ventricles—the valves between them then close with a “lub.” Your ventricles then pump the blood out into your blood vessels before closing more valves with a “dub.”
The speed at which this all happens (and as a result, your heart rate) is governed by messages from your hormones, as well as your autonomic nervous system. That’s the one that controls all the involuntary processes in your body, from digestion to heart rate to your sexual response. The autonomic nervous systems consists of two parts. The sympathetic, involved in your “fight or flight” response, revs your heart rate up. The parasympathetic, meanwhile, calms things down.
When you’re healthy, the balance between the two keeps your heart rate controlled and steady, says Dr. Allison Zielinski, a cardiologist at Northwestern Medicine and co-director of the sports cardiology program at Northwestern, in Chicago. Beyond the basics, there are a few other key terms that are helpful for runners to know.
Resting heart rate
This refers to the speed of your pulse when you’re not expending much effort—say, when you first get up in the morning or you’re kicking back on the couch with a good book or a Netflix binge.
Maximum (or maximal) heart rate
Just like it sounds, this number refers to the upper limit of how often your heart can beat in a minute. The only completely accurate way to measure it is in a lab, says Kimberly Gandler, M.S., human performance manager at the Memorial Hermann IRONMAN Sports Medicine Institute in Houston.
But a number of formulas and DIY tests allow you to make a pretty good estimate on your own (more on that in a minute). That’s likely to be good enough for most runners, says Heather Hart, an ACSM-certified exercise physiologist and co-founder of Hart Strength and Endurance Coaching in Myrtle Beach, South Carolina.
Most of these formulas use age. Your resting heart rate doesn’t change much through the years. But with time, your heart’s ability to pick up the pace decreases a bit. Experts believe that’s due to a loss of pacemaker cells or buildups of fat and fibrous tissue, which slows the muscles’ responses to the electrical signal.
Determining your maximum heart rate can help you define heart rate zones to guide your training—a way some coaches recommend planning your runs to get the most out of each workout. When you’re using this method, each zone has a specific purpose, and your goal is to keep your heart rate more or less within a prescribed range of beats.
Heart rate reserve
Subtract your resting heart rate from your maximum heart rate and you’ll get this number, your heart rate reserve. It represents the range between your highest and lowest heart rates, or how much you can rev up your pulse with exercise. This number is also often used in calculating heart rate zones for training.
Lactate threshold heart rate
When your muscles use oxygen to produce energy, they also churn out a variety of byproducts. One of those compounds, called lactate, is particularly important to athletes. Lactate builds up in the blood more quickly when your muscles are working hard, Hart says.
At one time, experts believed lactate primarily caused your legs to burn and your stomach to churn during hard efforts (partially true; it does cause fatigue)—and that it was largely to blame for post-run soreness (not true at all). Now, they know your body can actually use lactate for energy. In fact, the ability to do so swiftly is part of what can improve your running performance.
Your lactate threshold is the point at which your body produces lactate at the same rate it can process it; it’s a pace you could hold for about an hour. Lab tests (in which scientists take blood samples to measure lactate levels while you run) or field tests can pinpoint your heart rate at this amount of effort.
The closer your lactate threshold heart rate is to your maximum heart rate, the better for your running. And unlike max heart rate, which doesn’t change much based on your workouts, you can train to boost your body’s ability to clear lactate and improve your lactate threshold heart rate. Tempo runs, in which you hold a comfortably challenging pace, are the most powerful way to do this.
Heart rate variability
While your heart’s motion might seem as steady as a metronome when you’re taking your pulse, there’s actually a slight difference in the amount of time between one beat and the next.
This deviation, measured in milliseconds, results from the push and pull between your sympathetic and parasympathetic nervous systems. In this case, the higher the variability, the better. That means your body is ready to respond to whatever challenges come your way.
Your heart rate variability will likely decrease immediately after a hard run or race. But if you prioritize recovery, it can bounce back higher—much like your fitness.
What Is VO2 Max?
There’s one more definition that’s helpful to know when you’re thinking about how hard you can work on the run, and that’s VO2 max.
Simply put, VO2 max is a measure of how much of the oxygen you breathe can be used to power your activity, Hart says. Specifically, it’s measured in the number of milliliters of oxygen you use per kilogram of body weight in one minute, often expressed as one number (say, 30, 43.2, or 51).
Why is it important to athletes? Well, studies suggest VO2 max can account for 50 percent or more of the difference between performance in two long-distance runners. In fact, it’s considered one of the primary ways to measure your fitness. What’s more, improving your VO2 max is also linked to a longer, healthier life.
While VO2 max is related to heart rate—since the more oxygen-rich blood your heart pumps, the more your muscles can use—it’s not a one-to-one correlation. The efficiency with which your muscles extract and use that oxygen matters, too.
Your baseline VO2 max is partially based on genetics. In addition, men’s tend to be higher than women’s, and age plays a role, with declines of about 10 percent per decade after age 30. In large part, that tracks alongside the age-related declines in your maximum heart rate.
Your running watch or app may give you an estimate of this number, but it’s unlikely to be accurate, Hart says. The only way to know your VO2 max for certain is to have it tested in a lab, just like your lactate threshold. During this test, you’ll run on a treadmill or ride a bike with a mask on. By measuring how much oxygen you breathe in (and carbon dioxide you breathe out) physiologists determine how much of the precious gas you’re utilizing.
This information can be useful for your training—but unless you’re competing at an elite level, measures like the heart rate tests you can do on your probably work well enough, Hart says. More good news: Unlike max heart rate, you can typically improve your VO2 max with training, and you can do it even if you never visit an exercise science lab.
Unlike longer intervals that train your lactate threshold, it’s usually shorter, harder efforts that go farthest in improving your VO2 max. Try workouts like these:
- Uphill repeats, where you’re charging hard for 1 to 5 minutes at a time (and jogging downhill for at least half the amount of time you’re running)
- 800-meter repeats—at a pace you could hold for about 10 minutes—with the same amount of rest afterward as it took you to run the repeat
- Mile repeats at your 5K or 10K pace, with two to three minutes of rest in between