Listen to your heart, they say. That thumping in your chest can tell you when you’re excited, scared or in love. It can also tell you a lot about how hard you’re working during exercise; so much so that many people wear heart-rate monitors to quantify exactly how hard they’re training.
But is heart rate really the best indicator of exercise intensity? Should we trust that readout on the treadmill or that fancy wristwatch to tell us to push harder, back off or keep going?
Like many things in fitness, it depends. A basic understanding of what your heart does during different types of exercise helps determine whether you should worry about your heart rate or not.
WHAT YOUR HEART DOES
Your heart is a busy muscle. Its main job is to pump blood to other areas of the body that need it, delivering much-needed oxygen to muscles and organs. If you’re resting, the heart doesn’t pump very fast (the average HR of a healthy adult is about 60 beats per minute). But if you’re working hard, the heart beats faster to keep up with energy demands.
As you exercise, the heart pumps more blood to bring more oxygen to working muscles. Your HR is controlled by your sinoatrial node (SA node), also known as the body’s pacemaker. Luckily, you don’t have to consciously think about contracting your heart like you would your biceps; the SA node does that for you.
That said, your heart rate goes up during non-exercise activity, too. If you get scared, nervous or excited, there’s an increase in sympathetic nervous system activity, also known as “fight or flight” mode. While you’ll burn slightly more calories while excited, nobody got in shape by watching a scary movie, so we can’t rely on heart rate alone to determine exercise effectiveness.
So when does heart rate actually make a difference? It depends on what type of exercise you’re doing.
OPTION #1: AEROBIC EXERCISE
Aerobic exercise (literally, “with oxygen”), also known as cardio or long slow distance (LSD) training, typically comes in the form of jogging, biking, stair climbing or other similar exercises where you can sustain a steady pace for an extended period of time. HR is the most reliable measure of intensity for this type of exercise and is a surefire way to gauge your progress.
The reason HR works to quantify aerobic exercise intensity is because oxygen consumption (often called “VO2”) and HR are closely related during low-intensity exercise. HR and VO2 rise together steadily during aerobic exercise that isn’t too hard, meaning your heart is able to pump enough blood and deliver enough oxygen to meet the demands of your working muscles.
However, once exercise gets too hard (i.e. you start running faster, you run up a steep hill, etc.), it becomes anaerobic (literally, “without oxygen”) and your heart and lungs can no longer move enough blood and oxygen to meet energy demands. At this point, VO2 won’t increase (because you physically can’t consume any more oxygen), but your HR may continue to climb. It’s feasible that one’s HR might exceed 200 beats per minute during intense anaerobic exercise, but that doesn’t mean your VO2 keeps climbing.
That said, HR is a great indicator of aerobic exercise intensity, but it quickly becomes a poor indicator if exercise gets too hard and becomes anaerobic. So if you’re aiming to increase your aerobic fitness for endurance activities like jogging or biking, aim to keep your HR below 70% of your maximum heart rate (HRmax). How do you calculate your HRmax? The old-school method is:
HRmax = 220 – your age
This method is a bit dated, but works for most people. How do your find your HR? You can take your pulse at your wrist or neck for 15 seconds and multiply by four, or invest in a HR monitor for the most accurate measurement.
OPTION #2: HIGH-INTENSITY INTERVAL TRAINING
High-intensity interval training (HIIT) involves alternating short bursts of intense exercise (i.e. sprints, fast cycling, burpees, etc.) with longer periods of less intense exercise. This type of training is highly effective for losing fat because of the excess post-exercise oxygen consumption (EPOC) that it creates. It elevates your caloric expenditure for up to several hours after you stop exercising, making it a great bang-for-your-buck time investment. However, HIIT is anaerobic, meaning it’s too hard for oxygen consumption to match energy demands, making HR an unreliable measure of how hard you’re working.
As stated before, HR can keep climbing after you’ve reached your VO2 max (the most oxygen your body can possibly consume during exercise). We already know that at this point, HR and VO2 aren’t climbing together, and your only goal for HIIT is to consume as much oxygen as possible after exercise. So what’s the best way to measure your intensity? Rating of perceived exertion (RPE) is a good start, as well as keeping rest intervals short (60 seconds or less).
RPE is often measured on a scale from 1–10, with 1 being very easy and 10 being an all-out effort. For HIIT, keep your work intervals between 10–30 seconds at 8–10 RPE, and keep your rest intervals between 30–60 seconds at 4 RPE or less.
OPTION #3: LIFTING WEIGHTS
You often hear people say, “Wow, that heavy set of squats really got my heart rate up. I’m building strength and doing cardio!” This couldn’t be further from the truth. In fact, if you’re worried about your HR while lifting weights, you’re studying for the wrong test.
On the aerobic to anaerobic spectrum, lifting weights is as anaerobic as it gets. You’re using smaller muscle groups, heavier loads and higher efforts than you would while running or cycling, which makes lifting too hard to get enough oxygen to the working muscles to meet energy demands. If you’re actually lifting hard, your HR and VO2 never sync up, and that’s OK.
What’s more, while lifting weights, your muscles essentially “clamp down” on the surrounding veins and blood vessels, which reduces venous return (the rate of blood flow back to the heart). This increases blood pressure and therefore HR because the heart is trying like crazy to move blood around the body, but more and more blood is getting “trapped” in the working muscles. This partially explains that tight, burning feeling you get in your legs after a set of high-rep squats. This also explains why your HR skyrockets while lifting, but you’re not getting any cardio benefit from it.
So if HR doesn’t matter while lifting, how do you measure progress? Simple: lift more weight than you did before, or do more sets and reps with the same weight you did before. Follow this formula ad nauseam and you’ll make plenty of progress.
HAVE A HEART
It turns out HR isn’t the only way to measure exercise intensity. If you’re going for a jog, by all means track your HR and stay within your target HR zone. But if you’re doing HIIT or lifting weights, don’t worry about your HR and use other means to judge how hard you’re working.