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While power meters have revolutionized training methods in the past decade, the humble heart rate monitor remains a key component to getting the most out of your workouts.
The arrival of affordable power meters in the early 2000s heralded a revolution in cycling. Every coach and athlete with the resources got onboard the “power-is-everything” train, often tossing the old clumsy heart rate strap aside. In the frenzy, no one investigated whether power was actually a better training tool.
That is, not until two studies explored that claim. The first, conducted in 2009 by the Sports Science Institute of South Africa, used 21 well-trained cyclists. Half did intervals by power, the other half by heart rate. Statistically, the gains were equal but the group training with heart rate showed “a greater probability of a beneficial effect.” The second study, published in the Journal of Sports Science and Medicine in 2011, likewise found equal improvement in 11 recreational cyclists. Both research teams concluded that a power meter was not necessarily a better training tool.
Still, many coaches argue that a power meter is more precise and provides more immediate feedback to changes in effort. That’s true, but treating it as a replacement for heart rate assumes it measures the same aspects of training.
That’s a mistake, says prominent physiologist Dr. Iñigo San Millán, former director of the University of Colorado Sports Medicine and Performance Center. “With watts, we’re just looking at the end product, which is mechanical energy,” San Millán says. “But we might be missing what happens at the chemical or metabolic level.”
Put another way, power meters measure what’s happening with the bike. But heart rate measures the individual. Ultimately it’s the individual that needs to be trained.
Svein Tuft (Mitchelton-Scott) agrees, saying it can be dangerous to seek an arbitrary power number. “Watts are definitely a huge help,” Tuft says, “but it’s more important to understand your body and where you’re at in that moment rather than try to live up to some impossible expectation.”
Blood lactate is perhaps the best measure of what’s going on metabolically, but there is no on-the-road lactate monitor.
Fortunately, San Millán has thousands of data points showing that heart rate directly correlates with lactate, making it a great representation of what’s happening in the body. For example, riding at a heart rate just above or below maximal lactate steady state (MLSS) or anaerobic threshold offers distinct metabolic stresses.
Of course, you can identify your power at MLSS, but since it’s not a physiological measure, that number can vary dramatically through the season — up to 60 watts according to a 2000 study conducted by respected physiologist Alejandro Lucia. Threshold heart rate doesn’t change.
As a training metric, heart rate does have its limitations compared to power. You can’t say, “I’m riding at 185 beats per minute — I’m going to crush it this year!” But if you’re climbing 50 watts higher than last season and 30 watts better than your competition (all other things remaining equal), you can make that claim.
Ultimately, using a power meter and heart rate monitor in combination makes for a more complete toolbox. While power indicates how strong you’re riding, heart rate tells the metabolic cost of those watts. So, train by power zones but use heart rate to measure how your body is responding.
Building a more complete toolbox
Neither a heart rate monitor nor a power meter is beneficial unless they’re used right. Here are a few tips for making the most of whatever you keep in your toolbox.
Get accurate zones
No matter if you rely on watts or beats per minute, use this data within the context of your training zones and physiology. European-based pro ’cross racer Elle Anderson says that finding your heart rate at anaerobic threshold or MLSS is most important. In-lab testing is best, but an on-the-road time trial can work in a pinch.
Use heart rate’s consistency
Your power at threshold will change throughout the season, but heart rate won’t. Look for changes in your power-at-threshold heart rate to gauge your fitness. A gradual increase in your power relative to your heart rate means you’re getting stronger.
Race with heart rate
In most races, power can fluctuate dramatically due to accelerations out of corners, terrain, barriers, and responding to moves. This is one reason Anderson finds racing by heart rate to be more useful. “If it’s the first 20 minutes of a race and I look down and see my heart rate is pinned close to my max, that helps me to understand whether I need to back off the pace,” she says.
Do intervals by power zones; gauge with heart rate
Heart rate is slow to respond to increased efforts, making it hard to use for intervals under five minutes and useless for those under one minute. But heart rate is a great gauge for work at or below threshold. This work should be metabolically sustainable, says San Millán, meaning your heart rate should plateau. If your heart rate steadily rises during the effort, you’re targeting too high a wattage.
Gauge fatigue with morning heart rate
Because it’s a physiological measurement, heart rate can do something else that power can’t: indicate fatigue. One of the first signs of overtraining is a rise in your waking heart rate. Get a baseline when you’re rested. Then measure your heart rate regularly while still in bed. A rise of four or more beats per minute at rest suggests you may need to take a day off.
Gauge fatigue with heart rate and power
Using both power and heart rate provides a great way to gauge overtraining on the bike. When we are tired, our heart rate will drop relative to power, especially at higher intensities. If Anderson notices that her legs are tired and her heart rate is lower than normal for a given wattage, she backs off and rests.
Get advanced with heart rate variability (HRV)
HRV is becoming a common capability of new heart rate monitors. This is a measure of how variable the time interval between beats is, usually taken at rest. Both too much and too little variability are signs of being over-reached. A recent study by the Research Institute of Olympic Sports in Finland compared classically periodized runners to runners who trained by HRV. This HRV group was only prescribed high-intensity work when their seven-day HRV average was within the rested range. The HRV group did significantly fewer interval sessions over the four weeks but improved their 3,000-meter run time two percent while the traditionally trained group did not.
Use heart rate on long rides
Suppose you did a four-hour ride at a steady 160 watts. You might start the ride holding 145 beats per minute, but hours later your heart might be pumping 20 beats faster. This is known as cardiac drift. If you ride by power alone, you may be in the right physiological range initially but not at the end. Sticking to a heart rate range will keep you at the right intensity the entire ride.
Recognize artifacts when comparing the two
Since watts are a mechanical measurement, they remain static. 400 watts is always 400 watts. But a heart rate of 150 beats per minute doesn’t always mean the same thing. Many other factors can affect your heart rate. Fatigue lowers heart rate relative to power. Heat, dehydration, stress on race day, and cardiac drift all raise it. Coaches and athletes who prescribe to a power-only training method call these “artifacts.” In reality, they are important pieces of physiological bio-data that can help your training.
If your heart rate is abnormally high and you’ve ruled out everything else, remember a high heart rate appears a day or two before flu symptoms.
Trevor Connor is a long-time cycling coach and elite racer. He holds degrees in exercise physiology and nutrition from Colorado State University. He has served many roles in cycling, from team manager to coach at the National Centre in Canada.