Power is the King Metric in cycling these days, so much so that I sometimes run into riders who just don't even bother wearing their heart rate monitor once they get their first power meter.
But that's a mistake! Why?
Power is an output-based metric. That means it shows what kind of work you're putting into the bike. Obviously, that matters to anyone who wants to go fast on a bicycle for any reason... BUT, in the absence of other information, it doesn't tell you about what's going on inside your body.
What's going on inside your body matters to anyone who is training to go faster on a bicycle. To learn about what's going on inside your body, heart rate is the King Metric!
When you pair heart rate with power, and learn how they correlate, you are equipping yourself with powerful information about your training both in real-time and after the fact.
When you learn how you feel at those powers and heart rates, you've reached Zen Master status. With that information combined, you can make informed decisions about when you need to back off and change that hard workout for a lighter ride or even just turn around and go home. You can see when you might be starting to get sick. Conversely, you can see when you're particularly strong and should go for that extra interval.
But you can't get that kind of information without wearing your heart rate monitor!
OK, cool. That's all subjective, squishy stuff. What about the hard-core tangible stuff?
Track Your Training Zones (especially for those without power meters!)
Heart rate is one method to plan and execute your training, and it's been around a lot longer than power. Just as with power, you should conduct a test to determine your maximum heart rate and/or your heart rate at your lactate threshold (I prefer lactate threshold heart rate - LTHR - as my zone-setting metric). Then you can determine your training zones based on that data point, and away you go. But you have to test - more on this in the next article.
Pitfall #1: Heart rate training zones have one pitfall, and it's that heart rate responds somewhat slowly to changes in effort. For example, it may take several minutes for an athlete's heart rate to stabilize in a sub-threshold interval. This is what can cause many athletes to start longer efforts too hard as they try to ramp their HR up to their target quickly, instead of steadily allowing HR to build. They subsequently fatigue faster than is intended in the workout, and that can compromise the interval or even the entire workout.
You can prevent that by being cognizant of the fact that HR responds slowly, and build into your efforts more conservatively.
In intervals above threshold, it may never stabilize before the athlete fatigues - it just keeps going up to your maximum, or more likely until you can no longer sustain the effort.
Pitfall #2: Related to #1, because HR responds slowly to changes in effort, it is not ideal for short intervals, say intervals under 3 minutes. This is where power (or in the absence of power perceived exertion) really shines in training.
It's Good for Pacing Long, Steady State Endurance Rides
As I mentioned in my previous blog about nailing your long ride, I prefer heart rate over power when I'm doing my long, steady endurance riding (in "zone 2"). As I talked about above, heart rate tells me what's going on inside my body - it's a direct measurement of the stress my body is under, so I aim to keep it in a range of about 10 beats per minute (bpm) for the duration of my steady state ride. Power fluctuates a great deal when we're riding bikes outside, and you can drive yourself crazy trying to make a power number stay in a relatively narrow range. But keep that heart rate relatively steady and you're on your way to nailing the long ride!
Watch for Unusual Heart Rate Responses in Training
I touched on this in the introduction, but one of the most powerful things about understanding the trifecta of power, heart rate, and how you feel is being able to make informed decisions about your training. This is best explained with examples:
Example 1: You are going out for a 2-hour ride with some sweet spot intervals. A couple of people at work called out sick yesterday and you didn't get great sleep last night. As you warm up and get into some relatively easy endurance work, you notice that your heart rate is 128bpm at 170W. You know that normally after a few minutes at 170W, your heart rate is around 136bpm or so. You get into your first interval, and it feels really hard - harder than sweet spot typically feels... AND your heart rate is only 148bpm instead of the 157bpm you're used to seeing at this intensity after several minutes.
Your body is giving you clues that today's not the day for this workout - you don't feel 100%, your perceived exertion is high, and your HR won't elevate as normal. You should shut this workout down and ride easy or call it off altogether if you think you're getting sick.
Example 2: You are going out for a 3-hour ride, and feel great! 90 minutes into the ride, you're pushing 170W and you see your heart rate is still only 130bpm. Knowing that you like to ride by feel, you start pushing a little harder, and get up to 180W, and your HR elevates to 133bpm. You comfortably complete the ride with no issues! This can be an indication that you've adapted to your training and are getting fitter! (More on Efficiency Factor below.)
It takes time to learn how power and heart rate go together, and you have to be conscious of how you feel when you ride so you can learn to listen to your body in this way. Heart rate simply gives us another metric to pay attention to when your HR is abnormally low and you're not feeling it, it's a good indication that you might want to shut the ride down and take some rest.
Review Your Cardiac Drift
Now we're getting a bit more advanced... but for my athletes on their long steady-state rides, I look at their cardiac drift as an indicator of how their body is holding up to the duration and intensity of that ride. This requires power information as well; on Training Peaks, you can see it in your rides by looking at the Pw:HR metric. Ideally, we look for very little cardiac drift, say less than 5% drift relative to power, on longer rides.
I don't put a ton of stock into drift on individual rides, but I do watch trends. If I see a couple of 3-hour rides at 70% intensity, and I see 7% cardiac drift on both of those rides, that can be an indication that a rider needs to spend more time riding at that intensity before we start pushing out the duration of those rides.
Example of cardiac drift over a four-hour ride at relatively steady power.
In the chart above, you can see an increase in heart rate over the ride.
Coupled with power data, this is informative to see if the athlete was under more stress late in the ride or if they have adapted well to this length of ride.
Track Your Efficiency Factor
A core metric that again requires power data in tandem with heart rate, Efficiency Factor (EF) is simply the ratio of your normalized power to your heart rate over the course of a ride. As you become more aerobically fit, you'll see that you can put out more power at the same or lower heart rates, and thus your EF will go up over time. This is another key metric that I track for my athletes when I'm working to improve their aerobic energy system. As we know, improving aerobic capacity and aerobic endurance is the most important thing we do in almost every cycling discipline.
The chart above is an example of tracking metrics on long, relatively steady rides. Coupled with other information (for example, this athlete had COVID in December, which is why there is a gap in long rides and the corresponding EF dropped in January and February), and notes about the individual ride, a coach can make future training decisions about what the athlete may need to work on. Again, it's not about one individual ride or number, but rather trends in EF and Pw:HR along with the subjective information that matter.
OK, OK, so that's a lot to process... but the point is this: even if you have a power meter, always wear your HR monitor!!
While you might not be gathering information from it right now, as you learn (or if you ever hire a coach), there is real value in having HR data, even if you have a power meter.
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