Ask the Coach – the Scoop on Power Meters
What’s all this talk about power meters? How is it different from a heart-rate monitor as far as training goes? –Densie
Denise’s question is a good one. If you are in to training tools (some might call them gadgets) then you definitely want to keep reading. Even if you aren’t much into the electronics of training, it can’t hurt to know what’s out there! In this post I’ll explain what a power meter is and how it is used in training that is different from a heart-rate monitor. I’ll also post a few sample files for the data lovers.
When we talk about power meters, we are talking about measuring power output in cycling. Some power meters are on your rear hub (like the PowerTap) and others on your crank (SRM). When you have a power meter on your bike, it measures the amount of energy that you are putting forth. Power is measured in Watts, and if we remember basic physics, a Watt is a Joule per second. The Joule is a unit of energy (4.184kJ = 1 kilocalorie = 1 Calorie) and can also be written as N-m (N, newton, a measure of force) x (m, meter, a measure of distance). So a Watt is basically a measure of force x distance/time. But in short, it’s a true measure of your performance. There are other equations for circular motion (by pedaling and making a wheel rotate), but we’ll leave those alone.
How does this differ from heart rate? Heart rate is a physiological measurement; it’s a measure of your body’s response to the environment. Not only will your heart rate respond to your effort, but also to external conditions like temperature, if you are properly hydrated, and other such factors. Heart rate is also a lagging factor. If you start riding up a hill with a constant grade at constant effort, your heart rate will take a while to reach its maximum value for that effort. But if you are pedaling consistently, your power will remain constant over that interval. So if you want a tool that measures your true effort on the bike, you want a power meter.
To train with a power meter, the protocols are similar to training with a heart rate meter. You will perform a “field test” to determine power zones. Once these zones are determined (and there are several different methodologies to determine zones), you will perform workouts in these zones in order to improve your fitness. But because you have some maximum heart rate, watching power numbers increase will be much more telling of improved ability than trying to monitor improvements with heart rate.
One more note about training with power. Because your weight affects how much power a person can produce, we can’t compare pure power numbers from one athlete to another. A 180lb cyclist might be able to produce 340 Watts going up a hill, and a 135lb cyclist might only be able to produce 280 Watts on that same hill. But does that mean the 135lb cyclist isn’t as strong? Not necessarily. To compare wattage across athletes, we look at watts per kilogram. So take your weight in lbs and divide it by 2.2 to get your weight in kg. Then you want to compare watts produced over a time frame, typically we compare functional threshold power (FTP), or the power you’d be able to sustain for an hour. Take that power value and divide by your weight in kg and you get W/kg – a way to compare power output across athletes. So if our 180lb cyclist has an FTP of 340W, his power-to-weight ratio would be 4.1 (340W/82kg) at FTP. Our 135lb athlete with an FTP of 280W has a power-to-weight ratio at FTP of about 4.5 (280W/61kg). So even though our heavier athlete can generate more watts, the lighter athlete can generate more watts per kg and therefore would be faster at FTP, so for example in a 40k time trial.
Ok, so now we have a basic understanding of why we might want to train with a power meter, let’s look at some data.
Here’s a file from a workout I did recently in preparation for a ride with a lot of climbing. I did eight 6-minute hill repeats, trying to average around 210 watts. I did the first four repeats on one hill, then the next four on another nearby to work different cadences at the same power. I started out a little too strong, averaging about 220W, but then settled into an average closer to 210W. Repeat #5 was low, about 185W, so I thought I might be done. I tried one more and when I hit my goal, I knew my legs still had power in them so I continued with the intervals and finished consistently. Had #6 been similar to #5, I would have called it a day, as I was not able to generate more power in the desired zone. You can also see from the graph (in the link) how my heart rate lags.
Here’s a file from a cyclist in this year’s Tour de France that was posted and analyzed on the Training Peaks blog.
And here are a few links for more information on training with power.
The classic reference – Training and Racing with a Power Meter, Allen and Coggan
One final note: power meters aren’t cheap (they typically cost over $1000 new) so for most athletes, the “old-fashioned” heart-rate method is still a very viable way to train.
Please leave comments or questions in the comment section about this topic. If you have other questions that you would like to see answered in the ‘Ask the Coach’ column you can post them there as well. You can also contact Coach Nicole on facebook, twitter or via email at email@example.com.
Coach Nicole is the founder and head coach for NEO Endurance Sports & Fitness, a Colorado-based endurance sport coaching company. She is a USAT Level 1 Certified Coach and also coaches triathlon for Team In Training. Learn more at http://neoendurancesports.com/.