Understand the power meters on your road bike

Watts, kilojoules, FTP What does this data on your power meter mean? Read the answer in this quick guide to power on the bike.

If you want to get faster (and you do!), chances are you've decided to invest in a power meter. Once expensive equipment, power meters are now more affordable than ever and available for a variety of applications.

Power meters in the gears are most commonly used. But there are also other alternatives, integrated into pedals or the hubs of the wheel. Then once you've hooked up that power meter, you still have to learn to understand it to get the most out of it. Not surprisingly, the relationship between power and fitness is a complex topic that continues to be interpreted by coaches and athletes. Yet even the simplest power meter can provide valuable insight into your efficiency and potential on the bike. In this article we describe commonly used ones power metrics that are shown on the screen of the power meter.

Power

Power is the rate at which energy is used (energy over time) and is measured in watts. In cycling, energy is expressed in work (such as how hard you have to work to climb a mountain). It is a constant snapshot of your work speed at a given moment. It is the building block of all power-based training. A fun fact: a watt is a watt, whether you are cycling or powering your home. So when a professional cyclist unleashes 2,000 watts in a sprint, he or she can essentially power two houses with normal usage. Another equation: one horsepower is equal to 746 watts.

Average power

On your power meter, average power is probably not on your home screen, but perhaps in the second or third menu. This is exactly what it looks like: the average of your power over the entire ride, just like your average speed. But be careful: the average power measures everything during your ride. So you also coast and this is of course only a fraction of what you have driven. That's why we prefer to watch it normalized power.

A Tour de France rider will use an average of 220 to 320 watts for a four-hour stage; that's an intensity that most recreational riders can only maintain for an hour.

Kilojoules

The kJ, a basic unit of energy, is fortunate that for most rides, one kJ is approximately equal to one kilocalorie (or what nutritionists simply call a calorie). The actual rate is 4.18 kJs = 1 calorie, but people vary from 20 to 25 percent efficiency. So for every 100 calories burned during exercise, only 20 to 25 calories are propelling you and are measured with the power meter. The rest is converted into heat. If you know your power and time, you can calculate kJs, or calories burned.

If you're trying to lose weight, kJs are a benchmark to aim for. But the body burns a different ratio of fat to carbohydrates at different exercise intensities (and it continues to burn energy after the workout ends). So it always remains an approximation. A power meter keeps an honest record of how much work you actually did, and how much fuel you needed for that workout.

Tour riders typically produce more than 3,000 kJ during a stage.

Threshold power

This is an important measurement of how much power you can produce sustainably over an hour and is a fundamental metric within fitness. It is often expressed in watts per kilogram of body weight (see w/Kg). Do you want to test your own ability? The protocol is a 20-minute time trial, usually on a steady climb. But pros often do a specific series of tests that guarantee greater accuracy. After a 45-minute warm-up, perform maximum efforts in the following order, with full recovery (5 to 10 minutes of very gentle pedaling) between each effort:

5 seconds (neuromuscular strength)
5 minutes (aerobic power)
20 minutes (threshold power)
1 minute (anaerobic power)

Your functional threshold power is 95 percent of your 20-minute power.

Normalized power

Normalized power uses an algorithm to take into account variables such as time spent coasting, or interval work with its short, sharp peaks in power, to estimate the power you could have maintained for an entire ride (or segment of a ride such as a lap time) if you had averaged a constant power output. The power will almost always be higher than average, due to the coasting aspect. Average power is still important, but normalized power provides a more accurate representation of the true physiological demands of a ride.

Watts/Kg

Simply watts is not the most reliable measure of performance because cyclists produce different power outputs. A physically large cyclist can usually produce more watts on flat or rolling terrain than a smaller built climber. A larger cyclist must produce greater numbers of watts to maintain the same speed as the smaller cyclist. So a better measurement, especially on climbs, is watts produced per kilogram of body weight (which normalizes the size difference). This is usually used when dealing with threshold power, but it also plays a role in the other tests.

How much better are Tour riders than the rest of us? A general classification contender can produce just over 6 w/Kg on key climbs of the race. For comparison: a national professional can best produce 5 to 5.5 w/Kg; a good, competitive amateur or masters could probably produce around 4 w/Kg, and an untrained one would struggle to produce 2.5 w/Kg. That's right, a Tour pro can produce about 50 percent more watts than a competitive amateur racer and more than double what an untrained person could.

Heart rate

Just because power is a more accurate measure of exercise doesn't mean you have to give up heart rate (HR). It's still important because this is your body's response to work. And it is an important indicator of how you feel. Let's say you're going for a bike ride and you feel terrible on a climb; your heart rate spikes, but your strength just isn't there. Maybe you're overtraining or getting sick. An unusually high heart rate indicates that something is wrong; you won't get much out of training today, so you should go home and rest your body. Likewise, a low resting heart rate indicates that you have fully recovered from the last workout so you can get back in the saddle.

Cadence

Power equals work over time, right? So there are two ways to increase your power: you can increase the actual force on the pedal, or you can increase the number of times the pedal goes around in a minute. That's cadence. If you ride with a power meter, you'll notice that as you downshift and pedal at a higher cadence, it may feel a little easier at first, but your power will actually increase. Although you don't put the same amount of force on each pedal stroke, the increase in cadence means there are more pedal strokes per minute, so more total power.