Understand power meters on your road bike

Watts, kilojoules, FTP — what do these numbers on your power meter actually mean? Read the answer in this concise guide to cycling power.

If you want to get faster (and of course you do), chances are you’ve decided to invest in a power meter. What used to be expensive kit is now more affordable than ever, with power meters available for all kinds of setups.

Power meters built into the chainrings are the most commonly used. But there are also other options, integrated into pedals or the wheel hubs. Once you’ve connected your power meter, you still need to learn how to understand the data to get the most out of it. It’s no surprise that the relationship between power and fitness is a complex topic that is still interpreted by coaches and athletes. Even so, even the simplest power meter can offer valuable insight into your efficiency and potential on the bike.In this article, we explain commonly used power metrics shown on your power meter display.

Power

Power is the rate at which energy is used (energy over time) and is measured in watts. In cycling, energy is expressed as work (for example, how hard you need to work to climb a hill). It’s a constant snapshot of your work rate at any given moment. It’s the foundation of all power-based training. A fun fact: a watt is a watt, whether you’re riding your bike or powering your home. So when a pro rider puts out 2,000 watts in a sprint, he or she could essentially power two homes at normal consumption. Another comparison: one horsepower equals 746 watts.

Average power

Average power probably isn’t on your power meter’s home screen, but you may find it in the second or third menu. It’s exactly what it sounds like: the average of your power output over the entire ride, just like your average speed. But keep this in mind: average power measures everything during your ride. So that includes coasting too, and that’s naturally only a fraction of what you actually rode. That’s why we prefer to look at normalized power.

A Tour de France rider will average 220 to 320 watts over a four-hour stage; that’s an intensity most recreational riders can only hold for around an hour.

Kilojoules

kJ, a basic unit of energy, has the advantage that for most rides one kJ is roughly equal to one kilocalorie (or what nutritionists simply call a calorie). The actual conversion is 4.18 kJ = 1 calorie, but humans are only 20 to 25 percent efficient. So for every 100 calories burned during exercise, only 20 to 25 calories actually propel you forward and are measured by 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 useful metric to focus on. But the body burns a different ratio of fat and carbohydrates at different training intensities (and it continues to burn energy after the workout is over). So it always remains an estimate. A power meter does honestly track how much work you actually did, and how much fuel you needed for that ride.

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

Threshold Power

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

• 5 seconds (neuromuscular power)
• 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 account for variables such as time spent coasting, or interval work with its short, sharp spikes in power, to estimate the power you could have sustained over an entire ride (or a segment of a ride such as a lap time) if you had ridden at a constant average power. Normalized power will almost always be higher than average power because of the coasting aspect. Average power is still important, but normalized power gives a more accurate picture of the true physiological demands of a ride.

Watts/Kg

Simply put, watts alone are not the most reliable measure of performance, because cyclists produce different levels of power. A physically larger rider can generally produce more watts on flat or rolling terrain than a smaller-built climber. A bigger rider needs to produce higher watt numbers to maintain the same speed as the smaller rider. A better measurement, especially on climbs, is therefore watts produced per kilogram of body weight (as this normalizes the difference in size). This is commonly used when talking about threshold power, but it also plays a role in the other tests.

So how much better are Tour riders than the rest of us? A general classification contender can produce just over 6 w/Kg on the key climbs of the race. For comparison, a national-level pro may top out at 5 to 5.5 w/Kg; a strong, competitive amateur or masters rider can probably produce around 4 w/Kg, and an untrained person would struggle to produce 2.5 w/Kg. That’s right, a Tour pro can produce around 50 percent more watts than a competitive amateur racer and more than double what an untrained person could manage.

Heart rate

The fact that power is a more accurate measure of training does not mean you should ignore heart rate (HR). It still matters, because it reflects your body’s response to the effort. And it is an important indicator of how you feel. Say you go out for a ride and feel terrible on a climb; your heart rate spikes, but the power just is not there. You may be overtrained or getting sick. An unusually high heart rate shows that something is off; you are not going to get much out of training today, so you should head home and let your body rest. Likewise, a low resting heart rate indicates that you have fully recovered from the last training session, so you can get back in the saddle again.

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 is cadence. If you ride with a power meter, you will notice that when you shift down and pedal at a higher cadence, it may feel a little easier at first, but your power output actually goes up. Even though you are not applying the same amount of force on every pedal stroke, the increase in cadence means there are more pedal strokes per minute, so more total power.

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