Do you need a power meter to train for a triathlon?

A power meter sits on the bike, in the pedals, the cranks or the back wheel. There is nothing like it in the water. Your Apple Watch shows running power, but it works that out from your speed and movement rather than measuring it. Treat it as one more check. With six or eight hours a week, you want one way to judge effort that works in the swim, on the bike and on the run.

Two gauges work in all three sports, and on land a third joins them. The first is how long you could hold the effort. An easy effort lasts hours, threshold about twenty minutes and a very hard VO₂max effort a few minutes. The second is how hard it feels out of ten. Easy is about 3, threshold 7, VO₂max 8 and a sprint 10. The third is your breath, on the bike and the run. Easy is in and out through your nose. Threshold is in through the nose and out through the mouth.

On land, your heart rate checks these gauges. In the water it reads differently, so there the first two lead.

Most power plans are built on FTP, short for functional threshold power. It is meant to be the most power you could hold for about an hour. Researchers had 13 cyclists ride at their own FTP until they could not go on. They lasted about 34 minutes on average. The researchers concluded that FTP is not a reliable marker of the threshold it is meant to stand for. That is a problem with the test, not the meter. But every zone in a power plan is a percentage of that one number. If the number is off, so is every zone.

The meters themselves are good, though not perfect. A study checked 54 power meters from nine makers on a test rig in a lab. Each gave steady readings from one test to the next, and most were within a few percent. Six read more than 5 percent too high or too low. So a power meter is very good at comparing one ride with another on the same bike. It is less sure what your watts really are.

A 2022 review looked at the two simplest ways to judge effort. One is how hard it feels. The other is whether you can still talk in full sentences. Your breathing has two turning points, where it first quickens and where it starts to run away from you. Both gauges marked those points well, and neither needs a lab test first.

The question of how long you could last is good too. Twenty-five competitive cyclists rated how long they thought they could hold a series of efforts. From their answers, the researchers predicted how long they would last at one hard effort. One way of working it out gave about ten minutes, and another about fifteen. They lasted about eleven.

We found no long trial that trained one group of athletes by power and another by feel. What the studies do show is that feel finds the same efforts a lab does.

Watts are the same number whatever your day was like. You are not the same. Sixteen people cycled at one fixed power until they could not go on, twice. Before one ride, they spent 90 minutes on a hard mental task. Before the other, they watched documentaries. After the hard mental work they gave up about 15 percent sooner. Their heart and muscles worked the same. The ride simply felt harder all the way through.

So if you ride after a long day at work, last week's watts can be too much for today. Feel tells you that. Say an easy ride should feel like a 3 and it feels like a 5. Then you are carrying something from the rest of your life. Keep it at a 3, whatever speed that is.

On your next easy ride, keep your mouth closed. Breathe in and out through your nose the whole way. That is your 3, on any road and on any bike.

On race day you ride and then you run. On race day, ask the question the other way round. You know how long you must keep going. On the bike, that is the rest of the ride and the run after it. Ride at an effort you could hold that long. If it starts to feel like a 7, ease off. You could hold a 7 for only about twenty minutes. On a climb, give a little more and let the speed drop.

A power meter answers in a second. Heart rate takes a minute or more to catch up when you go harder. On a short climb or in a sprint, that instant number is useful. If your indoor trainer already shows watts, you have this too.

Use those watts as a record of what you produced. The effort is what you feel, checked against your heart rate. A power meter can make your riding more precise. It does not make you fitter by itself.

Which score out of ten to aim for at each effort is in How hard does it feel, out of ten?