Cycling: Power Meters and Training by Watts
Introduction
Cycling has an advantage no other endurance sport shares: you can directly measure the work you're doing, in watts, in real time. Not a consequence of effort like heart rate, nor a perception like feel: the power itself.
That changes how you train quite a lot. It also costs money, so it's worth knowing what it delivers before buying.
Heart rate tells you how your body responded. Watts tell you what you did. They aren't the same thing.
Why power beats heart rate
Heart rate is a response, and like any response it arrives late and gets contaminated.
It lags. You start an interval and heart rate takes 30 to 60 seconds to reflect it. In a two-minute set, half is over by the time the number arrives.
Everything affects it. Heat, dehydration, caffeine, sleep, stress, accumulated fatigue. The same effort can differ by ten beats between two days.
It drifts in long sessions. Over time heart rate climbs even when power stays flat.
Power has none of those problems: 200 watts are 200 watts today, tomorrow, in heat or when tired. What changes is how much they cost you, and that gap between what you produce and what it costs is itself useful information.
FTP and its limits
The central concept in power-based cycling is FTP (functional threshold power): the average power you could hold for roughly an hour.
How to estimate it without suffering for an hour: after a good warm-up, do 20 minutes at maximum sustainable effort and multiply your average power by 0.95. Ramp tests, which many smart trainers include, also exist.
That number defines training zones as percentages of FTP, and sessions get planned from it.
The honest caveat: FTP is a practical construct, not an exact physiological threshold. It was designed to be easy to measure in the field, and it approximates the second threshold with variable error between individuals. Physiologists prefer the concept of critical power, which is better founded and rather more cumbersome to measure.
For training, FTP works. For discussing physiology, it's worth knowing it's an approximation.
Watts per kilo
This is the metric that matters when there's a gradient. Climbing depends on how much power you produce relative to what you weigh.
You calculate it by dividing FTP by body weight. It's also why cycling places so much emphasis on weight, and that deserves care: chasing watts per kilo by losing weight at any cost is one of the most common routes to disordered eating and low energy availability in this sport. Raising the numerator is almost always healthier than lowering the denominator.
On flat terrain, by contrast, absolute power and aerodynamics govern, not the ratio to weight.
Do you need a power meter?
Honest answer: no, if you don't compete.
A power meter costs a fair amount, and for someone riding three times a week for health, the combination of heart rate plus perceived exertion works perfectly well. Thresholds can be estimated with the talk test and a 30-minute test, just as in running.
It's justified when:
- You do short intervals, where heart rate lag makes it useless.
- You train a lot indoors and want structure.
- You're preparing long events and need to pace precisely.
- Measuring motivates you and that makes you train more.
And an intermediate option: many interactive trainers estimate power with reasonable accuracy, so if you train indoors you may already have the data without buying anything else.
Basic weekly structure
With three sessions and no competition:
- One long, comfortable ride. Conversation possible, 60 to 120 minutes.
- One threshold session. Blocks of 10 to 20 minutes near FTP, with short recoveries.
- One interval session. Sets of 3 to 5 minutes above FTP, with full recovery.
And as always: make the easy genuinely easy.
A safety note
Road cycling carries a risk the gym doesn't, and it's traffic. Helmet always, lights even in daylight, familiar routes, and hard interval sessions aren't done in traffic: attention drops exactly when it's most needed. For hard work, use a trainer or closed roads.
Common mistakes
Chasing FTP as a number. It's a tool for pacing, not an identity marker.
Retesting every three weeks. It's exhausting and FTP doesn't change that fast. Every eight to twelve weeks is enough.
Comparing your watts with someone else's. Different power meters have different calibrations, and the relevant comparison is with yourself.
Ignoring your position on the bike. A bad fit causes more knee and back trouble than any training error.
Always riding at moderate intensity. The same error as in running, with more hours.
Conclusion
Watts measure what you do, not how you responded, and that makes them better for pacing. FTP is a practical approximation and sufficient for training, even if it isn't an exact physiological threshold.
If you don't compete, don't buy anything yet: do the 30-minute test with heart rate and perceived effort, define your territories, and structure the week with one long ride, one threshold session and one interval session. If after some months you find that heart rate lag limits you in short intervals, then a power meter solves a problem you actually have.
References
- Allen, H., Coggan, A. R., & McGregor, S. (2019). Training and Racing with a Power Meter (3rd ed.). VeloPress.
- Jones, A. M., et al. (2019). The maximal metabolic steady state: Redefining the "gold standard". Physiological Reports, 7(10).
- Borszcz, F. K., et al. (2018). Functional threshold power in cyclists: Validity of the concept and physiological responses. International Journal of Sports Medicine, 39(10).
- Poole, D. C., et al. (2016). Critical power: An important fatigue threshold in exercise physiology. Medicine & Science in Sports & Exercise, 48(11).
- Mountjoy, M., et al. (2018). IOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update. British Journal of Sports Medicine, 52(11).