Power Meters on Mountain Bikes

Should you run a power meter on your mountain bike?

I'll start this with a bit of troll-bait and say that if your primary training app is Strava, then no: you shouldn't. Strava is a social platform that, on its own, isn't super useful for structured training, even when you pay for the premium features. Strava's main features that support structured training are segments, laps, power curve, and "fitness". Forget about relative effort or "suffer score"--it's a useful metric only in hindsight.

Many of us run power meters on road bikes, and pretty much everyone who uses Zwift for any serious training is using some form of power-measurement, whether it's on the bike or build into the trainer. Power meters allow us to gauge our performance improvements (or losses) over time, train to specific targets like having a powerful sprint or being able to crush a time trial.

The data they provide can also be extrapolated to determine other datapoints like VO2Max, which is very basically a measure of how much oxygen your body can process, and is seen as a kind of performance cap.

They're also fantastic for interval training, which is generally easier for most folks to do on the road because the smoother surfaces tend to make it easier to hold a given power. Need a 5-minute interval? Go find a decently tall hill. Easy-peasy. Easier-peasier? Get on Zwift and do an interval session. Hard as hell and not worth the effort? Hold 5-minutes of steady power on a trail. We'll come back to that.

Zwift dangled a very silly carrot for me late last year that made me do interval training (which I HATE HATE HATE), and around the same time I started exploring yet another extrapolated datapoint from power meters: work (or W').

W' is, similar to VO2Max, a kind of functional upper limit on your body's capability. While VO2Max can change a little over time with fatigue and health, it generally isn't directly affected by your training load. W', however, is a measurement of your anaerobic capacity, expressed in kilojoules (kJ). It's like a reading of how much battery you have for work above critical power, which is roughly your threshold power. An amateur may have a W' value of 10 kJ, where a pro may come in closer to 25 kJ.

You only use this capacity when you're operating above your critical power, which is a computed value (similar to FTP) based on a complicated formula that uses your peak 2-minute and 10(ish)-minute power numbers to determine the specific rate of power drop-off in your power curve. Unlike most other metrics, though, you can recharge your W' battery by operating below critical power.

So say, for instance, your critical power (CP) is 290W, and you have a W' of 20kJ (wow--go you!). You're riding a tough, but manageable road race hiding in the group pedaling away at 250W for an hour. It's time for the big sprint, and when you give it the beans, you deplete that W' at ~1kJ/sec. Math will tell you that you have about 20 seconds to do your worst to the group, but after that you're at complete exhaustion.

...except you aren't completely exhausted, because just dialing it back to that steady 250W and not exceeding 290W will recharge your battery. After a few minutes you may be able to tackle another 15s sprint, and if you can sit-in long enough, maybe a full 20. But every time you rotate to the front and take a pull at 350W, you're taking a few joules.

Having the ability to measure and observe W' lets you see exactly what you have in the tank. It tells you when it's time to stop attacking and start saving for the big sprint, or when it's time to push the attack because you're sitting on TONS of kJ and nobody's making any moves.

Also similar to VO2Max, your W' can be trained, but changes are much slower than, say, FTP.

W' is complicated. It isn't sexy, it's not well explained on the internet, and isn't surfaced directly--to my knowledge--in any cyclocomputer screen or performance metrics. It can be inferred, though, with maths, and those maths can be added to a Garmin device thru a Connect IQ datafield.

And that means: if you have a power meter, you can monitor your real-time anaerobic capacity.

So while it may be nigh impossible to get direct useful insights into average power on a mountain bike ride, you can use that data to determine how hard you're going and when it's time to back off, which is dark magic when your power outputs are wildly variable.

You can also use that to gauge a race effort, which I covered earlier this year in my MonsterCross race report. And it's exactly why I threw hard-earned money at a mountain bike power meter.

My choice for the Lux wasn't anything crazy expensive: I went for the cheapest spider-based power meter I could find--even one that has a known propensity for being off by a couple %. Because I'm not interested in average power for that bike or even really in knowing specific peak values: I'm only after the amount of time I'm spending over critical power, and roughly how far over CP, to determine W' bal.

So should you run a power meter on your mountain bike? If you're willing to do enough testing to determine your CP values AND you're using a head-unit that can display W' AND you're using a training platform that supports additional datafields AND you're tired of blowing up in the middle of mountain bike races, then yes. Yes you should.