Can a power meter on an exercise bike give an accurate comparison to power on the road?


By: Gym Jones

I had the luck of having a nice high-tech exercise bike land in my basement, and I have been hitting it in the evenings. It seems, from what I've read, that power is the best measurement of work done on a ride. The bike has a power meter (and I think it is a decent one in the sense that it is truly using RPMs x Resistance vs. the ones I have seen in a gym that simply use the "level" you are riding on regardless of RPMs).

So basic question #1:

Can a power meter on an exercise bike give an apples to apples comparison to what would be measured on the road? (I am assuming that the resistance provided by the bike equates to the sum of wind, weight, road conditions this a decent assumption?)

Basic cycling question #2:

What is the seasoned cyclist's view on standing up in the seat on climbs? I'm ignorant here. For training/conditioning purposes, is it best to stay seated? Or do you care? Is the avg power generated at the end of the ride less reflective of conditioning if you are standing on the climbs? So far, I have been climbing out of the seat in order to maximize power and speed up the hills.

Lastly, basic question #3:

What would a cyclist (not a pro, but say a competitive Cat 4 racer) target for avg power over a 1 hour, 2 hour, 3 hour ride?

Any insight you can give would be greatly appreciated. Thanks for what you have built and cared for in Gym Jones.

All the best.


Oranges to Nectarines is probably more accurate. Power is a beautiful metric because it is objective: the same regardless of wind, incline, chip seal, heat, etc. On the actual bike power is measured at the rear hub or at the crank to provide a true measure of the force being generated by the rider. So despite the missing balance, proprioception, and other physical characteristics of an actual ride the power output comparison between real bike and indoor bike will be similar enough to learn some things about fitness. I have a power meter mounted on my real bike and whether it is locked down in a trainer, on rollers, or on the road I treat all data as if from the same source.

However, there is a large variation between the data supplied by the power meter on my spin bike and that from the device on my real bike so I treat those records separately. Speed calculated on the spin bike is irrelevant because it has no relationship to power output, only to the speed of the flywheel, thus the mileage is not comparable either. I can track and analyze training sessions on the spin bike in relation to other workouts done on that machine but the only correlation I draw between that and data derived from my real bike is heart rate. This is reasonably useful since many other factors are controlled while riding indoors but I can't use the numbers from the spin bike to accurately predict outdoor performance.

The seated vs. standing question is largely related to individual physiology. When standing the demand emphasizes muscle power and local muscular endurance because the cadence is necessarily slower (few could hit 100 RPMs standing) so muscle contraction times are longer and peak force per pedal stroke is higher (greater torque). Seated, one can produce the same power via a different mechanism: higher RPMs, shorter contraction times, more "rest" per minute of work, lower peak force development per pedal stroke, but also a much greater demand on the cardio-vascular system. Usually we say higher RPMs stress the heart/lungs, lower RPMs stress the muscles. Different riders will ride the same hill in their own way. The typical comparison is Armstrong with a very high cadence and Ulrich with a low cadence both riding similar speeds up the same climb. Each had their own, individual type of fitness.

As for the power output for a Cat 4 rider at different time intervals, it is most often measured as watts-per-kilogram of body weight. The common time durations used are 5 seconds, 30 seconds, one minute, five minutes and "functional threshold", closely linked to anaerobic threshold or a level of output one could maintain for 30 minutes to a couple of hours. Obviously there will be some changes with the longer durations as well, where fueling, hydration, and wise expenditure come into play. Last season I developed fine four-hour fitness, which I discovered during a stage race that took 6hrs 40min on day one and 7hrs 5min on day two. Both days showed a 40-50 watt decline in power right around four hours since that was within range of the standard training rides I could consistently do, which lasted 2-4 hours.

Much of the following is pulled from Andrew Coggan's book about using power to train on a bike:

For a Cat 4 Rider:
5 seconds: 14.5 - 15.5 watts/kg
1 minute: 7.5 - 7.9 watts/kg
5 minutes: 4 - 4.3 watts/kg
FT: 3.3 - 3.6 watts/kg

A good all-around rider would score at the lower end of the spectrum and be fairly even across the time durations. A sprinter would have higher numbers for the shorter durations and fall off towards the FT. A good time trial rider wouldn't have great sprint capacity but would show high 5 minute and FT numbers.

You can do a step or ramp style test on the machine to develop these numbers. Warm up 20-30 minutes
Go all out for 5 min (don't let the pace drop toward the end)
Active recovery 10-15 min
Go all out for 1 min
Active recovery 5 min
Go all out for 1 min
Active recovery 5 min
15 second sprint
Active recovery 2-3 min
15 second sprint
Spin 15-20 minutes to cool down

That takes care of the low end numbers.

Next, on another day, do a different test to get some longer range numbers. Warm up 20-30 min
Go all out for 5 min (don't let the pace drop toward the end)
Active recovery 10-15 min, i.e. at an "endurance pace" not just spinning aimlessly
20-min time trial (don't go out too fast, the point is to generate the maximum average power output over the time interval)
Active recovery 10-15 min, i.e. at an "endurance pace" not just spinning aimlessly
Spin 15-20 minutes to cool down

To be useful the machine must provide an average power number for the 20 minute TT. Take it, subtract 5% and this may be used as your FT number for future training sessions on the bike.

I hope this helps. If you have more questions fire away.

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