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This is a tough question to categorize because it involves fitness, physics, math, physiology, and probably more disciplines. But, it's being asked in the context of Fitness so I believe this is the best place to ask.

I began viewing watts back when I did cardio exercise on indoor cardio machines. For example, I used the Cybex Arc Trainer and would average around 250 watts, with short (30 second) spikes of up to 450 watts.

I also used the Schwinn Airdyne Pro trainer before, which is essentially a bike and 2 handles attached to a large fan which exponentially increases the resistance the faster a user pedals and pushes the handles. It is often used for High Intensity Interval Training (HIIT) for this reason. On this bike, I would average around 200 watts of output, with 20-30 second spikes up to the 700-900 watt range with what I would describe as "absolute, all out exhaustion." I would have to wait 2-4 minutes after these efforts before I could try for another major effort.

After these max efforts, my watts would dip to like 115 during the recovery period between intervals.

Fast forward a while, now I am a bicylist (road and gravel) and I have some questions that have arisen from my learning about power output applied to the bicycle.

Sadly, I do not have a power meter yet, but the Strava app based on speed, grade of the terrain, heart rate (I do have a HR monitor), and a few other metrics, will estimate my power output on an all-out sprint to be 450-480 watts. My perception of effort during these sprints is equal to the high intensity intervals that I used to do on the Airdyne bike; I can only sustain them for 20-30 seconds and I am near absolute exhaustion after. I absolutely cannot pedal any harder and many times the bike's gear is in the highest position (I have a "2 by" 34-50 crankset with 10 sprockets in the rear cassette, 11-34).

So I have a few questions related to this:

  1. In the real world, do watts on a bicycle outdoors accurately measure "level of effort?" For example, will my watt output be lower when I am up against wind resistance, rough road surface, etc...? If so, how would I measure the actual effort that my legs/body are having to output?

  2. Why is my max effort on a bicycle seemingly generating so much less power? Is it by nature of the bike that is such that it doesn't allow a human to as easily generate higher watts compared with a machine such as the Airdyne (perhaps because there is no upper body levers involved?)?

  3. Is it possible to be exerting less power on a given machine, but expending more calories than on another machine which I am exerting more power?

  4. I noticed that some "calorie calculators" such as the one on WebMD, use average speed as an estimation of calories burned during a given exercise. However, I noticed that it's entirely possible for me to ride my bicycle at a slow speed, but do so up a very steep hill which shoots my heart rate up and seems to be extremely taxing on my body. Is it generally possible on a bicycle to achieve high power output and calorie expenditure at slow speeds (say 6-7mph) if up against heavy wind resistance, hills, etc...?

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  • IIRC, watts correlate to calories almost directly as a 4x equivalent due to the drivetrain losses in power measurement. So 200 watts for an hour would be i800 calories. PM has an error rate of > 5%, while HR is error rate in the 10-15% range.
    – JohnP
    Oct 24, 2023 at 21:47

1 Answer 1

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I'm not fitness expert but I can answer the question from point of view of physics.

Watt is unit of power which says how much energy is being put to some system per unit of time. (Watt is equal to one Joule of energy per second.) In case of exercise machines and bicycles, you measure the power that goes into the machine. Power can be calculated as speed times force. (In case of exercise machines, you do circular motion, the machine doesn't measure speed directly but the RPM. However, the principle is the same, manufacturers just have to measure how power relates to RPM, which depends on how much air resistance is provided by the fan or what settings you use on brakes. The bike power meters measure RMP and also bending of the crank which is related to force you exert on pedal.) So your power output is quite good proxy for your "effort", even though it's not perfect as I'll discuss later.

Notice I have mentioned you measure "power that goes into machine". After holding 200W for a minute you might be tempted to calculate energy you burned according to formula I have provided above:

200W * 60s = 12000J = 2.87Cal

In practice you have burned much more energy, most of the energy is wasted as heat. Efficiency of human muscles during exercise like cycling is somewhere between 18-26%, so you actually spent somewhere around 11-16Cal. Somewhere inside that range should be the number shown on display of the exercise machine.

There are many factors influencing how much power you can put into machine or bike. You have mentioned that one of the exercise machines exercises the whole body. Obviously, when you employ more muscles, you are capable of providing more power, each muscle does part of the work. Although how much power you can output is not only limited by muscle fatigue and pain, but also by your level of cardio. You have only one pair of lungs that can only breathe so fast. And all the muscles have fight for available oxygen and sugar available in your blood. Plus you are generating lots of heat, which you have to dissipate to avoid cooking yourself from inside.

I would say that on normal bicycle you are mostly limited by how hard and fast you can push with your legs, while on the whole-body exercise machine you can output additional power using your arms and torso while being closer to limits posed by "cardio and physics stuff" mentioned in previous paragraph. In the second case you output significantly more power (at least for short time until biology and physics catches with you and you start to overheat and gasp for breath).

Let's ignore hands for rest of the answer and just focus on normal bicycles.

There might some external factors determining how much power you can output. Even small details like how comfortable is your position on seat can have huge impact on how efficient your muscles are and how much power you can produce, so differences between machines and real bike are to be expected.

Wind speed or incline shouldn't have any effect on how much power you can output, provided that you can always use gears to set comfortable cadence, and you ride in same position. You'll just go slower when going uphill or against the wind. Obviously, none of those assumptions are true though. As you said, sometimes you are limited by gear choice. Also, when you are doing hill-climb, you are sitting at different angle to your bike or you are often standing which changes your efficiency.

I'm not expert on mechanics on human body, so someone else will have to explain what's the difference in efficiency between standing and sitting on bike. But it's quite easy to show that there must be some optimal RPM only using simple physics.

Imagine you are in gear that's extremely high. It's basically brick wall. You are pushing with all your force into the pedal, let's say you are basically doing one-leg squat on it and push with force of 1600N. The gear is so high you basically can't move against any hill or air resistance, so speed (of the pedal going in circles, as that's where we measure the force) is zero. So you are providing 1600N * 0m/s = 0W to the bike even though your muscles are shaking, you are in pain and generating lots of heat and sweat.

Let's look at the other side of the spectrum. The gear is too low, it requires almost zero force to be pushed. Even if you go at your full speed, you are barely moving forward. (Near) zero force means you are outputting (near) zero watts to the bicycle, even though you are sweating. You are basically spending your energy spinning just your legs while doing nothing useful. Your efficiency is at 0% - all that energy is eventually wasted as heat.

So in both extremes, you are outputting 0W of power while suffering in different ways. This means there must be some sweet spot for your bike and your position on the bike that balances the RPM and force.

To answer question #3, some exercises are more efficient than others, it depends on how fast you are moving, which muscles you are using, under what angles are your joints etc. But in general, more power you measure using machine, the more calories you burn, as the efficiency doesn't vary that much between similar exercises.

To address the various apps, simplest way to calculate calorie consumption is to assume some activity level and multiply that by time. If the app asks you only for speed, then it either assumes you are riding on flat surface, or uses some kind of average value of how hilly the terrain is. If some app uses GPS, it can take slope and into account, plus it approximates aerodynamic drag based on your speed. I've seen apps that can download local weather to include the wind-speed in aerodynamic calculations (which turns out to be quite significant). Of course nothing beats power meter if you want accurate measurement, all those apps just do better or worse guesses.

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