ronfinch

Hi, spent an hour plus searching threads but can't find the answer to these specific questions, even though I am sure they must be here somewhere, so started thread...

1. How accurate can the calorie counter on the stationary exercise bike at the gym be?
2. Assuming the calorie counter is accurate, is it right to say that if I burned 360 calories over an hour I output 100 Watts / if I burned 720 calories I output power at 200 Watts over that hour / if I burned 1080 calories I output 300 Watts?

Thanks.

JerrySTL

1. Not very accurate. There's too many variables for such a simple machine to calculate accurately. Heck I find that their mileage readings are high when compared to real world efforts. Still it's better than nothing. Use it as a guide, similar to METS, and not the gospel truth.

2. No. The watts and calories relationship is not linear.

ronfinch

Cheers! Have seen a formula and various discussions regarding watts and calories so will go back to them. I have seen various bits and pieces on the one hour record recently, including the sort of watts they ouput, and am interested in how I compare to them (nowhere near is the answer of course, but I would like to see the figures anyway).

Carbonfiberboy 

Yes they are! 100 watts for an hour is 360 kj, 200 watts/hr is 720 kj, etc. kj converts 1:1 close enough to kilocalories for practical purposes, the body being somewhere around 25% efficient. Efficiency will vary maybe a couple percent between individuals but most folks disregard that, since it's going to stay about the same for each person. The OP is correct.

jsk

Agreed. Having said that, you have to take the power numbers displayed by an exercise bike at the gym with a grain of salt. They may not be based on a true power meter with strain guages, but rather just a "calibration curve" that estimates power.

Carbonfiberboy 

Oh, totally!

ronfinch

Oh right... Well 2 to 1 says it is a linear relationship so will go with that until I hear better or have confirmed it by looking at the formulas I saw before. Realise there is loads of stuff online regarding calorie counters, but got a manual of the machine at my gym, so will look at that and 'calibration curves' to see if can guess how far out it might be.

Edit - common sense initially to be basically impossible to confidently calculate calories burned for a layman. Oh well. I assume the people who confidently talk about watts have equipment that allows the user to input much more data, in order to get a much more accurate picture of calories etc...

Carbonfiberboy 

We have StepMills at our gym. They display watts among other information. Since they ask one to enter one's weight and don't take HR into the calculation, I think they are probably fairly accurate since, given weight and speed of ascent, watts are simple to calculate. Whether one could use one's HR at a particular stair climbing HR and apply that to guess at cycling watts is an open question.

In any case, watts figures vs. HR figures given by gym equipment are far more valuable as a measure of one's training success than they are for estimating calories burned. I use the scale for figuring my calorie balance. No time at all spent worrying about calories in or out. Just train and eat smart and track your progress.

gregf83 

They can be very accurate at measuring power but most aren't.

Not a good assumption. Measuring calories accurately requires a breathing apparatus, measuring power is much simpler. Once you have power you can get a reasonable estimate of calorie consumption.

ronfinch

I absolutely agree with this. My interest in watts was just piqued by reading about the watts output by people chasing the hour cycling record, other than that I am happy with the results my fairly low science approach to exercise gets me.


The exercise bikes at my gym aren't particularly good, and displasy what it claims are calories burned, but no watts. I will look online to see if I can find if there is an easy way to calculate watts on an exercise bike with fairly limited stats.

ronfinch

Hopefully last question on this - it seems like the bike can be set up to display watts (and mets). I am guessing that on an exercise bike it is realively simple to calculate watts based on resistance setting of flywheel v rpm. Is this right? If so this is all I need really..!

Thanks for replies btw, it is a confusing subject at first...

So if watts is calculated by speed at which I turn the cranks * force applied, am I right to assume that the force applied is equal to whichever gear the bicycle is in? I can see that I might be outputting more watts than this (say by heat), but am guessing this isn't taken into consideration. If the above is the case perhaps the exercise bike can give a fairly accurate reading of watts? (Assuming the actual readout is accurate, not high in order to please a gym-goer (as might be the case with the calorie counter...))

chaadster

I don't understand the question, but I'm pretty sure the answer is no.

What are you trying to do? Determine if the bike's power readout is accurate? If it's a typical gym bike, you're very safe assuming it is not accurate, but it would not be hard to look up the model specs and determine if it has a power meter. No power meter, no accuracy.

It's going to be impossible for you to compare your output to those riders you've been reading without access to a proper power meter.

Carbonfiberboy 

Well, if you were to hold a constant output, in terms of resistance setting and cadence, for some exact period of time and that resistance didn't change with wear and heat, you'd get a calorie count for that time period. You could calculate back to kj and thus to watts. However, since the calorie figure is probably wildly inaccurate, so would be the watts figure.

That said, inputting Garmin data for a road bike ride into Strava can allow you to get calculated power for different segments, and if a particular segment features a completely steady climb for a good ways, and you have accurately entered your and your bike's weight, Strava gives a reasonable estimate of your power for that segment. No, it's not "accurate" but it's reasonable. That's a little cheaper than a power meter, but it's not real-time so doesn't do you much good for training.

ronfinch

Ha, well yeah.
Thanks for the replies, I think the fact that I was generally just trying to get a handle on how watts are calculated meant my thoughts are badly formed...
If one Watt is the amount of work needed to move 1kg 1metre, I haven't found out why it would be so difficult to measure Watts being produced on a stationary exercise bike with a digital display etc. I can see different people would be more or less efficient, and any belt drive might be stretched or whatever, but there is no (or minimal) air resistance (maybe as the cranks spin!?), the 'terrain' is flat (or otherwise digitally monitorable (if using a programme simulating hills or whatever)), and although a belt may stretch I would hope the maintenance schedule for the unit meant that this was negligible.

I contacted the unit's manufacturer regarding how they calculate watts and the reply I got back tells me it is simply not reliable.
Obviously my initial interest was just in trying to compare myself to some of the hour record power output numbers, but that interest evolved a bit. I have given up on being able to measure myself against these others for the time being with the equipment available to me. If anyone posts regarding why stationary exercise bikes can't accurately measure watts I will read it with interest, otherwise no problem!
Thanks again for replies.

chaadster

Stationary exercise bikes *can* measure power accurately, and some *do*. See the Cycleops 400 Pro, for example.

So perhaps your question is, rather, why some cannot accurately measure watts? I don't get why it matters, and the main answer was given early upthread: they have no tool to measure force, which as @jsk noted, is commonly a set of strain guages for those that do. Wattbike use what they call a 'load cell', which I think is a collection of strain guages, and which they uniquely mount on the chain. Others mount on cranks and hubs.

There are, of course, other ways to measure watts, but I can only suppose they're difficult to imlement on a stationary bike, otherwise they'd exist.

Again, ai'm still not sure what your point is in all this, but if you're looking for solutions to the problem of getting power from a stationary bike withoit a power meter, an alternative for you may be to purchase a set of power meter pedals, such as Garmin Vectors, and mount those on your existing stationary bike. Those measure, obviously, force at the pedal, and you'd need an ANt+ enabled receiving unit to see the output, e.g. the Garmin Forerunner XT.

musiciansnicker

1. How accurate can the calorie counter on the stationary exercise bike at the gym be?

It depends on the digital meters, whether it supports user setting or not, if you can set up user settings (name, weight, height, age and gender), it will calculate the calorie burned based on the info you put in. The number of calories you burn with a stationary bike workout will depend on your weight, the length of the workout and the intensity of your effort during the workout.