andysummers

Say an out of shape fat dude gets on a bike (with a power meter) and rides for 30 min with an average power output of lets say 150 watts and an average heart rate of 160bpm. The same dude trains for a year, gets thin, gets on the bike again for 30 min/150watts average with an average heart rate of 140bpm.

Would the calories burned be approximately the same, since equal work was done, or would they be significantly different? How much different and why?

aham23

they would be less because it is now taking less effort to produce the same power. as for how much less, i dont know. later.

WickedShark

+1 you would burn less calories due to your effort being less to work at the same intensity.

TurboTurtle

I would say insufficient data.

A couple of things that I would think would be likely, though, would be:

- He' going much faster since he's putting out the same wattage and weighs significantly less.

- He has increased his cardio/pulmonary capacity since he is doing the same wattage at a lower heart rate (though with the variability of HR, it could be for dozens of other reasons).

TF

Pat

Well, if he weighs less, he would burn slightly fewer calories. On flat terrain, the difference in caloric burning though based on weight would be very little. Most of the work is done against wind resistance. If there was significant elevation change, the caloric burn would be more directly proportional to weight.

The shape the person is in has no real effect on calorie burn. He is doing pretty near the same work both times. Getting into better shape allows him to do the same amount of work with less effort.

asgelle

Since the same amount of work was done in both cases, (75 W-hrs to mix units) the only way to affect how many Calories were burned is through a change in efficiency. There is some controversy about how much an individual's efficiency can change over time (cf. the arguments over Coyle's article on Armstrong's data) I believe it is unlikely there will be an appreciable change over the course of one year. So the Calories consumed will be the same. Speed, perceived effort, and heart rate are another matter.

bwunger

+1 Work = Power * Time Same in both cases. As asgelle said, said person would have had to become more effecient at turning calories into power to change how much he burned.

tntyz

He's moving less weight so he's doing less work so he's burning fewer calories. Equal work is not being done.

Don't know how this translates into HR and how the human body actually works, though, which I suspect is what the OP is looking for. HRM's use HR as a factor in calculating calorie burn. My HR for the same amount of exercise (whatever that means) is lower now that I am in better shape, hence the calculated calories go down, too. Assuming you can trust the calorie calculation on a typical HRM, that is!

asgelle

How does 150 W for 30 minutes translate into different amounts of work depending on weight? Look back at the original hypothetical.

gregf83 

In addition to the 150W of useful work, the 'fat dude' is having to do a bunch of non-useful work moving his heavier legs around the cranks 90 times/min. Assuming the OP loses weight off his legs he will be more efficient in converting calories into useful work. So where a fit rider might be 25% efficient a non-fit rider may be 20% efficient.

Pat

Well, it matters some on how much of a weight difference there is. I mean if the fat dude is 350 lbs and he gets down to 170. Sure there would be a difference in calories burned on two rides of the same distance at the same speed. But if the weight loss is more normal, the difference will be pretty slight.

What the poster is asking is how can there not be a major difference in calories burned because when he was out of shape, he was working like all get out to do the distance at that speed. When he got into shape, he could do the same distance and speed with only moderate effort. The thing is that his body has changed. His muscles are toned up and his heart delivers blood to the muscles efficiently. He is probably burning about 90% of the calories as he did before but at a much lower intensity of effort.

It is amazing what getting into shape can do.