# Human Power (Cycling) & Energy Expenditure

1. Feb 4, 2016

### Sirsh

I have come across something that has me stumped which has something to do with the records of power output by cyclists over a one hour period.

The record of the most power (watts) generated by a cyclist is in the realm of 400-500 watts for an hour, so 400-500 Wh.

Now, we know that one watt (W) of energy is equal to one joule per second (J/s) and for one watt hour there would be the equivalent joules/hour which is 3,600 J/h.

If we are to convert the power output of the cyclist into joules/h then we would have (using 450 Wh) 1,620,000 joules/h or 1,620 kJ/h.

My confusion lies here, if the cyclist has expended/burnt 1,620 kJ in that hour of riding, this equates to approximately 368 calories/hour (using 4.4 kJ = 1 Calorie).

Now if you are to go on any website to figure out how many calories you have burnt over the course of riding for an hour, they generally will give you in excess of 3 times this amount.

Why is this? is there something wrong with what I have done, or is the power that the cyclist generated not depended on weight and other factors that would be incorporated in the calculation of energy expenditure?

2. Feb 4, 2016

### Staff: Mentor

The human body is far from 100% efficient.

3. Feb 4, 2016

### Sirsh

So the calculated calorie expenditure would incorporate things like respiration/body heat generation/increased blood flow etc.?

Whereas the 400-500W is purely the mechanical work that they can generate?

4. Feb 4, 2016

Correct.

5. Feb 7, 2016

### CWatters

And racing cyclists are probably a lot more efficient than the rest of us.