DRLski

How accurate is the calorie count on the Power-Tap Pro? I have a Garmin Edge 305 as well but I know that is WAY off. Is the Power-Tap anymore accurate?

DrPete 

The Powertap actually measures kilojoules (kJ) of work, which is actually about 22% of a kilocalorie. Because of the inherent inefficiencies of the human body, though, the kJ reading correlates pretty well with actual calories burned. That's a Friel/Coggan thing, not just me pulling it out of the air.

Snuffleupagus

Yes, generally.

It's a closer than the Garmin or Polar, as it measures work and then extrapolates kJ to kCal. It should be within 20%.

DrPete 

Polar is apparently a little more accurate than Garmin, at least in the higher-end models that use the OwnIndex to estimate VO2max. Unfortunately, as VO2max goes up, the calories burned at a given HR go down.

DRLski

Dr. Pete, sorry, I didn't clarify. I'm actually using Training Peaks to calculate the kCalories from my Power-Tap

DrPete 

For the power files I've uploaded to trainingpeaks, the calories and kJ are the same.

DRLski

k, I'm confused then...so, if I'm trying to figure out how many calories I burned so I can replenish myself after a workout would I use the kJ/kCal that the Power-Tap/TrainingPeaks gives me or do I have to calculate something else?

Snuffleupagus

Everything related to power training and nutrition would indicate that you'd want to replace your BMR calories, and any calories burnt through cycling. If you're doing any other strenuous activities like walking around at work, add those in too.

Adjust calories to the left or right if you're gaining or losing weight when you don't want to be.

DrPete 

I generally use the kJ measurement. Is that precise? Hell, I don't know... my Polar bases the kcal burned on heart rate, which probably overestimates because of coasting, etc., but intuitively I'd think that the powertap sort of underestimates because your body is still well above its usual resting state if you're coasting during a hard ride. The Garmin seems to be the highest and least accurate of the bunch from what I've read.

DRLski

k, thanks for the info DrPete and Snuffleupagus, I appreciate the help, I'm trying to keep the weight off this winter instead of gaining it so just trying to get a good idea of what I'm burning while on the bike.

Snuffleupagus

The PT is a great tool for it in my experience. I used to replace the calories that Fitday.com said I burnt on a 4 hour 20+ mph ride...yet I'd still put on a little weight.

Then I got a PT and saw the light I can't eat 7,000 calories, even after a hard 85 mile day

zimbo

Again, the PowerTap measures total work in kilojoules (i.e. power over time) with a high degree of accuracy. One kJ = 4.184 kCalories. Most people are about 21-25% energy efficient on the bike. Thus, it's pretty darn accurate to just use kJ in place of kC when determining how many calories you burn on the bike.

BTW, regardless of how much you weigh, you must average exactly 278 watts to burn 1000 calories in an hour.

--Steve

scirocco

Other way round, I think, and small calories, not Calories. One Calorie = 1 kcalorie = 4.184 kJ. (One kCalorie = 4184 kJ.)

Not "exactly" 278 watts. You've just told us that calories burnt depends on the efficiency of the rider. To burn 1000 Calories (kcal) in an hour a 21% efficient rider would only have to average 244W. The 25% rider would have to average 291W. Certainly it's nothing to do with weight.

Endox

Makes me wonder, is the body's efficiency genetic or can it be trained to an extent? I always thought it was pretty much spot on 25% for everyone but if it can vary by 5% it would surely make an appreciable difference to effective power output and overall performance.

cslone

Kjoule to Kcal is 1:4, and the body is about 25% efficient. So, roughly the same.

DrPete 

(educated guessing follows)

I think it can be trained to an extent. After all, the goal of training is to increase power at LT or VO2max, so in a way that's making a more efficient machine. Otherwise, though, the body is still doing all of its usual metabolic functions, and the pathways involved in turning food/muscle/fat to energy are all less than 100% efficient. So I'm guessing there's not much that can be done about the inherent inefficiency.

krazyderek

The only sure fire way to measure calories burned is thru dietary measurment, and weight loss over the course of a couple of weeks or months, and even one oversight there can screw you up. I use the 1:4 25% rule to say powertap is pretty much in the right ball park. Really keeping track of your calorie in/out to the nearest 25, or 50kcal is more then accurate enough for weight management.

merlinextraligh

It's as accurate as you're practically going to get. certainly a lot more accurate than HRM's, gym equipment calculations, on line calculators, etc.


In several threads on here people have gone to great lenghts quibling with the efficiency at which a given rider converts dietary calories to energy at the hub. Mostly, I think because they didn't like how few calories a power meter reveals you are actually burning (compared to the assumed calorie burn).