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Most accurate calorie counter?

Old 11-26-08, 06:02 PM
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CrimsonKarter21
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Most accurate calorie counter?

What's the most accurate monitor for counting calories lost? I've got the Kj thing on my PTap, but how accurate is it, especially when you're burning more energy because of temperature changes and such.

Does anyone have an idea?
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Old 11-26-08, 06:10 PM
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You are still going to have a problem with accuracy relating to the efficiency of the human body--only about 20-25% of the energy used gets transfered to mechanical energy.
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Old 11-26-08, 06:27 PM
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I'd be using it to just count how many calories are burned overall so I can setup an accurate diet. Also because I can't attach a PowerTap to every bike I ride on a regular basis. I'd also like to see how a 5-6 hour MTB ride can compare to a 2 hour criterium can compare to a madison race con the track can compare to a fun ride on the bike path can compare to a nasty, sandy, gritty rain ride on the rain bike.

Dig?
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Old 11-26-08, 06:35 PM
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without medical equipment that plugs into a wall, the PT is about as good as it'll get for estimating calories burned.

Apus, there are 4.18J in a Cal. But aerobic respiration is about 25% efficient. So divide by 4 and multiply by 4, and you have a pretty straight 1:1 conversion between KJ put on the ground and Calories burned.

More than temperature variation, going anaerobic for any period of time is much less efficient than 25% (not sure on the numbers but I think it's going to be closer to 4%), so a 100KJ sprint will burn a lot more calories than a 100KJ gradual hill.
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Old 11-26-08, 06:42 PM
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I see, but does 100KJ burned account for what's being lost through the body, or what's being put into the pedals?
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Old 11-26-08, 06:44 PM
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The KJ displayed on the PT is a measure of the energy being put through the pedals.

Joules and Calories are both different units of the same thing; energy.
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Old 11-26-08, 06:47 PM
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Okay, I gotcha. Thanks.
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Old 11-26-08, 07:17 PM
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You're only ~25% efficient, and given the unit differences between kJ and kcal, I usually multiply kJ by 1.1, and that seems quite accurate to me.
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Old 11-26-08, 07:48 PM
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So the 1.1 accounts for the ~75% of the energy going elsewhere?
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Old 11-26-08, 08:14 PM
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Math time.....

You climb a hill and your PT says you did 100kJ of work. that's 100000 joules.

Science says that there are 4180 Joules in a Calorie (note the capital C).

So your 100 KiloJoules is equal to 23.92 Calories of energy delivered to the road.

But your body is only 25-ish percent efficient. 25% = 1/4.

So in order to throw down 24 Calories, your body actually had to burn 4 times as much energy to make that happen.

24 * 4 = 96. Your body had to burn 96 Calories to do 24 Calories of work.

Lets recap, 100kJ ends up burning 96 Calories. Lets call that 100 and we're good.

100KJ on the PT = 100Cal burned. Yay physics.

Note that the 25% is an upper limit on the body's efficiency, so if you want to multiply by a larger factor (1.1) to make up for being 23.5% efficient, that's cool too. It's just more difficult than the just about accurate 1:1 ratio.

B.

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Old 11-26-08, 08:22 PM
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25% is on the high end of effeciency for cyclists. The numbers I have seen in articles is 18-23% with conjecture that the very best pros may get a point or two more. But assuming 25% makes the kJ -> Calories conversion simple and underestimates the Calories burned so you're more likely to lose weight using that number.
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Old 11-26-08, 08:43 PM
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Originally Posted by CrimsonKarter21 View Post
So the 1.1 accounts for the ~75% of the energy going elsewhere?
Heat and cooling that heat. If I'm pushing 300W, I know I'm generating ~1200W of energy, and most of it is a 900W light bulb in my core.
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Old 11-26-08, 08:53 PM
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So to the OP, I would suggest you get a good idea of how many calories you burn in an as compared to your percieved exertion or heart rate. Use your PT on your road bike to get the numbers. Then when you are on your mtn bike (or track bike), go by PE or HR to get your calories per hour. It isn't going to be really accurate, but probably the best you are going to do. And it should be good enough - most measures of calories in food aren't that accurate anyway.
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Old 11-26-08, 09:24 PM
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Originally Posted by CrimsonKarter21 View Post
I see, but does 100KJ burned account for what's being lost through the body, or what's being put into the pedals?
both. in kJ the number that your ptap gives you is the energy you have physically put into the pedals. if you put that same number into Calories it represents a rough estimate of the total energy your body has burned.

the factor of 1.1 that waterboy gave you accounts for the fact that most people are actually a little less than 25% efficient. the factor of 4 is what accounts for the ~75% of your energy that goes elsewhere.

can ya dig that?
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Old 11-26-08, 10:25 PM
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Doing calories in/out dieting can actually get quite complex, though pretty enlightening about the stuff you put into your body.

I did that two years ago when I was staring at a bunch of climbing races and needed to get really lean (BTW, if you're not a climber or within reach of the w/kg for climbing, with a bunch of climbing race staring you in the face, there are other things you can focus on that will be more productive).

Steps you need:

1) Look into metabolic rate testing. This will give you a 24 hour number from which all other numbers flow. There are cookie cutter formulas out there but I ended up nearly 20% below my age/size number. Like Eddie Merckx, I get fat quickly.

2) Food scale

3) Database

4) Spreadsheet for tracking.

The PT/SRM/WKO numbers are predicated on a set efficiency number. Your mileage may vary.
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