Sunday, May 13, 2012

How Sports Tracking Software Estimates Calories

A lot of fitness issues like weight are ruled by hormones. Similar to messaging software, objects talking to each other, hormones relay messages to the body about breaking down fats (growth hormone; glucagon), for example, or storing fats and grabbing amino acids from the bloodstream (insulin), or telling the brain that "I'm not hungry anymore" (leptin). Leptin is an interesting biochemical because it is mostly synthesized by fat itself, and is designed to inhibit hunger by acting on the hypothalamus.

This anatomical reality of hormones affecting hunger and fat storage makes counting calories alone a simplistic strategy for managing weight, but it still has its place.

A longterm calorie deficit, if somehow you manage not to put them back, will result in some weight loss. Or you might have your own reasons for aggregating calories burned, such as goal-reaching or motivation ("hey, I burned 30,000 extra calories last month!"). At any rate, your sports tracking software probably estimates the calories you are burning and spits out a number all the time.

How do they do it?

As I've mentioned before, they start with your basal metabolic rate (BMR), the baseline amount of calories you'll burn in one hour just keeping your internal systems (like the heart beating) going: about 70 calories for a 154 pound male.

The BMR is higher for a larger person, and slightly higher for males. A rough estimate of the BMR for women is 0.9 times your weight in kilograms (e.g., the hourly calorie burn for a woman weighing 64 kilograms or about 140 pounds is about 58). For men, the hourly BMR rate is 1.0 times their weight in kilograms.

You've probably entered your height, weight, and gender in the sports tracking software's preferences, and this "data" goes into the software's algorithms for estimating calories expended.

But this is only part of the picture; a bigger factor might be how you configure your sports activity first (such as running or cycling).

The way Endomondo seems to do it is by the activity you choose in the app, before it starts tracking the workout. For example, if I ride for one hour in "mountain biking" mode, as I did yesterday, the app estimates almost 850 calories expended. However, if I change the mode to "cycling transport" and ride for an hour, on the same course, the app only chalks up 350 calories. Huge difference. They don't seem to take into account factors such as elevation gained and lost, meaning how much climbing you're doing.

So there is obviously some software in there making assumptions about the exertion level of each mode of exercise. They figure a mountain-bike ride involves steep climbing, bouncing off roots, more mental concentration/stress, etc. For the sake of accuracy, you have to try to choose your exercise mode carefully, to get an accurate calorie-expenditure count, if you care about that.

Your tool might be overestimating your calorie count by a factor of 100 percent (I actually think Endomondo underestimates the calories expended by weight training).

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