A complicated fitness question
September 1, 2005
When I turned in my column Thursday, they handed me a letter that contained a fitness question sent from a gentleman in Dayton. It contained a question regarding fitness abilities and a challenge.
“How many calories are expended by a 160-pound man carrying a 40-pound backpack while hiking up a 1,000-foot mountain if his metabolism is 50 percent efficient?” Now, my background is medical, musical and physical, so I enlisted the help of my engineering husband for the technical data and several medical books and came up with some interesting facts.
First of all, your basic metabolic rate (or BMR) can be thought of as your resting or idling speed. It means how many calories your body needs to maintain itself if you are doing nothing. There are several factors that affect the BMR: your sex, weight, how much of your body is fat, your age, how much you sleep and the temperature. So we have only the sex and the weight to factor in. I am going to assume that it is an average summer day, the age is 30, there was an adequate amount of sleep time, and the body fat is average.
Next is the time element, which I assumed to be one-hour rise to 1,000 feet elevation. If he was climbing C Hill from King Street, that would be about a 1,000-foot rise. A 30-year-old man at 160 pounds has a resting BMR of 1,882. That is the number of calories needed per day. Divided by 24 hours, it equals 76 calories per hour at rest.
The way that you calculate this daily rate is to multiply your weight times 12, giving you your rate at age 20. Subtract 2 percent per each 10 years after age 20. Complicated, isn’t it?
Now we’re adding a 40-pound pack to the 160-pound man to equal 200 pounds. So the man is going to carry 200 pounds up 1,000 feet in one hour. To convert that to calories (KG-C), multiply 200 pounds times 1,000 feet equaling 200,000 foot-pounds. Then multiply that number by .000324, which is the conversion of foot-pounds to calories. And the answer to that, when rounded off, is 65 calories.
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In order to get the 200-pound man up the hill with only 50 percent efficiency, it will take twice as many calories, or 130. It doesn’t matter if it takes him 30 minutes or 30 hours, he will consume 130 calories carrying that weight up 1,000 feet. So the magic answer to the question is 130 calories plus the BMR (which is 76 calories per hour), or 206 calories. If he takes two hours to climb the 1,000 feet, his BMR will double to 152 calories, making his total consumption 152 plus 130, or 282 calories.
Didn’t think I could do this, did you? I couldn’t have without my husband’s math skills and the foot-pounds-to-calories conversion tables. It’s been a fun challenge and an educational one, too.
n Jerry Vance is owner of The Sweat Shop/Wet Sweat. She offers classes through Carson City Recreation and Aquatics Center and is a fitness instructor for the Senior Center.