Last week I went backpacking in the mountains with my son’s Boy Scout troop and took the week off from writing this column. We covered 50 miles over five rainy days with each of us carrying approximately 30 lbs of gear each in our backpacks. I suppose to some readers this sounds like a rather unappealing way to spend a week. For me, it was a nearly perfect week away from the stresses of everyday life and one in which I had some time to consider the human body as a machine which consumes energy and generates power.
Before we proceed with our discussion, I need to review some of the applicable units of measure for this discussion. First let’s consider a familiar unit of energy, the calorie. A calorie is the amount of energy needed to heat one gram of water by one degree centigrade. The energy contained in the foods that we eat can be measured in calories. Unfortunately, whenever you hear some one discuss a food calorie what they really mean is a thousand calories (a kilocalorie). Why someone decided that “kilocalorie” was too difficult to comprehend when discussing food is lost to history. So when you hear someone say that you should eat 2,000 calories a day in your diet, this is actually 2,000,000 calories. This distinction will be important in the discussion below.
Next let’s talk about units of power, which is energy expended per unit time. For example, an erg is a unit of power equal to the expenditure of one calorie per second. While knowing the definition of an erg can be quite useful if you are doing a crossword puzzle, the standard unit of measure for power is the Watt. To discuss Watts, first I need to introduce the Joule. There are 4.184 Joules in a calorie. I could explain why but this discussion of units is already probably becoming a bit tedious. A Watt is equal to one Joule per second.
OK, back to backpacking. I am 47 years old, weigh 160 lbs and am moderately active which means that my baseline expenditure of food calories every day is approximately 2,400. After doing a bit of math you can determine that my metabolism consumes an average power of 116 Watts, just a bit more than a standard light bulb. After determining my baseline power consumption, I let’s consider how much additional power I need to expend to walk, run, or backpack.
For a 160 lb person, walking 3.5 miles an hour requires 310 Watts of power and running 8 miles hour requires 1100 Watts. This large difference in power consumption corresponds to our intuitive understanding that running is much harder than walking. If I were walking for exercise I might do so for an hour which, after doing a bit of math, corresponds to an energy consumption of 267 calories. If instead I were running, I’d probably get tired after a half an hour and stop, by which time I would have burned 473 calories. Adding up the totals and rounding off a bit, on a day when I walk for an hour my body uses 2700 calories, 2400 as background metabolism and 300 more from walking. On a day when I ran for half an hour my total calorie consumption would be approximately 2900 calories.
As I was lugging my pack through the mountains, it felt like I had to generate significant power to keep moving and, therefore, would be expending quite a few calories. Due to this extra load and uneven footing on the trail, my walking speed was approximately 2 miles an hour. A 190 lb person – 160 lbs of me plus 30 lbs of backpack – walking at 2 miles and hour expends 210 Watts of power. On a typical day of backpacking I walk for six hours which corresponds to the consumption of 1084 calories. Of course the challenging part of backpacking is not the linear distance but the vertical climb. On a typical day I would average a cumulative elevation gain of 2,500 ft. Lifting myself and my pack up those hills requires another 506 calories.
Adding this altogether, on a backpacking day my body will expend approximately 4,000 food calories of energy. This calculation helps to explain an aspect of backpacking which appeals to those of us who have reached our 4th decade. While we are on the trail we can eat like teenagers again, which, at least for a little while, is kind of awesome.
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