We can start off easy by thinking about the sheer mechanical energy required for various activities. For example, if you were to bench press 100 pounds, the distance moved might be on the order of 18 vertical inches for a total of 1800 inch-lbs of energy. This would equate to about 204 joules or about 0.05 kcal or Calories. Now, this is the actual mechanical work associated with lifting that weight against gravity. If we take an estimate of 20% conversion efficiency in muscle as mentioned in the last post, then the food energy used by that single lift would be about 0.25 Calories.
Similarly, walking up a flight of stairs, say, 10 vertical feet, for a 190 lb man would equate to mechanical energy of about 0.62 kcal with a corresponding food energy use of 3.1 Calories.
It is more complicated to estimate energy associated with walking, running, or bicycling on flat ground. From the standpoint of mechanical work, moving horizontally entails no work at all. Of course we know that those activities use plenty of human energy. For the case of riding a bicycle, for example, the rider has to overcome rolling resistance and air friction. Rolling resistance depends on the type of tire, the inflation of the tire, the type of road surface, and the speed of the bicycle. The air friction depends heavily on the speed, as well as the density of the air, the cross-sectional area of the bike and rider, and the drag coefficient.
Just as an example, if a person were riding a bicycle at 10mph up a 2% grade, using typical values for rolling resistance and drag coefficient, the rate of mechanical work might be about 20 watts for rolling resistance, 25 watts for air drag, and 70 watts for going up the hill. This would correspond to about 99 kcal per hour for the mechanical work output. The food energy used might be in the ballpark of 500 Calories per hour. Or, if you prefer, 1.67 cheeseburgers per hour.
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