Actually, you expend far more calories than this! The calculation above assumes that we humans are 100% efficient at doing work. This is far from true...we use less than 25% of food calories to do mechanical work. In other words, for every ten joules of energy we take in from food, we can do 2.5 joules of work and 7.5 joules becomes heat. Given this 25% efficiency, how many pushups and knee bends would you have to do to burn 1 food calorie