Muscle can be regarded as a machine that converts chemical energy into work. The first and second laws of thermodynamics apply to muscle the same way as they apply to other machines. The first law states that the total energy remains constant when it changes from one form to another:
427.263 kg.m = 1.000 kilocalorie = 4.184 kilojoules
The ultimate source of our energy resides in the food that we eat. The energy values of the various foodstuffs are known since the beginning of the 20th century: When 1.0-g of carbohydrate was oxidized to CO2 and H2O in a bomb calorimeter 4.1 kcal was released. Values for 1.0-g fat or 1.0-g protein were 9.1 or 4.1 kcal, respectively. Similarly, the calorie values of various substances in the body were determined and the next job was to find the chemical substance that directly provides the energy for muscle work.
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