What makes the human machine perform? This question, involving the fundamental problem of the source of the energy in muscular work, has been asked and answered many times in the history of modern physiology. To one who does not appreciate the difficulties of the investigation of this subject it may seem strange that the fuel at the expense of which daily muscular tasks are accomplished cannot be spoken of with complete conviction. Instead of coal and wood, gas and petroleum, the animal organism is limited to the proximate principles—the calory-yielding foodstuffs. We have repeatedly referred to the hypothesis, so vigorously championed of late by the English physiologist and Nobel laureate A. V. Hill, that carbohydrate is always the immediate source of energy in muscular activity. There are unquestionably occasions when the readily available bodily supply of glycogen and dextrose cannot suffice for the task to be accomplished; yet the muscles
THE FUEL OF MUSCULAR ACTIVITY. JAMA. 1931;97(16):1154. doi:10.1001/jama.1931.02730160036013
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