This section is from the book "The Human Body: An Elementary Text-Book Of Anatomy, Physiology, And Hygiene", by H. Newell Martin. Also available from Amazon: The Human Body.
In the muscles we possess a set of organs capable of moving the body from place to place, of changing the relative positions of its parts, and of lifting external objects : as long as we are alive, more or fewer of our muscles are every moment doing some mechanical work. This fact suggests the question, where does this power of working come from ? In a few words, the answer is, it comes from the burning of parts of the body itself : in the burning, work-power or energy is set free and some of this is used by the muscles.
The different natural forces known to us are not nearly so numerous as the kinds of matter : we all, however, know several of them, as light, heat, electricity, and mechanical work. One of the greatest discoveries of the nineteenth century is that these different natural forces, or forms of energy, can be turned one into another, directly or indirectly: kinds of energy are transmutable, while, so far as we know at present, kinds of matter are not. We cannot, as the alchemists hoped, turn iron or mercury into gold, but we can turn light into heat, and heat into electrical force, or into mechanical work. When such transformations are made it is always found that a definite amount of one kind of energy disappears to give rise to a certain definite amount of another. In other words, it has been discovered that energy cannot be created: if we take a given quantity of heat we can turn some of it into mechanical work ; if we then turn all this mechanical work back into heat we get again exactly the quantity of heat which disappeared when the mechanical work appeared : and so with all other transformations of energy from one kind to another, and back again. This fact that energy or work-power can be turned from one kind into another, and often back again, but never created from nothing or finally destroyed, is known as the law of the conservation of energy.
What are the functions of the muscles? Are all of our muscles ever at rest at the same time ?
What question does the constant activity of our muscles suggest? How may this question be briefly answered ?
Name some forms of energy. Can they be turned from one form to another ?