What Is The Composition Of Muscle?

A muscle consists of sheathed fibers the diameters of which may range from 1/250th to 1/2500th of an inch, and these may be the full length of the muscle. The fibers are made up of parallel elements, 1/25,000th of an inch in diameter and these, in turn, consist of parallel actin and myosin filaments. It appears that the key to muscle action lies in these actin and myosin filaments. It is now understood that when a muscle contracts, one of these slides past the other.
Muscular activity consumes oxygen and nitrogen, causing a demand of these from the blood, and at the same time results in an increase of waste products, namely, carbon dioxide and lactic acid, which the blood carries away. It is the presence of lactic acid in the muscle that causes one to feel tired. The bloodstream serves both to feed the muscles and to carry away their waste products and does it without confusing the two. While this much is known, what remains to be understood is just how nerve force is able to change the products of food metabolism stored in the muscle to mechanical energy.