It’s so amazing how the central nervous system is constantly making decisions as to how many bundles of fibers it must signal to contract for the many works to be done by some 650 muscles in the body. Sensory organs in the fibers, called stretch receptors, monitor the fibers and send back reports to the central nervous system, and by this feedback they help in the decision-making. You may hate to make decisions, but unconsciously you are making millions of them constantly!
The more fibers that contract and tighten, the bigger and harder the muscle will be. For example, your biceps muscle in your upper arm contracts to lift your hand to scratch your head. Not many bundles of fibers are needed, and your biceps is quite soft. But in the same movement hoist a 30-pound weight to your shoulder, and your biceps will bulge and harden as many more fibers go to work.
Some muscles have a much finer and light control over tension than others. The fingers, for example, can close with an iron grip, or delicately handle thin-shelled eggs. Such muscles contain many fiber bundles, but each bundle with only a few fibers—some no more than 10. Other large muscles, as some in the legs, are incapable of such finely graded movements. They have fewer fiber bundles, but many more fibers in each—often more than 100.
Skeletal muscles have basically two kinds of fibers: dark ones for slower, steady use; white ones for quick bursts of action. (Called slow-twitch and fast-twitch fibers.) Some muscles are almost entirely made up of slow fibers, but others have slow and fast intermingled. Persons who are unusually quick in their movements have more white or fast fibers than those persons whose movements are slower. Agile, athletic gymnasts, for example, need fast fibers for the dazzling and explosive gyrations with which they amaze us. Also, outstanding sprinters have more of these fast fibers than do the long-distance runners. Training makes a difference, but it cannot change the proportion of fast and slow fibers—that’s hereditary, a gift.
Muscles are truly amazing. Their simplest movements are marvels we take for granted. With training, they perform feats of strength and endurance that are amazing. Running is no exception! It truly is a wonderful enjoyment, one of the best preventative medicines and an ability we should never take for granted.