What Muscles Can Do, and Can’t Do

Their simplest movements are marvels we take for granted. With training, they perform feats of strength and endurance that are amazing. But there is another kind of training that is far more important, that can do what muscles can never do!

A BOX is full of feathers. You want to pick it up. Your mind instructs the sets of muscles involved, and you pick up the feathers. Now the box is filled with lead bars. Your mind instructs the same set of muscles that lifted the feathers now to lift the lead, and they do so. Simple? Not at all.

A skeletal muscle fiber does not contract with less or more force for light or heavy weights. When a nerve ending tells the fiber to contract, it does so completely. If it contracts at all, it contracts all out. How is it, then, that at one command muscles exert only enough force to lift feathers, and at another command those same muscles muster the greater force needed to lift lead?

A muscle is made up of many bundles of tiny muscle fibers, each bundle being called a motor unit. To each unit there is a motor nerve, which at its tip branches out so that each muscle fiber has its own individual nerve ending to stimulate it. The electrochemical impulse is transmitted by chemicals from the nerve ending to the fiber, where it again becomes electrochemical. The fiber contracts. All the fibers in that bundle or motor unit contract.

Now, not all the bundles of fibers of a muscle contract when that muscle is used. If the mind knows it is only lifting feathers, the central nervous system signals only the comparatively few bundles needed to lift feathers. But if it is lead that is to be lifted, many more bundles will be stimulated to contract.

Sometimes the mind is fooled. If it thinks the box is full of feathers when it contains lead, not enough fibers are told to contract and the mind is surprised. It seems like the box has been nailed to the floor. But if the mind thinks the box is full of lead but has only feathers, many bundles of fibers are engaged to lift lead, and the box seems to fly up from the floor.

Decisions! Decisions!

The point is, 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 the 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, 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 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 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.

Where the Energy Comes From

ATP (adenosine triphosphate) is the energy-rich source for muscle action. It is produced in the muscle fibers by small bodies called mitochondria and is formed in several ways. Fats in muscle tissue (adipose tissue) are broken down to become free fatty acids in the muscle and also in the blood. Eventually, in the muscle fibers they are oxidized to release energy to make ATP. Glucose from the blood is also oxidized in the muscle fibers to form ATP. Some glucose from the blood is stored in the muscles as carbohydrates, called glycogen. Then, as ATP is needed, this glycogen is broken down into glucose, which, without the use of oxygen, produces ATP.

These methods for making ATP are used simultaneously, but in varying degrees, according to the circumstances. The kind of exercise, its intensity, its duration, the physical fitness of the individual​—all are factors governing how much ATP each method will supply at given times. Relative to distance running, however, when the exercise has been intense for a long time, the mainstay for ATP production is glycogen.

Marathoners often do what is called carbohydrate-loading. A few days before a race they gorge on carbohydrates, and in so doing they can increase the amount of glycogen stored in their muscles by as much as 300 percent. A by-product of this use of glycogen, however, is lactic acid, and it is its accumulation in the muscles that causes fatigue, and eventually muscle soreness.

Worship Muscles, or Their Maker?

Muscles can do many things: Throw a ball and make it curve and drop and slide. Balance the body upright on one hand. Send the body gracefully flying, flipping, spinning through the air. Muscles of one arm can hoist weights of hundreds of pounds overhead. Muscles in legs can propel the body over a bar seven feet high, or over the ground nearly 30 feet, or race 100 yards in nearly nine seconds, or a mile in less than four minutes, or 26 miles in a little over two hours. Or they can keep running for 50 miles, or 100. Tarahumara Indians in Mexico run 200 miles. The dubious claim is made that the Mahetangs, Tibetan monks especially trained for “swiftness of foot,” run 300 miles in 30 hours while repeating their sacred mantras in time to their strides and breathing.

Muscles are awesome. But muscles are not gods. Some runners seem to think so​—a minority, doubtless. One likens running to the search for the Holy Grail. Another runner claims that “the quest of spirit through the body has just begun.” Dr. George Sheehan, called by many the high priest of running, said: “My danger is that I will fail to reach my limits and find God. But here my running helps.” A woman jogger likened her running experience to a conversion. The wife of a runner said: “Tom used to be a Methodist. Now he’s a runner.” In his book on running, Joel Henning said: “It is indeed a form of worship, an attempt to find God.” Bob Anderson, editor of On the Run, declared: “Someone once said, ‘For humanity to survive, it will have to invent a new religion.’ The religion has been invented. It is the religion of the runner.”

But wait a minute! Muscles can’t save. Only their Maker can. Muscles reflect Jehovah’s creative wisdom. See his genius in their agility, speed, strength, endurance. See it in the electrochemical complexities, millions of reactions in millions of fibers, every second of the day, monitored and synchronized without thought from us. Without thought from us as they go about their business of keeping us alive: lungs breathing, heart beating, blood circulating, organs digesting, glands secreting, electrical circuits racing​—and much, much more we’re never aware of.

Muscular training is beneficial, but not to be compared to training in godly devotion. “Bodily training is beneficial for a little,” the apostle Paul wrote, “but godly devotion is beneficial for all things, as it holds promise of the life now and that which is to come.” (1 Tim. 4:8) Enjoy whatever kind of exercise you do. Relish its benefits. It can make you feel better. Godly devotion, however, can do what muscles can never do​—make you live longer, even forever. As the psalmist sang:

“The LORD sets no store by the strength of a horse and takes no pleasure in a runner’s legs; his pleasure is in those who fear him.”​—Ps. 147:10, 11, The New English Bible.

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WHAT AEROBIC EXERCISE DOES FOR MUSCLES

Muscle fibers become stronger and contract faster.

Mitochondria are increased in number​—they make ATP.

The number of enzymes needed by mitochondria to make ATP increases​—three times as much in distance runners as in sedentary persons.

The muscle’s myoglobin often doubles. It carries oxygen to the mitochondria. More myoglobin means more oxygen.

Arteries sprout new branches, capillaries increase, often doubling. This means more blood supply of oxygen to muscles.

Because of this increased circulation and more myoglobin, oxygen supply is more efficient and less blood flow is needed.

Amount of fats oxidized increases, supplying more ATP.

Oxidation of glucose increases​—another source of ATP.

Trained muscles contain higher concentrations of stored carbohydrates (glycogen)​—the main energy source for high-intensity exercise over long periods of time.

Trained muscles do not accumulate lactic acid as fast, and can hold more than untrained muscles; hence lessen fatigue.

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WHAT LACK OF EXERCISE DOES TO MUSCLES

Muscles become smaller, waste away, atrophy. Obvious when casts are removed from broken arms or legs​—muscles shriveled.

In one study, athletes spent 20 days in bed. Oxygen intake capacity dropped over one fourth. Heart’s pumping capacity dropped same amount. Red blood cells dropped 15 percent.