14.1: Muscle

Muscles make up about 45% of normal body weight in men and about 35% in women. There are various kinds, such as cardiac muscle that contracts the heart, and smooth muscles of blood vessels that contract or relax to alter the body’s distribution of blood. Most of our muscles are skeletal muscles—the ones attached by tendons to our skeleton.

We think of muscles as protein, but protein holds about three times its weight in water (Chap. 5). So muscle is mostly water—about 70% water and about 20% protein; the rest is fat and a bit of minerals and carbohydrate (glycogen). The exact content varies from person to person and muscle to muscle—much like the variation in different cuts of beef.

A muscle is made up of thousands of individual muscle cells. In addition to the standard cell parts, they have special proteins that enable the muscle to contract (Figure 14.1). Skeletal muscle cells are long and cylindrical, so are called muscle fibers. They are bunched together by thin wrappings of connective tissue. You can see the layout in meat. Carving tough meat across the fibers—cutting “across the grain”—makes it easier to chew.

Figure 14.1: Skeletal Muscle Contraction

The male sex hormone testosterone promotes muscle growth. Boys become more muscular at puberty, and men are more muscular than women. Women also make testosterone, but their levels are about 20-30 times lower than men. People vary a lot—men at the low end have testosterone levels near those of women who are at the high end.

As discussed in Chapter 9, there are different kinds of muscle fibers. Red fibers are geared for endurance and aerobic (oxygen-requiring) metabolism—they have more mitochondria (for oxygen-requiring metabolism), myoglobin (red, oxygen-carrying molecules), and fat droplets (main fuel for an endurance event). White fibers are more suited for bursts of activity (weightlifting, sprinting)—geared toward making ATP energy faster and without oxygen (anaerobic metabolism), using glucose as fuel.

Most of us have muscles with red and white fibers in about equal amounts. Elite strength-­and-power athletes have proportionately more white fibers in the relevant muscles; elite endurance athletes have more red. Training can enhance the aerobic (and possibly anaerobic) capacity of both kinds of fibers, but the proportion of each kind seems to be genetic.

Exercise

The number of muscle cells in a muscle also seems to be genetic. Thus, when we exercise or stop exercising, the size—rather than the number of cells—changes. When we regularly exercise a muscle, more of the contraction proteins are made, and the cell thickens and becomes stronger. A muscle’s strength is generally proportional to its size.

As food, the tenderest meat comes from the least-­exercised muscles. The muscle that lies along an animal’s back is not used much; it’s narrow and tender (e.g., pork tenderloin). In contrast, shoulder muscles are used a lot (exercised) and become thicker—and tougher (e.g., pork shoulder). Other things affect tenderness (e.g., marbling with fat) but, in general, the most tender meat comes from the least exercised muscles and the youngest animals (e.g., veal, roast suckling pig).

Muscle size adapts rapidly to use or disuse. Even the need to pull against gravity helps maintain muscle. Astronauts exercise regularly on space voyages, since weightlessness can cause rapid muscle loss. If you’ve had an arm or a leg in a cast, you’ve seen the marked contrast in muscle size between the limb just out of a cast and the other one. Likewise, the forearm of the arm that swings a tennis racket is more muscular than the opposite one. Body builders add bulk to specific muscles by exercising them. And as they know all too well, this bulk is quickly lost when they quit working out. (Use it or lose it!)

Building muscle means you need a little more protein in your diet, an amount easily met by a normal diet (Chap. 11). Contrary to what marketers would have you believe, protein or amino acid supplements do not in themselves increase muscle mass.

Anabolic Steroids

Anabolic steroids are used by some athletes to build muscle and strength; they are often simply called steroids. This can be confusing, since there are many hormones (e.g., estrogen, corticosteroids) that, technically, are steroids (i.e., hormones made from cholesterol), but have far different effects.

Anabolic steroids were first developed to treat patients with muscle-wasting diseases. Athletes take them at much higher doses than are prescribed for medical purposes. Their use to enhance athletic performance is generally illegal in the United States and in international competition.

Ben Johnson was stripped of a 1988 Olympic gold medal and his world record in the 100-meter dash when he tested positive for stanozolol. In 1990, Randy Barnes (a world record-holder in the shot put) was suspended from competition for 2 years for testing positive for methyltestosterone, and suspended again in 1998 for testing positive for androstenedione. Androstenedione is banned by the Olympics, the National Football League, and the National College Athletic Association. Mark McGwire’s use of it when he broke Roger Maris’s home run record in 1998 was legal—the steroid wasn’t banned at that time in professional baseball and was sold legally without a prescription in the U.S. as a dietary supplement.

Because anabolic steroids are similar to testosterone, the body reacts by making less testosterone (shrunken testicles is a side effect). Low testosterone doesn’t prove the use of these drugs, but provides supporting evidence.

Anabolic steroids are designed to retain or enhance testosterone’s growth-promoting effects on muscle while minimizing its other effects. Studies done on athletes in what was then East Germany clearly show that anabolic steroids taken in large doses during training promote increased muscle mass and strength. This was confirmed in a double-blind study in the U.S. Keep in mind that the dose matters. Steroids sold as dietary supplements advertised as “effective and without side effects” are usually one or the other—effective (effective dose) or without side effects (ineffective dose).

Some side effects of anabolic steroids are relatively mild (e.g., acne, lower-pitch voice, increased body hair, lower sex drive); others are serious. One of the more worrisome side effects is on the liver. There’s substantial evidence that anabolic steroids raise the risk of liver damage and liver cancer. Also, HDL-(good)-cholesterol is made in the liver, and anabolic steroids lower it to very low levels—a risk factor for heart disease. It’s ironic that a higher HDL is a benefit of physical fitness, and this benefit is canceled by taking steroids.

Another worrisome side effect is a severe psychiatric reaction of anger and hostility (‘roid rage) that occurs in some steroid users. Of those taking large doses, major depression or mania occurs in about 1 out of 5, and psychosis occurs in about 1 out of 10. Steroids may have a role in the problem of violent behavior.

Steroid use has spread beyond the fraternity of elite athletes. It’s now most common among men who aren’t athletes and use them to build muscle by lifting weights to “look good.” There’s concern about use among policemen.

Growth Hormone

Some athletes use human growth hormone made by biotech (in addition to or in place of anabolic steroids) to build muscle and speed recovery from muscle injury. Human growth hormone made by biotech is identical to that made by the body, so its illegal use by athletes is not easily caught. But the very high cost of human growth hormone limits its use among athletes.

Side effects from using large doses of growth hormone for athletic purposes are hard to establish, since athletes try to conceal their use. Anecdotal reports of abnormal bone growth are consistent with what we would expect from growth hormone use. We know the problems caused naturally by excessive production of growth hormone. In children, it accelerates bone growth, resulting in gigantism, characterized by gigantic height and sexual immaturity.

When excessive growth hormone production begins in adulthood (acromegaly), it causes the feet, hands, and the lower jaw to grow (the last bones to stop growing). There are also other problems, such as thickening (and sometimes darkening) of the skin. Nearly all women with this disease develop menstrual irregularities, and about a third of the men become impotent.

Age

Starting at about age 30, we gradually lose muscle protein, and the amount of fat in muscle increases. The fall in muscle protein leads to the weaker muscle strength seen in older adults.

But note that a physically fit 80-year-old can be stronger, more agile, and have more endurance than a sedentary 30-year-old. Mavis Lindgren at age 81 ran a marathon (26.2 miles) in 4 hours and 34 minutes, winning in her 70-81-year-old age group. There aren’t many sedentary 30-year-olds who can average a pace of 5.7 miles per hour for 4½ hours.

Muscle Contraction

Nerve cells usually provide the stimulus for muscle contraction, as when we walk or jerk our hand back from a hot pan. The heart muscle is stimulated rhythmically by pacemaker cells in the heart tissue itself. When these pacemakers don’t work properly because of disease or damage from a heart attack, normal rhythm can be restored by implanting an artificial pacemaker.

To contract, muscle cells need energy (ATP) and certain minerals (e.g., calcium ions) (Figure 14.1). ATP is made in breaking down glucose and fat (Chap. 9). We think of muscular contraction as movement, but it’s also needed just to maintain normal muscle tone, using some of the energy used in resting metabolism. Muscle cells contract asynchronously to sustain a tightness in the muscle—a tightness needed to maintain posture. Without muscle tone, your head would slump onto your chest.

Many athletes take protein or amino acid supplements because of a common misconception that protein is what fuels muscle contraction. Some amino acids are used for fuel when glycogen is depleted in endurance events, but the extra need for protein is modest and met by a normal diet (protein needs of athletes were discussed in Chap. 11). Athlete or not, almost all of the energy needed for muscle activity comes from the ATP generated by the metabolism of glucose and fat.