There are two general types of muscle in the body: cardiac or smooth muscle and skeletal or striated muscle. Cardiac muscle pumps the heart and is not under conscious, voluntary control. Skeletal muscle, on the other hand, is able to be controlled voluntarily. It is skeletal muscle that is responsible for creating movement in all parts of the body. First we will concentrate on skeletal muscle. Cardiac muscle will be discussed later when we deal with the heart and cardiovascular system.

Skeletal muscle accounts for forty percent of the weight of the average man. In athletes, depending on the sport, the total weight of muscle may be far higher.

Muscles are composed of multiple bundles of fibers. There is absolutely no way to increase the number of muscle fibers which exist in your body. What you are able to do, however, is dramatically increase the size and shape of the muscle fibers. This "muscle growth" process is known to scientists as hypertrophy (hi - pert - trophy). Hypertrophy is best accomplished by overloading the muscle with work. This is a common goal among weight lifters and other gym trained athletes.

Muscle creates movement in the body due to its ability to contract. When a group of fibers making up a certain muscle contracts than the joints or bones attached to that muscle will change position and movement results. The velocity of movement of the bones attached to the muscles are very important for sports performance. For example, throwing a 95 MPH major league fastball is not an easy task. Aside from lots of practice it requires the pitcher's arm to have a very fast movement when he releases the ball. The movement of his arm depends on two factors related to the muscles.

First, it depends on the total number of a certain type of muscle fiber called "fast twitch" muscle. There are two very general types of skeletal muscle. They are sometimes referred to by their "street" names; fast twitch and slow twitch. Fast twitch fibers or "white muscle" are responsible for rapid, sudden movements. Slow twitch fibers or "red muscle" is responsible for longer, sustained body movements. Individuals are born with a certain ratio of fast twitch versus slow twitch fibers and this ratio is unchangeable. The ratio does, however, vary from body location to body location. Two different athletes may both have 30% fast twitch fibers in bodies. The ratios in individual body parts could vary nonetheless. For instance, the first athlete might have 15% fast twitch in his quadricip muscle (thigh) while the second athlete could have 65% fast twitch in his quadricip. Generally, athletes with higher numbers of fast twitch fibers in their legs will be quicker sprinters while the slow twitch dominant athletes will either be poorer runners or better endurance runners. We would expect to find a far higher fast to slow twitch muscle ratio in sprinters as compared to marathon runners. The opposite is also true. We would expect to find a higher slow twitch to high twitch fiber ratio in marathon runners as compared to sprinters.

Second, it depends on how much the muscles opposing the forward velocity of the arm have been trained to relax. The muscles that perform the athletic task that we want done are called the "agonist" muscles while the muscles opposing the action are called the "antagonists."

In athletic movements the agonist muscles are always getting shorter (contracting) while the antagonist muscles are always resisting. Sore muscles often result from this stretch resistance. If the antagonist muscles stop resisting then the agonists will probably injure themselves by tearing or pulling the attached joint apart.

Lack of coordination between agonist and antagonist muscle groups can also cause muscle soreness. This can be seen clearly when an athlete changes types of exercise. Soreness results from swimmers who start running, bikers who start swimming, etc. This type of soreness may be avoided by introducing new types of exercise slowly thereby allowing the muscle groups to be coordinated with each other.

Muscle fibers need energy to contract. This is normally supplied by free fatty acids (fats), glucose or glycogen (carbohydrates). Energy for muscles can also come from amino acids (protein). Unfortunately, when energy is supplied from amino sources it is usually due to the unavailability of other nutrients and results in muscle breakdown and size loss. There is recent research suggesting that a certain group of aminos which in large part comprise muscle tissue (along with water) frequently supply energy to muscle during intense work. This group of aminos is called Branch Chain Aminos and include the main energy providing amino, Leucine. You will learn more about protein and aminos later in the sections that deal with Nutrition.

Related to the muscles are tendons and ligaments. Tendons are groupings of a type of connective tissue called collagen. The purpose of a tendon is to connect the muscle to the bone. Some tendons such as the deep flexor tendon of the leg also contain small percentages of muscle cells. Tendons can stretch up to 4 % above their normal size and return immediately to their normal shape and size with no negative after effects. In most well trained athletes, tendons can stretch as much as 8-10 percent beyond their normal size without injury. Once the tendon stretches too far, that is beyond 8-10 percent, some of the cross link fibers begin to break and the tendon can fail. Tendons, like muscles, become stronger with increased flexion or exercise. In general though tendons take twice as long as do muscles to become fit. Tendon damage is common in sports. Swelling is the body's first response to tendon injury. Swelling can cause serious, more permanent damage and should be immediately treated with ice or a doctor visit. Despite decreased pain as time continues, it may be as much as two weeks before the body begins to reinforce the tendon with new collagen. Tendon injuries and tendon inflammation or swelling (tendonitis) should be considered a serious condition and all muscle activity near the injured tendon should be limited.

Again, the best prevention for tendon problems is exercise. The more work you are able to slowly introduce to the tendons, the stronger they will become. Young athletes probably should not begin to weight train their muscles until their early teen years. This will allow for growth plate development and a healthy, normal skeletal system. When the young athlete decides that it is time to begin training his muscles, he should not immediately begin with a serious "overload" program. Rather he should begin with a training program centered around a higher number of repetitions per exercise set with less weight. He should also be careful not to neglect the muscles that are especially important for the performance of his specific sport. Later, after he becomes acclimated to gym training, he can slowly begin to cut down on the number of repetitions while at the same time increasing his working weights.

When in doubt about the appropriate time for beginning weight training with an individual athlete, simply schedule a physical with a well respected local sportmedicine physician. After examining the child he will be in a perfect position to make training recommendations.