Basics of Biomechanics, Muscle Physiology and Energy Systems for Athletes

Athletes who want to do their best need to have knowledge of at least some of the basic facts of anatomy, biomechanics, muscle physiology, and energy metabolism and how they influence strength training and endurance training programs. While most athletes may not need to know as much as the scientist in the laboratory, he or she needs to know enough to judge the suitability of different training methods for their given sport.

HOW THE BODY MOVES

The body moves when muscles contract. The muscles connect two or more bones that meet in articulating joints. One end of the muscle is attached by a tendon, a strong, rope-like cord made from collagen, to a bone at a point close to the center of the body. That point is referred to as the origin of the muscle. The other end of the muscle is attached by another tendon to a point on a bone further away from the center of the body. This point of attachment is called the insertion. The contracting muscle pulls the insertion point closer to the origin. The bones are used as levers to move the body and apply force to external objects. The bones are designed to fit together to provide ease of motion and stability. The ends of the bones are covered with articular cartilage that prevents friction from wearing down the parts of the bones that are in contact with each other. The bones are held together by ligaments and the joint capsule. Both are made of connective tissue, collagen fibers.

WHAT IS MUSCLE?

Muscles are comprised of long, thin cells collectively known as muscle fibers. The muscles contract when stimulated by a motor nerve. The motor nerve is comprised of bundles of cells called neurons that are designed to conduct changes in electrical charge along the surface of the cells. These charges are the result of the exchange of positive sodium ions on the outside of the cell membrane of the neuron for negatively charged potassium ions on the inside of the cell membrane of the neuron. Between each neuron are gaps called synapses through which the nerve impulse is conveyed via a neurotransmitter called acetylcholine. When the receptor sites on the end of a cell are filled with acetylcholine molecules, the nerve impulse is conveyed along the surface of the neuron to the next nerve junction until the last neuron intersects the neuromuscular junction. The nerve impulse stimulates the receptor sites to release calcium into the muscle cells, causing them to contract. Each neuron stimulates a certain number of muscle fibers. The strength of the impulse is determined by the number of neurons conveying the command to contract, which determines the force of the contraction.

The contractile element of the muscle fiber is called the myofibril. The myofibrils are composed of a linear element called actin, which are linked to each other by cross-bridges called myosin. Under the electron microscope, the interior of the muscle fiber resembles an aerial view of rowing crews. The cross bridges pull together, shortening the cell. As the muscle fiber contracts, the cross-bridges unlink, then link to a position farther down the actin filament, creating greater contraction. Each cell is linked longitudinally to the next. The muscle cells become shorter and thicker around the middle as they contract. This makes the entire muscle change shape the same way.

These fibers are grouped into bundles surrounded by a sheath of connective tissue. At each end, the muscle tapers to a tendon. The muscle contraction is comprised of the directed contraction of a however many of the muscle fibers as is necessary to perform the function required. The force of the contraction is directly proportional to the percentage of the muscle fibers that are contracting, and the amount of force each individual fiber can generate. Individual muscle fibers either contract completely, or not at all. That is called the all-or-none principle. In essence, the contraction of a muscle cell is like the state of an electric light. It is either on or it is off.

Most natural movements of the body are multi-joint and use a number of muscle groups working together. The muscle(s) that provides most of the function for a particular movement is known as prime movers. Muscles that assist the prime movers are called secondary movers. Muscles that oppose the prime movers are known as antagonists, muscles that statically hold the body in position are known as stabilizers.

MUSCLE FIBER TYPES AND FUNCTIONS

Muscles are comprised of two basic types of contractile fibers. Type I muscle fibers are also known as the oxidative type or slow-twitch fibers. They don't contract very fast or forcefully, but have great endurance. This high level of endurance is due to the abundance of fat-burning enzymes to help use fat as a fuel in moderate intensity, long-term exercise. It doesn't grow (hypertrophy) to any large degree. The other basic type of muscle fiber is the Type II or fast-twitch fibers. There are two subtypes of type I fiber. Muscle Fiber Type IIb contracts rapidly and with great force, growing large when subjected to high workloads. But, this type of fiber fatigues rapidly. Type IIb fibers have an abundance of glycolytic enzymes to burn sugar as fuel without oxygen. Muscle fiber type IIa has capabilities intermediate between the other type I and type IIb, but can convert to Type I with extensive and long-term endurance training. Type IIa and Type IIb fibers are collectively known as Fast-twitch fibers. Every muscle of every person has some of all three muscle fiber types. However, some muscles have a predominance of whatever type of fibers best fulfill the function of those muscles.

Weight Training with low resistance tends to develop strength of contraction in Type I fibers, while not stimulating any training effect on Type IIb fibers. Type I fibers are the first to contract in any movement. When the speed and force of contraction increases, Type IIa fibers contract as well as Type I fibers. Type IIb fibers only contract when the greatest amounts of speed and force are required.

Type I muscle fibers do not experience much hypertrophy (growth) in the contractile filaments when exercised, they primarily increase the number and size of the mitochondria. Mitochondria are subcellular structures responsible for oxidative production of energy in the muscle cell. Type IIa and IIb fibers respond to high intensity exercise by increasing the amount of contractile proteins in the cells and the intracellular fluid. Endurance exercise doesn't produce much muscular growth, while anaerobic and strength training does produce significant muscular growth. This is because the Type II muscle fibers, which are more likely to exhibit hypertrophy than Type I, are only used during high intensity exercise.

One can use the metaphor of electric lights in an office building to compare the operations of each muscle fiber type. Let us now suppose that there are three different types of lights. The emergency lights that illuminate the emergency exit signs are just bright enough to be seen, these lights are not very bright, but are always turned on first and they last for a very long time. These first lights are comparable to the Type I muscle fibers.

The second type of lights are normal florescent office lights. They are more powerful, but burn out more frequently than the emergency lights. These are turned on for usual workday use. Those lights are comparable to the Type IIa muscle fibers. Then there are powerful spotlights that are only used when an unusual amount of light is required. These are very powerful and produce 1,000,000 candlepower, but can only be used for a very short period of time before they burn out. The spotlights represent the Type IIb muscle fibers. The emergency exit lights are used first, and continue to be on whenever the lights are used. The fluorescent office lights are turned on next, with the emergency lights still burning. Finally, the spotlights are used only when the greatest amount of light is required, assisted by all the other lights still burning.

The average person has approximately 50% Type I fibers and 50% Type II fibers. Some people tend to have a predominance of one type and therefore have a greater potential to develop the performance characteristics possessed by that fiber type. Those who have a high percentage of Type I muscle fibers have an easier time developing endurance, while those who have a high percentage of Type II muscle fibers develop greater strength, speed and muscle size.

People with a high neuromuscular efficiency can contract a greater percentage of their muscle fibers with more synchronization than most other people. These people can generate more speed and force out of the fibers they possess. That can explain the unusually great strength seen in some average or small people. Other factors include leverages created by the insertion point of the muscle into the bone, and the relative lengths of limbs. These factors determine how much of the force is delivered in usable form to the object or body part on which the force is exerted.