There’s more to muscles than what’s walking around on the beach.
There are three types of muscle, striated, smooth and cardiac. All three have some things in common. Their contraction and relaxation creates mechanical energy that translates to movement of the body and internal body functions. All muscles have cells arranged in groups of fibers, imbedded in connective tissue and bound together by intercellular substance.
Striated muscles usually spend the most time with in an effort to be, as it’s known in Hawaii, “luking goood.” Striated muscle tissue consists of bundles of fibers called fasiculi made up of long nucleated cells with myofibrils in alternating dark and light bands that give them a striped appearance. Groups of fibers lie parallel to each other and each is enclosed in a membrane called perimysium. Each complete muscle is encased in a sheath called epimysium which is encased in sheets of connective tissue, deep fascia, that separate individual muscles. These are the muscles of the skeleton, eyelids, tongue and soft palate, eye, scalp, pharynx and upper esophagus. These are the voluntary muscles and are stimulated by impulses from fibers of the cerebrospinal nerves. If nerves of these muscles are severed, the muscle will atrophy and paralysis results.
The voluntary actions of the skeletal muscles is involved in the control, posture and movements of the body such as walking, talking, moving the fingers, swallowing, movements of the eyes, and the voluntary action of the abdominal muscles for respiration and elimination.
Skeletal muscle have the ability to contract and extend, to stretch and twist, the ability to regain their original shape and size after being stretched and to respond to a stimulus. The usual muscle stimulus consists of a nerve impulse resulting from a change in the internal or external environment due to a chemical, electrical, mechanical or thermal influence. A weak stimulus will produce a weak muscle contraction involving only a few fibers. If the stimulus is strong the reaction may involve all the fibers of the muscle.
A single stimulus will invoke a contraction and then an immediate release. A continuous or rapidly repeating stimuli will result in a sustained contraction called a tetanus (as in lockjaw). If this condition is continued for a period of time the muscle gradually loses the ability to contract and fatigue sets in. Fatigue is due to the muscles’ inability to synthesize adenosine triphosphate (ATP). ATP is essential for the production of energy used in muscle contractions. Acute, physiologic fatigue is usually a temporary condition with the muscle(s) regaining their ability to contract after a period of rest. Chronic fatigue is usually a symptom of underlying disease or pathological conditions.
Skeletal muscles work on the principal of leverage with the bones acting as the lever or fulcrum. There are three classes of levers: class 1 is where the fulcrum lies between the lever and the resistance. The triceps muscle extending the arm below the elbow is an example. Class 2 lever is when the resistance is between the power and the fulcrum. Example is when the gastrocnemius (calf) muscle raises the weight of the body on the toes. Class 3 lever is when the power applied is between the resistance and the fulcrum. When the biceps flexes the arm at the elbow is an example. Most levers of the human body are types 1 or 3. All three types are essential since different lever types allow for speed at the expense of power, power at the expense of speed or the ability to change the direction of movement.
Skeletal muscles have two attachment points. The end that remains relatively still is known as the origin end and the other is the insertion end, or moveable end. When a muscle contracts the insertion end moves closer to the origin end. Either end in some muscles in the extremities can function as the origin. Some muscles, the biceps brachii for example which acts on both the humerus and radius, can traverse two joints bringing about an action at either or both ends.
There are four types of skeletal muscles: prime movers, antagonists, synergists and fixators. Prime movers are muscles that bear the responsibility for a particular movement. The biceps, which flexes the forearm, is this type muscle. Antagonists are muscles that produce an opposite action to the prime mover. The triceps, when straightening the forearm is an example. Synergists prevent undesirable actions. Example: a pronator that resists undesirable action by a prime mover. Fixators are muscles that stabilize a part. Example: stopping the shoulder from moving when the arm is flexed.