The human body has three types of muscles: Skeletal, smooth, and cardiac. Because Sports Massage Therapists are mainly concerned with skeletal muscles, they will be the ones primarily discussed throughout this text.
Skeletal muscles are primarily attached to the bones of the body and, unlike smooth and cardiac muscles, are under voluntary control. They comprise most of the flesh of the body and constitute about 40% to 50% of a person’s total body weight. Skeletal muscles perform the following functions:
Produce movement of joints by contracting.
Prevent undesired movement of joints.
Produce heat (through the splitting of Adenosine Triphosphate (ATP) during contraction.)
Skeletal muscle is comprised of long cylindrical multinucleate cells which lie parallel to each other. Each cell is surrounded by a thin elastic membrane called the sarcolemma which encloses its contents. Within the membrane is the fluid protoplasm, or the sarcoplasm, of the cell which contains myofibrils (discussed later in this text) and the sarcoplasmic reticulum, comprised of a network of small channels and fluid-filled sacs.
Skeletal muscle cells are also called fibers. Overlaying the sarcolemma of each fiber is a thin layer of connective tissue called the endomysium. The fibers are grouped together into many individual bundles which are covered with a layer of connective tissue called the perimysium. These bundles are known as fascicles and together they form a muscle. The entire muscle is covered by yet another thin layer of connective tissue called the epimysium, which itself is covered by connective tissue called fascia. These last two protective layers are tapered at the ends and form the tendons which attach a muscle to bone, cartilage or connective tissue.
Each individual muscle fiber contains very fine, long protein strands called myofibrils, which are aligned side-by-side and extend the length of the fiber. They are the units which lengthen and contract the muscle. The myofibrils are “actually chains of tiny contractile units, called sarcomeres, which are aligned end-to-end like boxcars in a train along the length of the myofibrils.” The Sarcomeres are formed by even finer strands known as myofilaments. The myofilaments are comprised of proteins which form dark thick strands—A bands—and light thin strands—I bands—and are what give skeletal muscle its striated appearance.
The I bands are also called actin filaments because they are primarily made of a protein called actin, but they also contain two other proteins—troponin and tropomyosin. The A bands are also known as myosin filaments because they are formed from a protein called myosin; they contain ATPase enzymes which split ATP to produce the energy for muscle contraction. The myosin filaments contain “lollipop” projections, referred to as cross-bridges or myosin heads, which spiral around its length. During muscle contraction, these projections attach to binding (active) sites on actin filaments to produce movement. In resting muscle, the troponin and tropomyosin cover the active sites and inhibit the myosin heads from bonding to the actin filaments, thereby preventing muscle contraction.
Skeletal muscle is stimulated to contract by impulses transmitted by specialized nerve cells called motor neurons. The cell body of a motor neuron resides in the central nervous system and its axon extends to the muscle. In the muscle, the axon is divided into numerous axonal terminals, each of which connects with individual muscle fibers. The intersection where the axonal terminals and muscle fibers connect is called the neuromuscular junction. A motor neuron and the muscle fibers to which it connects are together known as a motor unit.
When an action potential travels down the muscle fiber membrane and reaches the axonal terminals, the neurotransmitter known as acetylcholine stimulates the release of calcium ions into the sarcoplasm from the sarcoplasmic reticulum. Calcium ions quickly attach to troponin in the actin filaments, which causes troponin to pull on the tropomyosin (to which troponin is attached) thereby exposing the active sites of the actin filament and allowing it to interact with the myosin heads. As a result, the ATPaze enzymes on the myosin heads become activated and split ATP, which energizes the link between the actin and myosin filaments and causes muscle contraction. Once a muscle has contracted, calcium is reabsorbed back into the sarcoplasmic reticulum, which allows troponin and tropomyosin to once again inhibit the link between actin and mysoin. As a result, the muscle once again returns to a relaxed state.
Although it is not known precisely how actin and myosin produce muscle contractions, the “sliding filament theory” proposed by H. E. Huxley in the 1950’s is a possible explanation:
(The theory) suggests that stimulation of the (muscle) fiber prompts the… tiny crossbridges that extend from the myosin filament (to) attach to active sites on the actin filament. The release of calcium ions within the muscle fiber exposes these active sites, facilitating the attachment of the two (filaments) to one another. Each crossbridge exerts a pull on the actin filament, causing the actin and myosin filaments to slide past one another. Under the influence of (ATP) released in the binding process, each crossbridge is then disconnected from its binding site on the actin filament and moves to a neighboring site. Since the process happens simultaneously in all of the cells of a muscle, the entire muscle contracts.
As healthcare professionals, Therapeutic & Sports Massage therapists need to be as educated and knowledgeable about the workings of the human neuromuscular and skeletal systems as possible. This will help aid in treatment and in discussions with other healthcare professionals.
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