Information compiled by Mr. Paul Riley: Naginata Shugyo; Aurora, CO
Know your body, Know your limitations,
Then dare to stretch your horizons...
Information compiled by Mr. Paul Riley: Naginata Shugyo; Aurora, CO
Contractions, Filaments, and Stretch ReflexIf you have been following these presentations then you are aware that we are attempting to give the reader only a cursory introduction to Human Physiology. It is necessary to understand, to some degree, the how and why of movement before we can accept the position that "cellularly speaking" we are not all created equal. Being aware of the limitations inherent to body structure, we hope, will aid anyone in realizing their potential, and allow them to "safely" stretch those limits.
Muscles and bones comprise what is known as the "musculoskeletal system". The bones provide posture and structural support for the body and the muscles provide the body with the ability to move (by contracting, and thus generating tension). The musculoskeletal system also provides protection for the body's internal organs. In order to serve their function, bones must be joined together by something. The point where bones connect to one another is called a "joint", and this connection is made mostly by "ligaments" (along with the help of muscles). Muscles are attached to the bone by "tendons". Bones, tendons, and ligaments do not possess the ability (as muscles do) to make your body move.
Muscle ContractionMuscles produce force through the process of contraction. When a muscle contracts it may change its shape, but it never changes its volume; it does not become smaller. During the process of contraction the energy from chemical reactions in the muscle can be converted into useful work. Thus, contraction of muscle enables us to move about and perform direct actions on the environment. A great variety of muscles are found in the bodies of different animals, and the structure of each muscle is suited to its particular function. Despite this great diversity among muscles, it appears that the basic mechanism of the contractile process is the same (the coupling of the splitting of adenosine triphosphate (ATP) to the interaction of two proteins, myosin and actin).
Movements within the body are produced by the contraction of cardiac and smooth muscle. The heart consists mainly of cardiac muscle, which has properties that enable the heart to operate like a pump. Its rhythmic pattern of contraction, for example, is an intrinsic characteristic of cardiac muscle tissue; signals from the nervous system can modify the contractions but are not needed to keep a normal heart beating. Smooth muscle is found in the walls of the digestive tract, reproductive system, and blood vessels. The functioning of cardiac and smooth muscle is coordinated with other functions of the body by the action of the nervous system and hormones.
Locomotion of the body is produced by the complex cooperation among the skeletal muscles and other systems, including skeletal, nervous, and circulatory systems. The skeleton provides a rigid framework to which the skeletal muscles are attached by tendons. Usually a muscle spans, and causes movement of, more than one joint. Muscle contraction can cause a pulling action but not a pushing action. Even the simplest movement, therefore, requires two muscles (one muscle to bend a joint and a separate one to straighten it out again). When muscles act in this way, they are said to be antagonists. Movements of the body usually involve the action of many muscles. The nervous system controls the action of the muscles so that the force and movement are matched to the task to be performed. The brain and spinal cord accomplish this coordinating by sending signals in the form of action potentials that travel along the nerve fibers to the muscles. Nerves connect the spinal column to the muscle. The place where the nerve and muscle meet is called the "neuromuscular junction". When an electrical signal crosses this junction it is transmitted deep inside the muscle fibers. Inside the muscle fibers, the signal stimulates the flow of calcium which causes the thick and thin myofilaments to slide across one another. When this occurs, it causes the sarcomere to shorten, which generates force. When billions of sarcomeres in the muscle shorten all at once it results in a contraction of the entire muscle fiber. An individual nerve fiber and the muscle fibers it activates are called a motor unit. Different amounts of muscle are made to contract, and movements are coordinated by varying the number of motor units that are activated and the frequency of activation of single motor units.
Chemical reactions supply the energy for muscle contraction. The circulation of the blood transports the fuel for these reactions to the muscle and removes the by-products.
FilamentsA skeletal muscle consists of a number of cells, called muscle fibers. Muscle fibers contain many of the same chemicals, ions, and organelles as other cells. For example, they contain mitochondria in which ATP is made. The structures that are particularly characteristic of muscle fibers and important for their function are the filaments and the membranes.
The filaments, which are directly involved in the contractile process, are of two sizes: thick filaments, which are about 1.6 micrometers (1 micrometer = 0.001 mm) in length, and thin filaments, which are about 2 micrometers in length. They are arranged parallel to the long axis of the fiber in cylindrical- shaped columns called myofibrils. These structures run the length of the fiber and are 1 or 2 micrometers in diameter. Along the length of each myofibril, the sets of filaments interlock, forming the alternate regions in which thick or thin filaments are either present or overlap. These regions repeat along the myofibril to give the striped, or striated appearance that can be seen under the light microscope.
The sarcomere is the name given to the "unit pattern" that is repeated along the myofibril. Each sarcomere has an A band that contains thick filaments, and thin filaments overlapping with them, and an I band region that contains only thin filaments. The H zone is the part of the center of the A band and contains only thick filaments. The M line is in the center of the A band and consists of material connecting the thick filaments together. At the edges of the sarcomeres, structures called Z lines connect the thin filaments.
It is the splitting of the ATP molecule along with the interaction of myosin with actin that is the fundamental process of contraction. This process is controlled by the action of troponin, tropomyosin, and calcium ions. Interaction of actin and myosin can only occur when calcium ions are bound to particular sites on troponin; in relaxed muscle, calcium ions are not bound to these sites, and troponin and tropomyosin act together to prevent the interaction of actin and myosin.
Two separate membrane systems in skeletal muscle are involved in controlling contraction. The first consists of the plasma or cell membrane, which surrounds each fiber, and the transverse tubules, which are tube-shaped continuations of cell membrane. The tubules penetrate into the fiber and form rings around the myofibrils.
The second membrane system is the sarcoplasmic reticulum, which is analogous to the endoplasmic reticulum of other cells. It is like a closed bag inside the fiber itself; it has no opening to the outside of the cell. The calcium ions that act as a trigger for the contraction are stored in the sarcoplasmic reticulum when the muscle is resting. At particular locations the walls of the transverse tubules and the sarcoplasmic reticulum lie very close together in a characteristic pattern that is called the triad. All these membranes are involved in transmitting the signal to contract from the nerve to the contractile elements in the muscle.
SignalThe communication between nerve and muscle actually occurs at the neuromuscular junction, the specialized area in which the nerve endings lie close to the endplate region of the muscle fiber. When an action potential (electrical signal) reaches the nerve endings, a chemical transmitter, Acetylcholine, is released, and it diffuses across the small gap between the nerve and the muscle. If enough Acetylcholine reaches the endplate region of the muscle fiber, an action potential is triggered that spreads over the cell membrane and down into the fiber along the transverse tubules. When the action potential reaches the triads, the signal is communicated to the sarcoplasmic reticulum by a mechanism that is not, as yet, understood. Calcium ions are then released from the sarcoplasmic reticulum, and the level of calcium in the region of the filaments increases. Calcium ions combine with the troponin in the thin filaments, and troponin is stopped from inhibiting the myosin-actin interaction. This allows the thick and thin filaments to interact, and contraction takes place. When the "command" for contraction ceases, the release of calcium ions ends and, by an active process using the energy from ATP splitting, calcium ions are pumped back into the sarcoplasmic reticulum. As this process lowers the level of calcium ions in the sarcoplasm, the calcium ions dissociate from the troponin and the myosin-actin interaction is inhibited. As a result, tension produced by the muscle diminishes, and it relaxes back to the resting state.
Sliding Filament TheoryA major advance in the understanding of muscular contraction was the realization that the thick and thin filaments slide past each other during contraction. This "sliding filament" theory was proposed independently by A.F. Huxley and R. Niedergerke, and by H.E. Huxley and Jean Hanson in the early 1950s. Essentially, the theory holds that the filaments do not change in length during contraction. Instead, a muscle shortens when the thin filaments slide over the thick ones, so that they penetrate farther into the region of the thick filaments (A) band. The region containing only thin filaments (I band) becomes smaller. The size of the A band remains constant and equal to the length of the thick filaments.
A part of each myosin molecule in the thick filament is able to bind to actin in the thin filament and to act as an enzyme to catalyze the splitting ATP. This region of the myosin molecule protrudes from the main backbone of the thick filament and forms what is called a cross-bridge. It appears that during contraction a cross-bridge splits a molecule of ATP and stores the energy from this reaction. The cross-bridge then interacts with actin, forming a physical link between them; this link then changes shape so that the filaments slide past each other to produce shortening and tension. At this step, the energy from ATP splitting appears as mechanical work being done by the muscle. The link between the myosin and actin then breaks. The distance that the filaments move in one such cycle is very small compared to the total shortening that can occur. Thus each cross-bridge goes through this cycle repeatedly in order to shorten the muscle a large distance.
Fast and Slow Muscle FibersThe energy which produces the calcium flow in the muscle fibers comes from "mitochondria", the part of the muscle cell that converts glucose (blood sugar) into energy. Different types of muscle fibers have different amounts of mitochondria. The more mitochondria in a muscle fiber, the more energy it is able to produce. Muscle fibers are categorized into "slow-twitch fibers" and "fast-twitch fibers". Slow twitch fibers (also called "type 1 muscle fibers") are slow to contract, but they are also very slow to fatigue. Fast-twitch fibers are very quick to contract and come in two varieties: "Type 2A muscle fibers" which fatigue at an intermediate rate, and "Type 2B muscle fibers" which fatigue very quickly. The main reason the slow-twitch fibers are slow to fatigue is that they contain more mitochondria than fast-twitch fibers and hence are able to produce more energy. Slow-twitch fibers are also smaller in diameter than fast-twitch fibers and have increased capillary blood flow around them. Because they have a smaller diameter and an increased blood flow, the slow twitch fibers are able to deliver more oxygen and remove more waste products from the muscle fibers (which decreases their "fatigability").
These three muscle fiber types (Types 1, 2A, and 2B) are contained in all muscles in varying amounts. Muscles that need to be contracted much of the time (like the heart) have a greater number of Type 1 (slow) fibers. When a muscle first starts to contract, it is primarily Type 1 fibers that are initially activated, then Type 2A and Type 2B fibers are activated (if needed) in that order. The fact that muscle fibers are "recruited" in this sequence is what provides the ability to execute brain commands with such fine-tuned muscle responses. It also makes the Type 2B fibers difficult to train because they are not activated until most of the Type 1 and Type 2A fibers have been recruited.
ATP Splitting and Metabolic ReactionsThe splitting of ATP to form adenosine diphosphate (ADP) and inorganic phosphate has a central role in the metabolism of living systems. It is of critical importance in the functioning of muscle, because this is the only reaction that can directly supply energy to myosin and actin for normal muscular contraction. Thus it is essential that the muscle have an adequate supply of ATP to fuel the contractile process. This requirement is met by the actual rebuilding of ATP as it is used rather than by storing a very large amount of ATP in the muscle.
During brief contractions, ATP is rebuilt by the transfer of a phosphate group from phosphocreatine to ADP to form ATP and creatine. This reaction is catalyzed by the enzyme creatine phosphokinase; it is so rapid and complete that not until 1962 was it finally proved experimentally by D.F. Cain and R.E. Davies that ATP is split during contraction of intact muscle. If contractions last more than a few seconds, ATP is supplied by a more complex set of reactions, including the glycolytic and oxidative metabolic pathways. The original starting materials for all metabolic reactions are the food ingested and the oxygen breathed.
Types of Muscle ContractionsThe contraction of a muscle does not necessarily imply that the muscle shortens; it only means that tension has been generated. Muscles can contract in the following ways:
This is a contraction in which no movement takes place, because the load on the muscle exceeds the tension generated by the contracting muscle. This occurs when a muscle attempts to push or pull an immovable object.
This is a contraction in which movement does take place, because the tension generated by the contracting muscle exceeds the load on the muscle. This occurs when you use your muscles to successfully push or pull an object.
Isotonic contractions are further divided into two types:
A contraction in which the muscle decreases in length against an opposing load, such as lifting a weight up.
A contraction in which the muscle increases in length as it resists a load, such as pushing something down.
During a concentric contraction, the muscles that are shortening serve as the agonists and hence do all the work. During an eccentric contraction the muscles that are lengthening serve as the agonists (and do all of the work).
Reciprocal InhibitionWhen an agonist contracts, in order to cause the desired motion, it usually forces the antagonists to relax. This phenomenon is called "reciprocal inhibition" because the antagonists are inhibited from contracting. This is sometimes called "reciprocal innervation" but that term is really a misnomer since it is the agonists which (relax) the antagonists. The antagonists do not actually innervate (cause the contraction of) the agonists.
Such inhibition of the antagonistic muscles is not necessarily required. In fact, co-contraction can occur. When you perform a sit-up, one would normally assume that the stomach muscles inhibit the contraction of the muscles in the lumbar, or lower, region of the back. In this particular instance however, the back muscles (spinal erectors) also contract. This is one reason why sit-ups are good for strengthening the back as well as the stomach.
When stretching, it is easier to stretch a muscle that is relaxed than to stretch a muscle that is contracting. By taking advantage of the situations when reciprocal inhibition does occur, you can get a more effective stretch by inducing the antagonists to relax during the stretch due to the contraction of the agonists. You also want to relax any muscles used as synergists by the muscle you are trying to stretch. For example, when you stretch your calf, you want to contract the shin muscles (the antagonists of the calf) by flexing your foot. However, the hamstrings use the calf as a synergist so you want to also relax the hamstrings by contracting the quadriceps (keeping your leg straight).
Exercise Part 1
Exercise Part 2
Exercise Part 4
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