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Old 05-12-2006, 05:39 PM
EricT EricT is offline
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Section: 1 Physiology of Stretching
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The purpose of this chapter is to introduce you to some of the basic physiological concepts that come into play when a muscle is stretched. Concepts will be introduced initially with a general overview and then (for those who want to know the gory details) will be discussed in further detail. If you aren't all that interested in this aspect of stretching, you can skip this chapter. Other sections will refer to important concepts from this chapter and you can easily look them up on a "need to know" basis.

Section: 1.1 The Musculoskeletal System
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Together, muscles and bones comprise what is called the "musculoskeletal system" of the body. 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. Muscles are very unique in this respect.

Section: 1.2 Muscle Composition
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Muscles vary in shape and in size, and serve many different purposes. Most large muscles, like the hamstrings and quadriceps, control motion. Other muscles, like the heart, and the muscles of the inner ear, perform other functions. At the microscopic level however, all muscles share the same basic structure.

At the highest level, the (whole) muscle is composed of many strands of tissue called "fascicles". These are the strands of muscle that we see whenwe cut red meat or poultry. Each fascicle is composed of "fasciculi" whichare bundles of "muscle fibers". The muscle fibers are in turn composed oftens of thousands of thread-like myofybrils", which can contract, relax,and elongate (lengthen). The myofybrils are (in turn) composed of up tomillions of bands laid end-to-end called "sarcomeres". Each sarcomere is made of overlapping thick and thin filaments called "myofilaments". The thick and thin myofilaments are made up of "contractile proteins",primarily actin and myosin.

Section: 1.2.1 How Muscles Contract
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The way in which all these various levels of the muscle operate is as follows: 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 the neuromuscular 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.

When a muscle fiber contracts, it contracts completely. There is no such thing as a partially contracted muscle fiber. Muscle fibers are unable to vary the intensity of their contraction relative to the load against which they are acting. If this is so, then how does the force of a muscle contraction vary in strength from strong to weak? What happens is that more muscle fibers are recruited, as they are needed, to perform the job at hand. The more muscle fibers that are recruited by the central nervous system, the stronger the force generated by the muscular contraction.

Section: 1.2.2 Fast and Slow Muscle Fibers
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The 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
Varietie "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 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.

`HFLTA' states that the the best way to remember the difference between muscles with predominantly slow-twitch fibers and muscles with predominantly fast-twitch fibers is to think of "white meat" and "dark meat". Dark meat is dark because it has a greater number of slow-twitch muscle fibers and hence a greater number of mitochondria, which are dark. White meat consists mostly of muscle fibers which are at rest much of the time but are frequently called on to engage in brief bouts of intense activity. This muscle tissue can contract quickly but is fast to fatigue and slow to recover. White meat is lighter in color than dark meat because it contains fewer mitochondria.

Section: 1.3 Connective Tissue
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Located all around the muscle and its fibers are "connective tissues". Connective tissue is composed of a base substance and two kinds of protein based fiber. The two types of fiber are "collagenous connective tissue" and "elastic connective tissue". Collagenous connective tissue consists mostly of collagen (hence its name) and provides tensile strength. Elastic connective tissue consists mostly of elastin and (as you might guess from its name) provides elasticity. The base substance is called "mucopolysaccharide" and acts as both a lubricant (allowing the fibers to easily slide over one another), and as a glue (holding the fibers of the tissue together into bundles). The more elastic connective tissue there is around a joint, the greater the range of motion in that joint. Connective tissues are made up of tendons, ligaments, and the fascial sheaths that envelop, or bind down, muscles into separate groups. These fascial sheaths, or "fascia", are named according to where they are located in the muscles:

"endomysium"
The innermost fascial sheath that envelops individual muscle fibers.

"perimysium"
The fascial sheath that binds groups of muscle fibers into individual
fasciculi (see Section 1.2 [Muscle Composition]).

"epimysium"
The outermost fascial sheath that binds entire fascicles (see Section 1.2 [Muscle Composition]).

These connective tissues help provide suppleness and tone to the muscles.

Section: 1.4 Cooperating Muscle Groups
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When muscles cause a limb to move through the joint's range of motion, they usually act in the following cooperating groups:

"agonists"
These muscles cause the movement to occur. They create the normal range of movement in a joint by contracting. Agonists are also referred to as "prime movers" since they are the muscles that are primarily responsible for generating the movement.

"antagonists"
These muscles act in opposition to the movement generated by the agonists and are responsible for returning a limb to its initial
position.

"synergists"
These muscles perform, or assist in performing, the same set of joint motion as the agonists. Synergists are sometimes referred to as "neutralizers" because they help cancel out, or neutralize, extra motion from the agonists to make sure that the force generated works within the desired plane of motion.

"fixators"
These muscles provide the necessary support to assist in holding the rest of the body in place while the movement occurs. Fixators are also sometimes called "stabilizers".

As an example, when you flex your knee, your hamstring contracts, and, to some extent, so does your gastrocnemius (calf) and lower buttocks. Meanwhile, your quadriceps are inhibited (relaxed and lengthened somewhat) so as not to resist the flexion (see Section 1.6.4 [Reciprocal Inhibition]). In this example, the hamstring serves as the agonist, or prime mover; the quadricep serves as the antagonist; and the calf and lower buttocks serve as the synergists. Agonists and antagonists are usually located on opposite sides of the affected joint (like your hamstrings and quadriceps, or your triceps and biceps), while synergists are usually located on the same side of the joint near the agonists. Larger muscles often call upon their smaller neighbors to function as synergists.

The following is a list of commonly used agonist/antagonist muscle pairs:

* pectorals/latissimus dorsi (pecs and lats)

* anterior deltoids/posterior deltoids (front and back shoulder)

* trapezius/deltoids (traps and delts)

* abdominals/spinal erectors (abs and lower-back)

* left and right external obliques (sides)

* quadriceps/hamstrings (quads and hams)

* shins/calves

* biceps/triceps

* forearm flexors/extensors

Section: 1.5 Types of Muscle Contractions
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The 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:

"isometric contraction"
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.

"isotonic contraction"
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:

"concentric contraction"
This is a contraction in which the muscle decreases in length (shortens) against an opposing load, such as lifting a weight up.

"eccentric contraction"
This is a contraction in which the muscle increases in length (lengthens) as it resists a load, such as lowering a weight down in a slow, controlled fashion.

During a concentric contraction, the muscles that are shortening serve as the agonists and hence do all of the work. During an eccentric contraction the muscles that are lengthening serve as the agonists (and do all of the work). See Section 1.4 [Cooperating Muscle Groups].

Section: 1.6 What Happens When You Stretch
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The stretching of a muscle fiber begins with the sarcomere (see Section 1.2 [Muscle Composition]), the basic unit of contraction in the muscle fiber. As the sarcomere contracts, the area of overlap between the thick and thin myofilaments increases. As it stretches, this area of overlap decreases, allowing the muscle fiber to elongate. Once the muscle fiber is at its maximum resting length (all the sarcomeres are fully stretched), additional stretching places force on the surrounding connective tissue (see Section 1.3 [Connective Tissue]). As the tension increases, the collagen fibers in
the connective tissue align themselves along the same line of force as the tension. Hence when you stretch, the muscle fiber is pulled out to its full length sarcomere by sarcomere, and then the connective tissue takes up the remaining slack. When this occurs, it helps to realign any disorganized fibers in the direction of the tension. This realignment is what helps to rehabilitate scarred tissue back to health.

When a muscle is stretched, some of its fibers lengthen, but other fibers may remain at rest. The current length of the entire muscle depends upon the number of stretched fibers (similar to the way that the total strength of a contracting muscle depends on the number of recruited fibers contracting). According to `SynerStretch' you should think of "little pockets of fibers distributed throughout the muscle body stretching, and other fibers simply going along for the ride". The more fibers that are
stretched, the greater the length developed by the stretched muscle.
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If you act sanctimonious I will just list out your logical fallacies until you get pissed off and spew blasphemous remarks.

Last edited by EricT; 05-25-2006 at 01:26 PM.
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