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Old 05-12-2006, 07:24 PM
EricT EricT is offline
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Section: 1.6.1 Proprioceptors
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The nerve endings that relay all the information about the musculoskeletal system to the central nervous system are called "proprioceptors". Proprioceptors (also called "mechanoreceptors") are the source of all "proprioception": the perception of one's own body position and movement. The proprioceptors detect any changes in physical displacement (movement or position) and any changes in tension, or force, within the body. They are found in all nerve endings of the joints, muscles, and tendons. The proprioceptors related to stretching are located in the tendons and in the
muscle fibers.

There are two kinds of muscle fibers: "intrafusal muscle fibers" and "extrafusal muscle fibers". Extrafusil fibers are the ones that contain myofibrils (see Section 1.2 [Muscle Composition]) and are what is usually meant when we talk about muscle fibers. Intrafusal fibers are also called "muscle spindles" and lie parallel to the extrafusal fibers. Muscle spindles, or "stretch receptors", are the primary proprioceptors in the muscle. Another proprioceptor that comes into play during stretching is located in the tendon near the end of the muscle fiber and is called the
"golgi tendon organ". A third type of proprioceptor, called a "pacinian corpuscle", is located close to the golgi tendon organ and is responsible for detecting changes in movement and pressure within the body.

When the extrafusal fibers of a muscle lengthen, so do the intrafusal fibers (muscle spindles). The muscle spindle contains two different types of fibers (or stretch receptors) which are sensitive to the change in muscle length and the rate of change in muscle length. When muscles contract it places tension on the tendons where the golgi tendon organ is located. The golgi tendon organ is sensitive to the change in tension and the rate of change of the tension.

Section: 1.6.2 The Stretch Reflex
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When the muscle is stretched, so is the muscle spindle (see Section 1.6.1 [Proprioceptors]). The muscle spindle records the change in length (and how fast) and sends signals to the spine which convey this information. This triggers the "stretch reflex" (also called the "myotatic reflex") which attempts to resist the change in muscle length by causing the stretched muscle to contract. The more sudden the change in muscle length, the stronger the muscle contractions will be (plyometric, or "jump", training is based on this fact). This basic function of the muscle spindle helps to maintain muscle tone and to protect the body from injury.

One of the reasons for holding a stretch for a prolonged period of time is that as you hold the muscle in a stretched position, the muscle spindle habituates (becomes accustomed to the new length) and reduces its signaling. Gradually, you can train your stretch receptors to allow greater lengthening of the muscles.

Some sources suggest that with extensive training, the stretch reflex of certain muscles can be controlled so that there is little or no reflex contraction in response to a sudden stretch. While this type of control provides the opportunity for the greatest gains in flexibility, it also provides the greatest risk of injury if used improperly. Only consummate professional athletes and dancers at the top of their sport (or art) are
believed to a actually possess this level of muscular control.

Section: 1.6.2.1 Components of the Stretch Reflex
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The stretch reflex has both a dynamic component and a static component. The static component of the stretch reflex persists as long as the muscle is being stretched. The dynamic component of the stretch reflex (which can be very powerful) lasts for only a moment and is in response to the initial sudden increase in muscle length. The reason that the stretch reflex has two components is because there are actually two kinds of intrafusal muscle fibers: "nuclear chain fibers", which are responsible for the static component; and "nuclear bag fibers", which are responsible for the dynamic component.

Nuclear chain fibers are long and thin, and lengthen steadily when stretched. When these fibers are stretched, the stretch reflex nerves increase their firing rates (signaling) as their length steadily increases. This is the static component of the stretch reflex.

Nuclear bag fibers bulge out at the middle, where they are the most elastic. The stretch-sensing nerve ending for these fibers is wrapped around this middle area, which lengthens rapidly when the fiber is stretched. The outer-middle areas, in contrast, act like they are filled with viscous fluid; they resist fast stretching, then gradually extend under prolonged tension. So, when a fast stretch is demanded of these fibers, the middle takes most of the stretch at first; then, as the outer-middle parts extend, the middle can shorten somewhat. So the nerve that senses stretching in these fibers fires rapidly with the onset of a fast stretch, then slows as the middle section of the fiber is allowed to shorten again. This is the dynamic component of the stretch reflex: a strong signal to contract at the onset of a rapid increase in muscle length, followed by slightly "higher than normal" signaling which gradually decreases as the rate of change of the muscle length decreases.

Section: 1.6.3 The Lengthening Reaction
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When muscles contract (possibly due to the stretch reflex), they produce tension at the point where the muscle is connected to the tendon, where the golgi tendon organ is located. The golgi tendon organ records the change in tension, and the rate of change of the tension, and sends signals to the spine to convey this information (see Section 1.6.1 [Proprioceptors]). When this tension exceeds a certain threshold, it triggers the "lengthening reaction" which inhibits the muscles from contracting and causes them to relax. Other names for this reflex are the "inverse myotatic reflex",
"autogenic inhibition", and the "clasped- reflex". This basic function of the golgi tendon organ helps to protect the muscles, tendons, and ligaments from injury. The lengthening reaction is possible only because the signaling of golgi tendon organ to the spinal cord is powerful enough to overcome the signaling of the muscle spindles telling the muscle to contract.

Another reason for holding a stretch for a prolonged period of time is to allow this lengthening reaction to occur, thus helping the stretched muscles to relax. It is easier to stretch, or lengthen, a muscle when it is not trying to contract.

Section: 1.6.4 Reciprocal Inhibition
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When an agonist contracts, in order to cause the desired motion, it usually forces the antagonists to relax (see Section 1.4 [Cooperating Muscle Groups]). 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 inhibit (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 quadricep (i.e., keeping your leg straight).

Section: 2 Flexibility
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Flexibility is defined by Gummerson as "the absolute range of movement in a joint or series of joints that is attainable in a momentary effort with the help of a partner or a piece of equipment." This definition tells us that flexibility is not something general but is specific to a particular joint or set of joints. In other words, it is a myth that some people are innately flexible throughout their entire body. Being flexible in one
particular area or joint does not necessarily imply being flexible in another. Being "loose" in the upper body does not mean you will have a "loose" lower body. Furthermore, according to `SynerStretch', flexibility in a joint is also "specific to the action performed at the joint (the ability to do front splits doesn't imply the ability to do side splits even though both actions occur at the hip)."

Section: 2.1 Types of Flexibility
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Many people are unaware of the fact that there are different types of flexibility. These different types of flexibility are grouped according to the various types of activities involved in athletic training. The ones which involve motion are called "dynamic" and the ones which do not are called "static". The different types of flexibility (according to Kurz) are:

"dynamic flexibility"
Dynamic flexibility (also called "kinetic flexibility") is the ability to perform dynamic (or kinetic) movements of the muscles to bring a limb through its full range of motion in the joints.

"static-active flexibility"
Static-active flexibility (also called "active flexibility") is the ability to assume and maintain extended positions using only the tension of the agonists and synergists while the antagonists are being stretched (see Section 1.4 [Cooperating Muscle Groups]). For example, lifting the leg and keeping it high without any external support (other than from your own leg muscles).

"static-passive flexibility"
Static-passive flexibility (also called "passive flexibility") is the ability to assume extended positions and then maintain them using only your weight, the support of your limbs, or some other apparatus (such as a chair or a barre). Note that the ability to maintain the position does not come solely from your muscles, as it does with static-active flexibility. Being able to perform the splits is an example of static-passive flexibility.

Research has shown that active flexibility is more closely related to the level of sports achievement than is passive flexibility. Active flexibility is harder to develop than passive flexibility (which is what most people think of as "flexibility"); not only does active flexibility require passive flexibility in order to assume an initial extended position, it also requires muscle strength to be able to hold and maintain that position.

Section: 2.2 Factors Limiting Flexibility
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According to Gummerson, flexibility (he uses the term "mobility") is affected by the following factors:

* Internal influences

- the type of joint (some joints simply aren't meant to be flexible)

- the internal resistance within a joint

- bony structures which limit movement

- the elasticity of muscle tissue (muscle tissue that is scarred due to a previous injury is not very elastic)

-the elasticity of tendons and ligaments (ligaments do not stretch much and tendons should not stretch at all)

- the elasticity of skin (skin actually has some degree of elasticity, but not much)

- the ability of a muscle to relax and contract to achieve the greatest range of movement

- the temperature of the joint and associated tissues (joints and muscles offer better flexibility at body temperatures that are 1 to 2 degrees higher than normal)

* External influences

- the temperature of the place where one is training (a warmertemperature is more conducive to increased flexibility)

- the time of day (most people are more flexible in the afternoon than in the morning, peaking from about 2:30pm-4pm)

- the stage in the recovery process of a joint (or muscle) after injury (injured joints and muscles will usually offer a lesser degree of flexibility than healthy ones)

- age (pre-adolescents are generally more flexible than adults)

- gender (females are generally more flexible than males)

- one's ability to perform a particular exercise (practice makes perfect)

- one's commitment to achieving flexibility

- the restrictions of any clothing or equipment

Some sources also the suggest that water is an important dietary element with regard to flexibility. Increased water intake is believed to contribute to increased mobility, as well as increased total body relaxation.

Rather than discuss each of these factors in significant detail as Gummerson does, I will attempt to focus on some of the more common factors which limit one's flexibility. According to `SynerStretch', the most common factors are: bone structure, muscle mass, excess fatty tissue, and connective tissue (and, of course, physical injury or disability).

Depending on the type of joint involved and its present condition (is it healthy?), the bone structure of a particular joint places very noticeable limits on flexibility. This is a common way in which age can be a factor limiting flexibility since older joints tend not to be as healthy as younger ones.

Muscle mass can be a factor when the muscle is so heavily developed that it interferes with the ability to take the adjacent joints through their complete range of motion (for example, large hamstrings limit the ability to fully bend the knees). Excess fatty tissue imposes a similar restriction.

The majority of "flexibility" work should involve performing exercises designed to reduce the internal resistance offered by soft connective tissues (see Section 1.3 [Connective Tissue]). Most stretching exercises attempt to accomplish this goal and can be performed by almost anyone, regardless of age or gender.

Section: 2.2.1 How Connective Tissue Affects Flexibility
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The resistance to lengthening that is offered by a muscle is dependent upon its connective tissues: When the muscle elongates, the surrounding connective tissues become more taut (see Section 1.3 [Connective Tissue]). Also, inactivity of certain muscles or joints can cause chemical changes in connective tissue which restrict flexibility. According to M. Alter, each type of tissue plays a certain role in joint stiffness: "The joint capsule (i.e., the saclike structure that encloses the ends of bones) and ligaments are the most important factors, accounting for 47 percent of the stiffness, followed by the muscle's fascia (41 percent), the tendons (10 percent), and skin (2 percent)".

M. Alter goes on to say that efforts to increase flexibility should be directed at the muscle's fascia however. This is because it has the most elastic tissue, and because ligaments and tendons (since they have less elastic tissue) are not intended to stretched very much at all. Overstretching them may weaken the joint's integrity and cause destabilization (which increases the risk of injury).

When connective tissue is overused, the tissue becomes fatigued and may tear, which also limits flexibility. When connective tissue is unused or under used, it provides significant resistance and limits flexibility. The elastin begins to fray and loses some of its elasticity, and the collagen increases in stiffness and in density. Aging has some of the same effects on connective tissue that lack of use has.

Section: 2.2.2 How Aging Affects Flexibility
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With appropriate training, flexibility can, and should, be developed at all ages. This does not imply, however, that flexibility can be developed at the same rate by everyone. In general, the older you are, the longer it will take to develop the desired level of flexibility. Hopefully, you'll be more patient if you're older.

According to M. Alter, the main reason we become less flexible as we get older is a result of certain changes that take place in our connective tissues. As we age, our bodies gradually dehydrate to some extent. It is believed that "stretching stimulates the production or retention of lubricants between the connective tissue fibers, thus preventing the formation of adhesions". Hence, exercise can delay some of the loss of flexibility that occurs due to the aging process.

M. Alter further states that some of the physical changes attributed to aging are the following:

* An increased amount of calcium deposits, adhesions, and cross-links in the body

* An increase in the level of fragmentation and dehydration

* Changes in the chemical structure of the tissues.

* Loss of "suppleness" due to the replacement of muscle fibers with fatty collagenous fibers.

This does *not* mean that you should give up trying to achieve flexibility if you are old or inflexible. It just means that you need to work harder, and more carefully, for a longer period of time when attempting to increase flexibility. Increases in the ability of muscle tissues and connective tissues to elongate (stretch) can be achieved at any age.

Section: 2.3 Strength and Flexibility
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Strength training and flexibility training should go hand in hand. It is a common misconception that there must always be a trade-off between flexibility and strength. Obviously, if you neglect flexibility training altogether in order to train for strength then you are certainly sacrificing flexibility (and vice versa). However, performing exercises for both strength and flexibility need not sacrifice either one. As a
matter of fact, flexibility training and strength training can actually enhance one another.

(To be continued)
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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 02:28 PM.
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