Understanding Stretching & Flexibility

This paper is a modest attempt to answer some frequently asked questions about stretching and flexibility. It is organized into sections covering the following topics:

• Physiology of Stretching
• Flexibility
• Types of Stretching

Physiology of Stretching

The purpose of this section 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 will be discussed in further detail.

• The Musculoskeletal System
• Muscle Composition
• Connective Tissue
• Co-operating Muscle Groups
• Types of Muscle Contractions
• What happens When You Stretch?

The Musculoskeletal System

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.

Muscle Composition

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 muscle is composed of many strands of tissue called fascicles. Each fascicle is composed of fasciculi which are bundles of muscle fibres. The muscle fibres are in turn composed of tens of thousands of thread-like myofibrils, which can contract, relax, and lengthen. The myofibrils are composed of up to millions 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.
How Muscles Contract

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 fibres. Inside the muscle fibres, 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 fibre.

A muscle fibre contracts completely. There is no such thing as a partially contracted muscle fibre. Muscle fibres 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 fibres are recruited, as they are needed, to perform the job at hand. The more muscle fibres recruited by the central nervous system, the stronger the force generated by the muscular contraction.

Fast and Slow Muscle Fibres

The energy which produces the calcium flow in the muscle fibres comes from mitochondria, the part of the muscle cell that converts glucose (blood sugar) into energy. Different types of muscle fibres have different amounts of mitochondria. The more mitochondria in a muscle fibre, the more energy it is able to produce. Muscle fibres are categorized into slow-twitch fibres and fast-twitch fibres.

Slow-twitch fibres (Type 1) are slow to contract, but they are also very slow to fatigue. Fast-twitch fibres are very quick to contract and come in two varieties: Type 2A muscle fibres which fatigue at an intermediate rate, and Type 2B muscle fibres which fatigue very quickly. The main reason the slow-twitch fibres are slow to fatigue is that they contain more mitochondria than fast-twitch fibres and hence are able to produce more energy. Slow-twitch fibres are also smaller in diameter than fast-twitch fibres and have increased capillary blood flow around them. Because they have a smaller diameter and an increased blood flow, the slow-twitch fibres are able to deliver more oxygen and remove more waste products from the muscle fibres.

These three muscle fibre 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) fibres. According to Health for Life Training Advisor:

When a muscle begins to contract, primarily Type 1 fibres are activated first, and then Type 2A, then 2B. This sequence of fibre recruitment allows very delicate and finely tuned muscle responses to brain commands. It also makes Type 2B fibres difficult to train; most of the Type 1 and 2A fibres have to be activated already before a large percentage of the 2B fibres participate.

Health for Life Training Advisor further states that the best way to remember the difference between muscles with predominantly slow-twitch fibres and muscles with predominantly fast-twitch fibres is to think of “white meat” and “dark meat”. Dark meat is dark because it has a greater number of slow-twitch muscle fibres and hence a greater number of mitochondria, which are dark. White meat consists mostly of muscle fibres 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 colour than dark meat because it contains fewer mitochondria.

Connective Tissue

Located all around the muscle and its fibres are connective tissues. Connective tissue is composed of a base substance and two kinds of protein based fibre. The two types of fibre are collagenous connective tissue and elastic connective tissue. Collagenous connective tissue consists mostly of collagen and provides tensile strength. Elastic connective tissue consists mostly of elastin and provides elasticity. The base substance is called mucopolysaccharide and acts as both a lubricant, allowing the fibres to easily slide over one another, and as glue, holding the fibres 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 fibres.
• Perimysium – The fascial sheath that binds groups of muscle fibres into individual fasciculi.
• Epimysium – The outermost fascial sheath that binds entire fascicles.

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

Co-operating Muscle Groups

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.

When you flex your knee for example, your hamstring contracts, and, to some extent, so does your gastrocnemius and lower buttocks. Meanwhile, your quadriceps are inhibited (relaxed and lengthened somewhat) so as not to resist the flexion (see section Reciprocal Inhibition). In this example, the hamstring serves as the agonist, or prime mover; the quadriceps 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 neighbours 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

Types of Muscle Contractions

The contraction of a muscle does not only mean that the muscle shortens; it 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.
• 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.
• 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 pushing something down.
What Happens When You Stretch?

The stretching of a muscle fibre begins with the sarcomere, the basic unit of contraction in the muscle fibre. 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 fibre to elongate. Once the muscle fibre is at its maximum resting length, additional stretching places force on the surrounding connective tissue. As the tension increases, the collagen fibres in the connective tissue align themselves along the same line of force as the tension. Hence when you stretch, the muscle fibre 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 fibres 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 fibres lengthen, but other fibres may remain at rest. The current length of the entire muscle depends upon the number of stretched fibres.
Picture little pockets of fibres distributed throughout the muscle body stretching, and other fibres simply going along for the ride. Just as the total strength of a contracting muscle is a result of the number of fibres contracting, the total length of a stretched muscle is a result of the number of fibres stretched — the more fibres stretched, the more length developed by the muscle for a given stretch.

• Proprioceptors
• The Stretch Reflex
• The Lengthening Reaction
• Reciprocal Inhibition

Proprioceptors

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 fibres.

There are two kinds of muscle fibres: intrafusal muscle fibres and extrafusal muscle fibres. Extrafusal fibres are the ones that contain myofibrils and are what is usually meant when we talk about muscle fibres. Intrafusal fibres are also called muscle spindles and lie parallel to the extrafusal fibres. 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 fibre 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 fibres of a muscle lengthen, so do the intrafusal fibres. The muscle spindle contains two different types of fibres (or stretch receptors) which are sensitive to the change in muscle length and the rate of change in muscle length. When muscles contract, tension is placed 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.

The Stretch Reflex

When the muscle is stretched, so is the muscle spindle. The muscle spindle records the change in length (and speed) and sends signals to the spine which convey this information. This triggers the stretch reflex (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 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 and reduces its signalling. 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. Professional athletes and martial artists at the top of their sport are believed to actually possess this level of muscular control.

Components of the Stretch Reflex

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 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 fibres: nuclear chain fibres, which are responsible for the static component; and nuclear bag fibres, which are responsible for the dynamic component.
Nuclear chain fibres are long and thin, and lengthen steadily when stretched. When these fibres are stretched, the stretch reflex nerves increase their firing rates as their length steadily increases. This is the static component of the stretch reflex.

Nuclear bag fibres bulge out at the middle, where they are the most elastic. The stretch-sensing nerve ending for these fibres is wrapped around this middle area, which lengthens rapidly when the fibre is stretched. The outer-middle areas, in contrast, act like they are filled with viscous fluid; they resist fast stretching, gradually extend under prolonged tension. So, when a fast stretch is demanded of these fibres, 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 fibres fires rapidly with the onset of a fast stretch, then slows as the middle section of the fibre 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” signalling which gradually decreases as the rate of change of the muscle length decreases.

The Lengthening Reaction

When muscles contract, 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. 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-knife 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 signalling of Golgi tendon organ to the spinal cord is powerful enough to overcome the signalling 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.

Reciprocal Inhibition

When 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 inhibit the antagonists. The antagonists do not actually innervate 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 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.

Flexibility

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 are:

• 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. For example, lifting the leg and keeping it high without any external support.
• 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). 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.

Factors Limiting Flexibility

Flexibility (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 warmer temperature 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 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, I will attempt to focus on some of the more common factors which limit one’s flexibility. The most common factors are:

• Bone structure.
• Muscle mass.
• Excess fatty tissue.
• Connective tissue.
• Physical injury or disability.

Depending on the type of joint involved and its present condition, 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. Most stretching exercises attempt to accomplish this goal and can be performed by almost anyone, regardless of age or gender.
How Connective Tissue Affects Flexibility

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 tauter. Also, inactivity of certain muscles or joints can cause chemical changes in connective tissue which restrict flexibility. To quote Michael J Alter directly:
“A question of great interest to all athletes is the relative importance of various tissues 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). However, most efforts to increase flexibility through stretching should be directed to the muscle fascia. The reasons for this are twofold. First, muscle and its fascia have more elastic tissue, so they are more modifiable in terms of reducing resistance to elongation. Second, because ligaments and tendons have less elasticity than fascia, it is undesirable to produce too much slack in them. Overstretching these structures may weaken the integrity of joints. As a result, an excessive amount of flexibility may destabilize the joints and increase an athlete’s 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.

How Flexibility Affects Ageing

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 Michael J 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:

“The primary factor responsible for the decline of flexibility with age is certain changes that occur in the connective tissues of the body. Interestingly, it has been suggested that exercise can delay the loss of flexibility due to the aging process of dehydration. This is based on the notion that stretching stimulates the production or retention of lubricants between the connective tissue fibres, thus preventing the formation of adhesions.”

Michael J 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 fibres with fatty, collagenous fibres.

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 can be achieved at any age.

Strength and Flexibility

One of the best times to stretch is right after a strength workout such as weightlifting. Static stretching of fatigued muscles performed immediately following the exercise(s) that caused the fatigue, helps not only to increase flexibility, but also enhances the promotion of muscular development, and will actually help decrease the level of post-exercise soreness. Here’s why: After you have used weights (or other stimuli) to overload and fatigue your muscles, your muscles retain a “pump” and are shortened somewhat. This “shortening” is due mostly to the repetition of intense muscle activity that often only takes the muscle through part of its full range of motion. This “pump” makes the muscle appear bigger. The “pumped” muscle is also full of lactic acid and other by-products from exhaustive exercise. If the muscle is not stretched afterward, it will retain this decreased range of motion and the build-up of lactic acid will cause post-exercise soreness. Static stretching of the “pumped” muscle helps it to become “looser”, and to “remember” its full range of movement. It also helps to remove lactic acid and other waste-products from the muscle. While it is true that stretching the “pumped” muscle will make it appear visibly smaller, it does not decrease the muscle’s size or inhibit muscle growth. It merely reduces the “tightness” (contraction) of the muscles so that they do not “bulge” as much.

Also, strenuous workouts will often cause damage to the muscle’s connective tissue. The tissue heals in 1 to 2 days but it is believed that the tissues heal at a shorter length (decreasing muscular development as well as flexibility). To prevent the tissues from healing at a shorter length, physiologists recommend static stretching after strength workouts.

Why Contortionists Should Strengthen

You should be “tempering” (or balancing) your flexibility training with strength training (and vice versa). Do not perform stretching exercises for a given muscle group without also performing strength exercises for that same group of muscles. Judy Alter, in her book Stretch and Strengthen, recommends stretching muscles after performing strength exercises, and performing strength exercises for every muscle you stretch. In other words: “Strengthen what you stretch, and stretch what you strengthen!”

The reason for this is that flexibility training on a regular basis causes connective tissues to stretch which in turn causes them to loosen and elongate. When the connective tissue of a muscle is weak, it is more likely to become damaged due to overstretching, or sudden, powerful muscular contractions. The likelihood of such injury can be prevented by strengthening the muscles bound by the connective tissue. Dynamic strength training consisting of light dynamic exercises with weights (lots of reps, not too much weight), and isometric tension exercises. If you also lift weights, dynamic strength training for a muscle should occur before subjecting that muscle to an intense weightlifting workout. This helps to pre-exhaust the muscle first, making it easier (and faster) to achieve the desired overload in an intense strength workout. Attempting to perform dynamic strength training after an intense weightlifting workout would be largely ineffective.

If you are working on increasing (or maintaining) flexibility, then it is very important that your strength exercises force your muscles to take the joints through their full range of motion. Repeating movements that do not use a full range of motion in the joints (e.g., bicycling, certain techniques of Olympic weightlifting, pushups) can cause a shortening of the muscles surrounding the joints of the working limbs. This shortening is a result of setting the nervous control of length and tension in the muscles at the values repeated most often or most strongly. Stronger stimuli are remembered better.

Over flexibility

It is possible for the muscles of a joint to become too flexible. There is a trade-off between flexibility and stability. The looser you get, the less support offered to the joints by their adjacent muscles. Excessive flexibility can be just as much of a liability as not enough flexibility. Either one increases your risk of injury.

Once a muscle has reached its absolute maximum length, attempting to stretch the muscle further only serves to stretch the ligaments and put undue stress upon the tendons (two things that you do not want to stretch). Ligaments will tear when stretched more than 6% of their normal length. Tendons are not even supposed to be able to lengthen. Even when stretched ligaments and tendons do not tear, loose joints and/or a decrease in the joint’s stability can occur vastly increasing the risk of injury.

Once you have achieved the desired level of flexibility for a muscle or set of muscles and have maintained that level for a solid week, you should discontinue any isometric or PNF stretching of that muscle until some of its flexibility is lost.

Types of Stretching

Just as there are different types of flexibility, there are also different types of stretching. Stretches are either dynamic (meaning they involve motion) or static (meaning they involve no motion). Dynamic stretches affect dynamic flexibility and static stretches affect static flexibility (and dynamic flexibility to some degree).

The different types of stretching are:

• Ballistic stretching.
• Dynamic stretching.
• Active stretching.
• Passive (or relaxed) stretching.
• Static stretching.
• Isometric stretching.
• PNF stretching.

Ballistic Stretching

Uses, the momentum of a moving body or a limb in an attempt to force it beyond its normal range of motion. This is stretching, or “warming up”, by bouncing into (or out of) a stretched position, using the stretched muscles as a spring which pulls you out of the stretched position. (E.g. bouncing down repeatedly to touch your toes.) This type of stretching is not considered useful and can lead to injury. It does not allow your muscles to adjust to, and relax in, the stretched position. It may instead cause them to tighten up by repeatedly activating the stretch reflex.

Dynamic stretching

Moving parts of your body and gradually increasing reach, speed of movement, or both. Dynamic stretching consists of controlled leg and arm swings that take you to the limits of your range of motion in a controlled fashion. In dynamic stretches, there are no bounces or “jerky” movements. An example of dynamic stretching would be slow, controlled leg swings, arm swings, or torso twists.

Dynamic stretching improves dynamic flexibility and is quite useful as part of your warm-up for an active or aerobic workout (such as Karate, MMA, Football or Rugby).

Dynamic stretching exercises should be performed in sets of 8-12 repetitions:
Tired muscles are less elastic, which causes a decrease in the amplitude of your movements. Do only the number of repetitions that you can do without decreasing your range of motion. More repetitions will only set the nervous regulation of the muscles’ length at the level of these less than best repetitions and may cause you to lose some of your flexibility. What you repeat more times or with a greater effort will leave a deeper trace in your kinaesthetic memory. After reaching the maximal range of motion in a joint in any direction of movement, you should not do many more repetitions of this movement in a given workout. Even if you can maintain a maximal range of motion over many repetitions, you will set an unnecessarily solid memory of the range of these movements. You will then have to overcome these memories in order to make further progress.

Active Stretching

Also referred to as, static-active stretching. An active stretch is one where you assume a position and then hold it there with no assistance other than using the strength of your agonist muscles for example, bringing your leg up high and then holding it there without anything other than your leg muscles themselves to keep the leg in that extended position. The tension of the agonists in an active stretch helps to relax the muscles being stretched (the antagonists) by reciprocal inhibition.

Active stretching increases active flexibility and strengthens the agonistic muscles. Active stretches are usually quite difficult to hold and maintain for more than 10 seconds and rarely need to be held any longer than 15 seconds.
Many of the movements (or stretches) found in various forms of yoga are active stretches.

Passive Stretching

Referred to as relaxed stretching or static-passive stretching. A passive stretch is one where you assume a position and hold it with some other part of your body, or with the assistance of a partner or some other apparatus. For example, bringing your leg up high and then holding it there with your hand. The splits are an example of a passive stretch in this case the floor is the “apparatus”
Slow, relaxed stretching is useful in relieving spasms in muscles that are healing after an injury. Obviously, you should check with your doctor first to see if it is okay to attempt to stretch the injured muscles.

Relaxed stretching is also very good for “cooling down” after a workout and helps reduce post-workout muscle fatigue, and soreness.

Static Stretching

Many people use the term “passive stretching” and “static stretching” interchangeably. However, there are a number of people who make a distinction between the two.

• Static stretching involves holding a position. That is, you stretch to the farthest point and hold the stretch.
• Passive stretching is a technique in which you are relaxed and make no contribution to the range of motion. Instead, an external force is created by an outside agent, either manually or mechanically.

Notice that the definition of passive stretching given in the previous section encompasses both of the above definitions. Throughout this document, when the term static stretching or passive stretching is used, its intended meaning is the definition of passive stretching as described in the previous section. You should be aware of these alternative meanings, however, when looking at other references on stretching.

Isometric Stretching

A type of static stretching which involves the resistance of muscle groups through isometric contractions (tensing) of the stretched muscles). The use of isometric stretching is one of the fastest ways to develop increased static-passive flexibility and is much more effective than either passive stretching or active stretching alone. Isometric stretches also help to develop strength in the “tensed” muscles (which helps to develop static-active flexibility), and seems to decrease the amount of pain usually associated with stretching.

The most common ways to provide the needed resistance for an isometric stretch are to apply resistance manually to one’s own limbs, to have a partner apply the resistance, or to use an apparatus such as a wall or the floor to provide resistance.

Isometric stretching is not recommended for children and adolescents whose bones are still growing. These people are usually already flexible enough that the strong stretches produced by the isometric contraction have a much higher risk of damaging tendons and connective tissue. Precede any isometric stretch of a muscle with dynamic strength training for the muscle to be stretched. A full session of isometric stretching makes a lot of demands on the muscles being stretched and should not be performed more than once per day for a given group of muscles, ideally, no more than once every 36 hours.

The proper way to perform an isometric stretch is as follows:

• Assume the position of a passive stretch for the desired muscle.
• Tense the stretched muscle for 7-15 seconds (resisting against some force that will not move, like the floor or a partner).
• Finally, relax the muscle for at least 20 seconds.
Some people seem to recommend holding the isometric contraction for longer than 15 seconds; research has shown that this is not necessary. So you might as well make your stretching routine less time consuming.

How Isometric Stretching Works

Recall, there is no such thing as a partially contracted muscle fibre: when a muscle is contracted, some of the fibres contract and some remain at rest (more fibres are recruited as the load on the muscle increases). Similarly, when a muscle is stretched, some of the fibres are elongated and some remain at rest. During an isometric contraction, some of the resting fibres are being pulled upon from both ends by the muscles that are contracting. The result is that some of those resting fibres stretch.

Normally, the fibres that stretch during an isometric contraction are not very significant. The true effectiveness of the isometric contraction occurs when a muscle that is already in a stretched position is subjected to an isometric contraction. In this case, some of the muscle fibres are already stretched before the contraction and if held long enough the initial passive stretch overcomes the stretch reflex and triggers the lengthening reaction inhibiting the stretched fibres from contracting.

At this point:

When isometrically contracted, some of the resting fibres would contract, many of the resting fibres would stretch, and many of the already stretched fibres, which are being prevented from contracting by the inverse myotatic reflex [the lengthening reaction], would stretch even more. When the isometric contraction was relaxed and the contracting fibres returned to their resting length, the stretched fibres would retain their ability to stretch beyond their normal limit. I.e. The whole muscle would be able to stretch beyond its initial maximum, and you would have increased flexibility.

The reason that the stretched fibres develop and retain the ability to stretch beyond their normal limit during an isometric stretch has to do with the muscle spindles: The signal which tells the muscle to contract voluntarily, also tells the muscle spindle’s (intrafusal) muscle fibres to shorten, increasing sensitivity of the stretch reflex. This mechanism normally maintains the sensitivity of the muscle spindle as the muscle shortens during contraction. This allows the muscle spindles to habituate to an even further-lengthened position.

Proprioceptive Neuromuscular Facilitation (PNF) Stretching

PNF stretching is currently the fastest and most effective way known to increase static-passive flexibility. PNF is an acronym for proprioceptive neuromuscular facilitation. It is not really a type of stretching but is a technique of combining passive stretching and isometric stretching in order to achieve maximum static flexibility. Actually, the term PNF stretching is itself a misnomer. PNF was initially developed as a method of rehabilitating stroke victims. PNF refers to any of several post-isometric relaxation stretching techniques in which a muscle group is passively stretched, then contracts isometrically against resistance while in the stretched position, and then is passively stretched again through the resulting increased range of motion. PNF stretching usually employs the use of a partner to provide resistance against the isometric contraction and then later to passively take the joint through its increased range of motion. It may be performed, however, without a partner, although it is usually more effective with a partner’s assistance.
Most PNF stretching techniques employ isometric agonist contraction/relaxation where the stretched muscles are contracted isometrically and then relaxed. Some PNF techniques also employ isometric antagonist contraction where the antagonists of the stretched muscles are contracted. In all cases, it is important to note that the stretched muscle should be rested (and relaxed) for at least 20 seconds before performing another PNF technique. The most common PNF stretching techniques are:

The hold-relax

This technique is also called the contract-relax. After assuming an initial passive stretch, the muscle being stretched is isometrically contracted for 7-15 seconds, after which the muscle is briefly relaxed for 2-3 seconds, and then immediately subjected to a passive stretch which stretches the muscle even further than the initial passive stretch. This final passive stretch is held for 10-15 seconds. The muscle is then relaxed for 20 seconds before performing another PNF technique.

The hold-relax-contract

This technique is also called the contract-relax-contract, and the contract-relax-antagonist-contract (or CRAC). It involves performing two isometric contractions: first of the agonists, then, of the antagonists. The first part is similar to the hold-relax where, after assuming an initial passive stretch, the stretched muscle is isometrically contracted for 7-15 seconds. Then the muscle is relaxed while its antagonist immediately performs an isometric contraction that is held for 7-15 seconds. The muscles are then relaxed for 20 seconds before performing another PNF technique.

The hold-relax-swing

This technique (and a similar technique called the hold-relax-bounce) actually involves the use of dynamic or ballistic stretches in conjunction with static and isometric stretches. It is very risky, and is successfully used only by the most advanced of athletes that have managed to achieve a high level of control over their muscle stretch reflex). It is similar to the hold-relax technique except that a dynamic or ballistic stretch is employed in place of the final passive stretch.

Notice that in the hold-relax-contract, there is no final passive stretch. It is replaced by the antagonist-contraction which, via reciprocal inhibition serves to relax and further stretch the muscle that was subjected to the initial passive stretch. Because there is no final passive stretch, this PNF technique is considered one of the safest PNF techniques to perform as it is less likely to result in torn muscle tissue. Some people like to make the technique even more intense by adding the final passive stretch after the second isometric contraction. Although this can result in greater flexibility gains, it also increases the likelihood of injury.

Even more risky are dynamic and ballistic PNF stretching techniques like the hold-relax-swing, and the hold-relax-bounce. If you are not a professional athlete, you probably have no business attempting either of these techniques as the probability of injury is great). Even professionals should not attempt these techniques without the guidance of a professional coach or training advisor. These two techniques have the greatest potential for rapid flexibility gains, but only when performed by people who have a sufficiently high level of control of the stretch reflex in the muscles that are being stretched.
Like isometric stretching PNF stretching is also not recommended for children and people whose bones are still growing (for the same reasons. Also like isometric stretching, PNF stretching helps strengthen the muscles that are contracted and therefore is good for increasing active flexibility as well as passive flexibility. Furthermore, as with isometric stretching, PNF stretching is very strenuous and should be performed for a given muscle group no more than once per day (ideally, no more than once per 36-hour period).

The initial recommended procedure for PNF stretching is to perform the desired PNF technique 3-5 times for a given muscle group, resting 20 seconds between each repetition. However, a 1987 study whose results suggest that performing 3-5 repetitions of a PNF technique for a given muscle group is not necessarily any more effective than performing the technique only once. As a result, in order to decrease the amount of time taken up by your stretching routine, without decreasing its effectiveness), perform only one PNF technique per muscle group stretched in a given stretching session.

How PNF Stretching Works

During an isometric stretch, when the muscle performing the isometric contraction is relaxed, it retains its ability to stretch beyond its initial maximum length. PNF takes immediate advantage of this increased range of motion by immediately subjecting the contracted muscle to a passive stretch.
The isometric contraction of the stretched muscle accomplishes several things:
It helps to train the stretch receptors of the muscle spindle to immediately accommodate a greater muscle length.

The intense muscle contraction, and the fact that it is maintained for a period of time, serves to fatigue many of the fast-twitch fibres of the contracting muscles. This makes it harder for the fatigued muscle fibres to contract in resistance to a subsequent stretch.
The tension generated by the contraction activates the Golgi tendon which inhibits contraction of the muscle via the lengthening reaction. Voluntary contraction during a stretch increases tension on the muscle, activating the Golgi tendon organs more than the stretch alone. So, when the voluntary contraction is stopped, the muscle is even more inhibited from contracting against a subsequent stretch.

PNF stretching techniques take advantage of the sudden “vulnerability” of the muscle and its increased range of motion by using the period of time immediately following the isometric contraction to train the stretch receptors to get used to this new, increased, range of muscle length. This is what the final passive (or in some cases, dynamic) stretch accomplishes.

How to Stretch

Stretching can do more than just increase flexibility. Benefits of stretching include:

• Enhanced physical fitness.
• Enhanced ability to learn and perform skilled movements.
• Increased mental and physical relaxation.
• Enhanced development of body awareness.
• Reduced risk of injury to joints, muscles, and tendons.
• Reduced muscular soreness.
• Reduced muscular tension.
• Increased suppleness due to stimulation of the production of chemicals which lubricate connective tissues.
• Reduced severity of painful menstruation (dysmenorrhea) in females.

Unfortunately, even those who stretch do not always stretch properly and hence do not reap some or all of these benefits. Some of the most common mistakes made when stretching:

• improper warm-up
• inadequate rest between workouts
• overstretching
• performing the wrong exercises
• performing exercises in the wrong (or sub-optimal) sequence

Acknowledgement.
• Sport Stretch, by Michael J. Alter.
• Stretching Scientifically, by Tom
• SynerStretch for Total Body Flexibility, from Health for Life.
• The Health for Life Training Advisor, also from Health for Life.
• Mobility Training for the Martial Arts, by Tony Gummerson.
• Bradford D. Appleton.

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