The Physiology Of Stretching

Tiger Athletic Fitness & Conditioning is a private, appointment only strength & conditioning gym in the heart of Sandton offering tailor made, goal-oriented fitness programs. This is the first of three resources, a modest attempt to address some of the frequently asked questions about stretching and flexibility, beginning with the physiology of stretching.

Physiology of Stretching

An introduction to some of the basic physiological concepts that come into play when a muscle is stretched as well as a general overview of basic concepts.

  • The Musculoskeletal System – 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. 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 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.
  • 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 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
  • What happens When You Stretch?

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 are 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 can 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 can 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 can 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 must 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.

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 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 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 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 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 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 possess this level of muscular control.

The stretch reflex has both a dynamic component and a static component. The static component of the stretch reflex persists if 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 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 can 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 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, 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 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.

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