The stretching of a muscle fiber
begins with the sarcomere (see section 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 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. According to SynerStretch:
Picture little pockets of fibers distributed throughout the muscle body stretching, and other fibers simply going along for the ride. Just as the total strength of a contracting muscle is a result of the number of fibers contracting, the total length of a stretched muscle is a result of the number of fibers stretched -- the more fibers stretched, the more length developed by the muscle for a given stretch.
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 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.
When the muscle is stretched, so is
the muscle spindle. 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 actually 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 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.