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.