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The purpose of this chapter 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
(for those who want to know the gory details) will be discussed in
further detail. If you aren't all that interested in this aspect of
stretching, you can skip this chapter. Other sections will refer to
important concepts from this chapter and you can easily look them up on
a "need to know" basis.
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
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 (whole) muscle is composed of many strands of
tissue called fascicles. These are the strands of muscle that we
see when we cut red meat or poultry. Each fascicle is composed of
fasciculi which are bundles of muscle fibers. The muscle
fibers are in turn composed of tens of thousands of thread-like
myofybrils, which can contract, relax, and elongate (lengthen).
The myofybrils are (in turn) 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.
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 fibers. Inside the muscle fibers, 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 fiber.
When a muscle fiber contracts, it contracts completely. There is no such
thing as a partially contracted muscle fiber. Muscle fibers 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 fibers are recruited, as they are needed, to perform
the job at hand. The more muscle fibers that are recruited by the
central nervous system, the stronger the force generated by the muscular
The energy which produces the calcium flow in the muscle fibers comes from
mitochondria, the part of the muscle cell that converts glucose
(blood sugar) into energy. Different types of muscle fibers have
different amounts of mitochondria. The more mitochondria in a muscle
fiber, the more energy it is able to produce. Muscle fibers are
categorized into slow-twitch fibers and fast-twitch fibers.
Slow-twitch fibers (also called Type 1 muscle fibers) are slow to
contract, but they are also very slow to fatigue. Fast-twitch fibers
are very quick to contract and come in two varieties: Type 2A
muscle fibers which fatigue at an intermediate rate, and Type 2B
muscle fibers which fatigue very quickly. The main reason the
slow-twitch fibers are slow to fatigue is that they contain more
mitochondria than fast-twitch fibers and hence are able to produce more
energy. Slow-twitch fibers are also smaller in diameter than fast-twitch
fibers and have increased capillary blood flow around them. Because they
have a smaller diameter and an increased blood flow, the slow-twitch
fibers are able to deliver more oxygen and remove more waste products
from the muscle fibers (which decreases their "fatigability").
These three muscle fiber 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)
fibers. When a muscle first starts to contract, it is primarily Type 1
fibers that are initially activated, then Type 2A and Type 2B fibers
are activated (if needed) in that order. The fact that muscle fibers are
recruited in this sequence is what provides the ability to execute
brain commands with such fine-tuned tuned muscle responses. It also makes
the Type 2B fibers difficult to train because they are not activated
until most of the Type 1 and Type 2A fibers have been recruited.
HFLTA states that the the best way to remember the
difference between muscles with predominantly slow-twitch fibers and
muscles with predominantly fast-twitch fibers is to think of "white
meat" and "dark meat". Dark meat is dark because it has a greater number
of slow-twitch muscle fibers and hence a greater number of mitochondria,
which are dark. White meat consists mostly of muscle fibers 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 color
than dark meat because it contains fewer mitochondria.
Located all around the muscle and its fibers are connective
tissues. Connective tissue is composed of a base substance and two
kinds of protein based fiber. The two types of fiber are
collagenous connective tissue and elastic connective tissue.
Collagenous connective tissue consists mostly of collagen (hence its
name) and provides tensile strength. Elastic connective tissue consists
mostly of elastin and (as you might guess from its name) provides
elasticity. The base substance is called mucopolysaccharide and
acts as both a lubricant (allowing the fibers to easily slide over one
another), and as a glue (holding the fibers 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
- The innermost fascial sheath that envelops individual muscle fibers.
- The fascial sheath that binds groups of muscle fibers into individual
fasciculi (see section Muscle Composition).
- The outermost fascial sheath that binds entire fascicles (see section Muscle Composition).
These connective tissues help provide suppleness and tone to the
When muscles cause a limb to move through the joint's range of motion,
they usually act in the following cooperating groups:
- 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.
- These muscles act in opposition to the movement generated by the
agonists and are responsible for returning a limb to its initial
- 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.
- 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.
As an example, when you flex your knee, your hamstring contracts, and,
to some extent, so does your gastrocnemius (calf) 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 quadricep 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 neighbors to function as
The following is a list of commonly used agonist/antagonist muscle
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)
The contraction of a muscle does not necessarily imply that the muscle
shortens; it only 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.
- 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.
During a 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 (and do all of the work). See section Cooperating Muscle Groups.
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 (similar to the way that
the total strength of a contracting muscle depends on the number of
recruited fibers contracting). According to SynerStretch you
should think of "little pockets of fibers distributed throughout the
muscle body stretching, and other fibers simply going along for the
ride". The more fibers that are stretched, the greater the length
developed by the stretched muscle.
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 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.
When the muscle is stretched, so is the muscle spindle
(see section 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 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
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 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-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 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.
When an agonist contracts, in order to cause the desired motion, it
usually forces the antagonists to relax (see section 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
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
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