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Chapters
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 respect.
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 contraction.
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 muscles:
- endomysium
- The innermost fascial sheath that envelops individual muscle fibers.
- perimysium
- The fascial sheath that binds groups of muscle fibers into individual fasciculi
(see section Muscle Composition).
- epimysium
- The outermost fascial sheath that binds entire fascicles (see section
Muscle Composition).
These connective tissues help provide suppleness and tone to the muscles.
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.
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 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
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 lowering a weight down
in a slow, controlled fashion.
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 length decreases.
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 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 (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 straight).
Go to the previous,
next chapter.
by Brad Appleton
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