Download Postural and Phasic Muscles

Muscle Classification System:
A Position Statement
By Rodney Corn MA, PES, CSCS
and the NASM Performance Team
When reviewing the variety of terms associated with muscle classification, it
is apparent that many inconsistencies exist. This can be detrimental to the
health and fitness professional who desires to effectively communicate with
other related professionals. Furthermore, it can cause confusion and
increase anxiety that may exist when learning the intricacies of kinetic chain
concepts. Thus it is imperative that the National Academy of Sports
Medicine (NASM) provides a simplistic position on this matter. It must be
stressed, however, that categorizations or generalizations made about the
human body cannot encompass the vast variability that exist within this
complex system. Any generalizations made in this statement are for ease
of explanation, description and communication and should be interpreted
with caution.
Background
Many different approaches have been taken to categorize muscle function
and dysfunction. These include distinguishing muscles based more on
their proposed functional abilities (postural/phasic and/or
stabilizer/mobilizer) and reaction capacity (tight/overactive/hypertonic and
weak/inhibited).1,2 fiber type distribution (slow twitch/type I/slow
oxidative/SO; fast twitch/type IIa/fast oxidative glycolytic/FOG; fast
twitch/type IIb/fast glycolytic/FG)3,4 and structural locale (local/global).5,6,7
Position
While each of these can be rationalized for validity, the NASM has chosen
to use the terms “local” and “global” muscles suggested by Bergmark. Error!
Bookmark not defined. The following text will expand the meaning of the terms
local and global and explain the characteristics these muscles show a
tendency to display in response to their environment.
Definition
It is often suggested that there are two interdependent muscular systems
that enable our bodies to maintain proper stabilization while concurrently
distributing forces for the production of movement. Crisco and Panjabi 8
have stated that this concept may stem from the great Leonardo Da Vinci.
Da Vinci suggested that muscles located more centrally to the cervical
spine (local) provided intersegmental stability (support from vertebrae to
vertebrae) while the more lateral muscles (global) supported the cervical
column as a whole to produce movement.
Local Muscles
The local muscles are predominantly involved in joint support or
stabilization.Error! Bookmark not defined.,9,10 They are not typically movement
producers, but provide stability to allow movement of a joint. They usually
are located in close proximity to the joint and often have a poor mechanical
advantage for movement production.11 They also have a broad spectrum
of attachments to the passive elements of the joint that make them ideal for
increasing joint stiffness and thus stability.Error! Bookmark not defined.,Error! Bookmark
not defined.,12,13,14
A list of common local muscles include the:Error! Bookmark not defined.,Error!
Bookmark not defined.,15











Deep cervical flexors
Rotator cuff
Rhomboids
Mid and lower trapezius
Transversus abdominis
Multifidus
Diaphragm
Muscles of the pelvic floor
Gluteus medius and minimus
External rotators of the hip
Vastus medialis obliquus
Global Muscles
The global muscles are predominantly larger and responsible for
movement. They consist of more superficial musculature that attach from
the pelvis to the rib cage and/or the upper and lower extremities. Error!
Bookmark not defined.,Error! Bookmark not defined.,Error! Bookmark not defined.,Error! Bookmark not
defined.,i16
They are associated with movement of the trunk and limbs and
equalizing external loads placed upon the body.Error! Bookmark not defined. They
also are important for transferring and absorbing forces from the upper and
lower extremities to the pelvis.Error! Bookmark not defined.
The major global muscles include the:Error! Bookmark not defined.,Error! Bookmark not
defined.,Error! Bookmark not defined.


Sternocleidomastoid
Upper trapezius













Levator scapulae
Pectoralis major
Deltoid
Latissimus dorsi
Rectus abdominis
External obliques
Erector spinae
Gluteus maximus
Hamstrings
Rectus femoris
Iliopsoas
Adductors
Gastrocnemius/soleus
Concepts
To properly rationalize a general classification scheme for muscle function
and dysfunction, it is important to review some basic concepts that will help
to illuminate how muscles move and respond to movement and their
environment. First and foremost, we must highlight some very important
constants concerning the human body and movement: Error! Bookmark not
defined.,17,18,19,20
1.
2.
3.
4.
All humans have similar structure and function
All humans act under the constant force of gravity
All muscles are capable of providing stabilization in some capacity
All movement and muscle is controlled by the nervous system
The Nervous System
Ironically, it is the fourth constant, the nervous system, that allows for the
most variability within the human body and is often the most overlooked.
Panjabiii21 has alluded to the importance of the nervous system working in
concert with muscular and articular systems in controlling stabilization and
movement. Bullock-SaxtonError! Bookmark not defined. noted that some muscles
as a result of the location would work against gravity more so than other
muscles. In turn, this will influence the sensory input into the nervous
system from muscles and joints that can alter interpretation and responding
actions.
Research has also demonstrated that by changing the frequency of
stimulation to a motor unit, the biochemical properties can change (i.e. slow
twitch muscle fiber that is rapidly stimulated converts to fast twitch fiber and
vice versa).22,23,24,25 This type of response has been noted in the
transversus abdominis.Error! Bookmark not defined.
The nervous system also plays a major role in the inhibition of muscles
either through pain or as a result of reciprocal inhibition.Error! Bookmark not
defined.,Error! Bookmark not defined.,26,27,28,29 Inhibition of a muscle is a decrease in
the neural drive to that muscle that reduces its ability to respond to stimuli
with proper timingError! Bookmark not defined.,Error! Bookmark not defined. and can thus
result in a loss of proper strength (weakness).
Pain is highly influential on the nervous system. Research has
demonstrated alterations to afferent and efferent motor responses in the
presence of pain.30,31,32 This often effects the local muscles as they have
been shown to have a propensity to inhibition as a result of pain. Error!
Bookmark not defined.,Error! Bookmark not defined.,Error! Bookmark not defined.,iii33
Reciprocal inhibition is a principle whereby a tight muscle will cause
decreased neural input to its functional antagonist (inhibition).Error! Bookmark
not defined.,Error! Bookmark not defined.,Error! Bookmark not defined.,Error! Bookmark not
defined.,Error! Bookmark not defined.,Error! Bookmark not defined.,Error! Bookmark not defined.
Electromyographic (EMG) data has demonstrated that tight muscles have a
propensity to activate (simulate concentric action) easier and at times when
they would normally remain less active.Error! Bookmark not defined.,Error! Bookmark not
defined.,Error! Bookmark not defined. Tightness is characterized by a decrease in the
resting length of a muscle as well as the common occurrence of overactivity
(heightened neurological state).Error! Bookmark not defined.,Error! Bookmark not
defined.,Error! Bookmark not defined.,Error! Bookmark not defined.,Error! Bookmark not defined.
Global muscles show a propensity to becoming tight.
Ultimately, the nervous system dictates the status of muscles and their
function. It is the nervous system that creates inhibition either through pain
or as a result of antagonistic muscle tightness/overactivity.
Rationale
It is known that non-diseased humans have near identical
neuromusculoskeletal structure and perform a variety of similar activities
under the constant force of gravity. Thus it can be deduced that the human
body will respond to stimuli in a similar manner. Therefore, generalizing
musculature within the human body can be justified for ease of description
and communication.
The NASM has chosen to address and categorize muscles as “local” and
“global”. These terms, when defined, promote an awareness of the
muscles location that has a major influence on their biomechanical
function.Error! Bookmark not defined.,Error! Bookmark not defined.,Error! Bookmark not defined. In
contrast, muscles that are labeled as “postural” and “phasic” or “stabilizers”
and “mobilizers” make reference to a specific action that can more easily be
misconstrued.
Postural and Phasic
Support for this statement lies in the previously mentioned third and fourth
constants. The terms postural and phasic denote actions performed based
upon fiber type. Postural being predominantly slow twitch/type I/SO and
phasic being fast twitch/type IIa/FOG.Error! Bookmark not defined.,Error! Bookmark not
defined. However, in the fourth constant it is stated that all movement and
thus muscle is controlled by the nervous system. As the nervous system is
designed to be highly adaptable, it can alter the stimulation and response of
effector motorneurons.Error! Bookmark not defined.,Error! Bookmark not defined.,Error!
Bookmark not defined.,34
Research has demonstrated that change in nervous stimulation to a motor
unit, such as that seen in disuse or injury, can alter the physical
characteristics of that motor unit.Error! Bookmark not defined.,Error! Bookmark not
defined.,Error! Bookmark not defined. For example, a type I motor-unit stimulated at
a high frequency will change in physical characteristics to a type II and/or
vice versa.35
Therefore it is the nervous system and not necessarily the fiber type
distribution within the muscle that is ultimately responsible for the muscle
action.
Stabilizer and Mobilizer
The terms stabilizer and mobilizer again refer to a specific action performed
by the muscle. Inconsistency can arise from the third constant, which
stated that all muscles are capable of providing stabilization in some
capacity. The best way around this is delineating primary, secondary and
tertiary stabilizers, which many professionals do use. Thus it might be
better to just use the term “stabilizers” with varying degrees of stabilization
(primary, secondary and tertiary). In either case, the premise is still being
placed on the action of the muscle that can be directly influenced and
changed by neural input.
Local and Global
The terms local and global simply refer to the location of the muscle in
relation to the joint of motion. Local and global muscles are deemed more
prone to stabilization not based upon fiber type, rather on their
biomechanical advantage (or disadvantage) relative to the joint. The
smaller the moment arm (leverage system) of the muscle, the less torque
or motion (concentric/eccentric action) it will be able to induce. Thus by
default, they may be better delegated to stabilizing (isometric action).
Conversely, a larger moment arm generally indicates a muscle’s greater
distance from the joint and the greater potential to manipulate
movement.Error! Bookmark not defined.,Error! Bookmark not defined.,36
Conclusion
There is much confusion among heath and fitness professionals pertaining
to terminology used for muscle classification. Many professionals use
terms that are pertinent to specific actions of muscles based upon their
fiber type. However, as all motion and muscle is controlled by the nervous
system, research has shown that these actions and characteristics can be
altered via neural input.Error! Bookmark not defined.,Error! Bookmark not defined.,Error!
Bookmark not defined.,Error! Bookmark not defined.,Error! Bookmark not defined.,Error! Bookmark not
defined.
Therefore, classification based upon specific fiber type actions may
be misleading.
The NASM has chosen to use the terms “local” and “global” muscles to
denote differences in musculature. This system of classification is based
upon physical location and biomechanical properties rather than fiber type
distribution.
Local muscles are biomechanically less advantageous to manipulate
movement of a joint and thus may be better suited for stabilization. These
muscles show a propensity to inhibition, defined as a decrease in the
neural drive to a muscle that reduces its ability to respond to stimuli with
proper timingError! Bookmark not defined.,Error! Bookmark not defined. and can thus result
in a loss of proper strength (weakness).
Global muscles have greater biomechanical advantages to manipulate
movement of a joint(s). These muscles show a propensity to become tight,
defined as a decrease in the resting length of a muscle as well as the
common occurrence of overactivity (heightened neurological state).Error!
Bookmark not defined.,Error! Bookmark not defined.,Error! Bookmark not defined.,Error! Bookmark not
defined.,Error! Bookmark not defined.
Many classifications exist and all can be rationalized to make sense in
certain populations. The key is to develop them so they make sense in any
population. Ultimately, this can only be achieved through proper definition
and rationale that comes as a result of a genuine concern to illuminate the
most pertinent applicable information. The NASM has taken the industry
up on this offer and delivered a proposal for muscle classification. Please
remember that the human body is very interdependent and complex. No
categorization that attempts to generalize the systems of the body will be
able to precisely simplify this complexity. However, for ease of explanation,
education and communication, we must strive to create accurate simplicity
of the human body.
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