When reviewing the variety of terms associated with muscle classification, it is apparent that many inconsistencies exist. This can be detrimental to fitness professionals who desire 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, a simplistic position on this matter is imperative. 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.
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), fiber type distribution (slow twitch/type I/slow oxidative or SO; fast twitch/type IIa/fast oxidative glycolytic or FOG; fast twitch/type IIb/fast glycolytic or FG) and structural locale (local/global).
While each of these can be rationalized for validity, this article will use the terms “local” and “global” muscles suggested by Bergmark. The following text will expand the meaning of the terms local and global and explain the characteristics these muscles typically display in response to their environment.
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 have stated that this concept may stem from the great Leonardo Da Vinci who 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.
The local muscles are predominantly involved in joint support or stabilization. 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. 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.
A list of common local muscles include the following:
- Dep cervical flexors
- Rotator cuff
- Mid and lower trapezius
- Transversus abdominis
- Muscles of the pelvic floor
- Gluteus medius and minimus
- External rotators of the hip
- Vastus medialis obliques
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. They are associated with movement of the trunk and limbs and equalizing external loads placed upon the body. They also are important for transferring and absorbing forces from the upper and lower extremities to the pelvis.
The major global muscles include the following:
- Upper trapezius
- Levator scapulae
- Pectoralis major
- Latissimus dorsi
- Rectus abdominis
- External obliques
- Erector spinae
- Gluteus maximus
- Rectus femoris
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:
- 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. Panjabi has alluded to the importance of the nervous system working in concert with muscular and articular systems in controlling stabilization and movement. Bullock-Saxton 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). This type of response has been noted in the transversus abdominis.
The nervous system also plays a major role in the inhibition of muscles, either through pain or as a result of reciprocal inhibition. Inhibition of a muscle is a decrease in the neural drive to that muscle that reduces its ability to respond to stimuli with proper timing 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. This often affects the local muscles as they have been shown to have a propensity to inhibition as a result of pain.
Reciprocal inhibition is a principle whereby a tight muscle will cause decreased neural input to its functional antagonist (inhibition). 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. Tightness is characterized by a decrease in the resting length of a muscle as well as the common occurrence of over activity (heightened neurological state). 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.
It is known that non-diseased humans have near identical neuromusculoskeletal structures 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.
I've 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. 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. 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.
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. 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. 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.
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. Therefore, classification based upon specific fiber type actions may be misleading.
By using the terms “local” and “global” muscles to denote differences in musculature, 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 timing 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).
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. 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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