Lesson 5: Using the Updated Stabilizer

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Muscle Roles: What is an Agonist, Antagonist, Stabilizer, Fixator or Neutralizer Muscle? Ground Up Strength Categories » Physiology and Anatomy » Anatomy » Biomechanics » Kinesiolgy » Muscle Roles: What is an Agonist, Antagonist, Stabilizer, Fixator or Neutralizer Muscle? Why Can You Lift More On Flat Bench Press Than On Incline? You Can’t Isolate a Muscle, But Does it Matter? Perhaps the biggest misunderstanding about how skeletal muscles function to produce the body’s movements concerns their particular role. Most people think that a muscle performs ONE particular and very defined role and that they always perform this role.

This is not how it works. Muscles must work together to produce different bodily movements and a particular muscle’s role may change depending on the movement. The most important aspect to understand about how muscles function to produce a joint movement is synergy. Synergy means that two or more things work together to produce a result that is greater than any of those things could do alone, so that the whole result is greater than the sum of the individual effects of the agents involved. Even the simplest joint movement requires muscles working together in this synergistic or cooperative fashion. Usually, the muscles that are directly involved in producing a certain joint movement are called agonists and muscles that are indirectly involved, by some other role, are called synergists.

However, even if a muscle adds directly to a joint’s movement by adding its own torque, it can still correctly be called a “synergist”. Students of strength training are always having great difficulty in distinguishing the difference between agonists, synergists, stabilizers, fixators, etc. Muscle synergy, as above, is an important concept, but the word synergist, used to describe a muscle’s role, is a silly word that is used in different ways by different texts. We’d do well to abandon it. Although the word is not useful, it is largely used so we cannot simply ignore it even though we could easily side-step it by simply describing the different roles a muscle may take in helping to produce a movement.

This would simply cause more confusion, not less, and it is not our place, here at GUS, to decide whether the term should be abandoned. Do not be too surprised by this. As you study human movement you will find contradictions to be the rule. It is not always completely decided how terms should be used and, to be frank, many of the most popular usages are incorrect ones.

The first definition we can easily render incorrect, as will be seen since it incorrectly uses the word agonist to include muscles that cannot be considered agonists. The second definition is better as it uses the word agonist correctly but it still uses the confusing word synergist, which we have to deal with. So, we will deal with it by accepting it but insisting upon using it properly. While some muscles work together, in a concentric fashion, to produce a movement, others work in other ways to help cancel out other movements, such as the unwanted movement of another bone that the muscle attaches to, or by opposing the movement that could occur in an undesired plane of motion.

Lesson 5: Using the Updated Stabilizer

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The movement produced is the net result of all the different forces produced by the muscles. As stated above, agonist muscles are muscles that are responsible for causing a certain joint motion. However, the term is often defined incorrectly to mean ALL the muscles that have a role in producing a movement. By this definition stabilizers, neutralizers, and fixators are also agonists. An agonist is a muscle that is capable of increasing torque in the direction of a limb’s movement and thus produce a concentric action. In other words, the muscle can produce a force that accelerates a limb around its joint, in a certain direction.

Lesson 5: Using the Updated Stabilizer

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This does NOT mean that this direction is the only one the muscle can produce force in but only that it is capable of this and thus is directly involved in producing a certain movement, making it a prime mover. Many people refer to muscles having a redundant role in producing torque about a joint as being synergistic agonists but with one of these muscles being the prime mover. This is a silly and arbitrary distinction since there are many instances where a muscle with a redundant role can take over for a paralyzed one, making that muscle the “prime mover”. Agonist and “prime mover” simply speaking, means the same thing and the terms are interchangeable. The brachialis, for instance, is another elbow flexor, located inferior to the biceps on the upper arm. Unlike the biceps, which inserts onto the radius, which is able to rotate, the brachialis inserts onto the ulna which cannot rotate. This, it can be said that the brachialis is the only pure flexor of the elbow joint whereas the larger biceps can also supinate the forearm.

But the brachialis is the only pure elbow flexor. A muscle can only be referred to as an agonist in relation to a movement or another muscle. It is never proper to call any one muscle an agonist unless we are describing its role in a movement or we are referring to it in terms of a muscle on another side of the joint, known as an anatagonist. The biceps brachii is an agonist for elbow flexion. It is assisted by the brachialis and the brachioradialis.

Lesson 5: Using the Updated Stabilizer

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These are the agonists of elbow flexion, all of which are capable of flexing the elbow joint to some extent. Some muscles involved in a joint action do not directly contribute a torque force to the movement but assist the movement in indirect ways. These roles that are commonly referred to as synergist muscles, as explained above, but that we are calling the agonist’s synergists. These roles are many but some of the basic terms used to describe these muscles are stabilizer, neutralizer and fixator.

Lesson 5: Using the Updated Stabilizer

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However, the term stabilizer, for our purposes, means the same thing as fixator. There is more than one way to categorize the functional role of muscles. We may look at the muscles in terms of their function in specific movements or we may look at them in terms of the entire body as a system, complete with many subsystems. The latter view is not what we are concerned with in this explanation but the when viewed this way muscles are classified according to their function rather than their role in a particular movement.

This concept was first proposed by Rood and furthered by the work of Janda and Sahrmann as well as by Comerford and Mottram who proposed the concept of local and global stabilizers and global mobilizers. Although, the concept of a stabilizing muscle can still be viewed in terms of a single movement in this system, certain muscles are considered to have the primary function of stabilizers in the body, being, by virtue of their position, shape, angle or structure, more suited to work as a stabilizer than as a mobilizer. For instance, this view teaches us that the abdominal group of muscles, once primarily thought of as a muscle we perform situps with, is much more important as a major stabilizer of the spine. The type of stabilizer we will discuss here, however, are fixators, which are active during one movement and at one joint. There are certain muscles that act primarily as stabilizes because of their angle of pull. An example of such muscles is a group of muscles known as the rotator cuff muscles of the shoulder girdle. A fixator is a stabilizer that acts to eliminate the unwanted movement of an agonist’s, or prime mover’s, origin.

Many muscles are attached to more than one bone. When this happens the muscles are said to be multiarticulate or multijoint muscles. When these muscles contract they tend to move both bones to which they are attached. This would, of course, make everyday movements quite impossible.

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For instance, consider elbow flexion by the biceps brachii. When you do a curl, the biceps acts to flex the elbow. However, the biceps is attached at two places, proximally and distally. Its distal attachment, the insertion, is to the radius. It’s the radius bone we want to move when we curl a dumbbell.

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One of its proximal attachments, though, the origin, is to the scapula. The scapula is one heck of a mobile bone. In fact, it has no real bony attachments of its own. Neutralizers, like fixators, act to prevent unwanted movement.

Lesson 5: Using the Updated Stabilizer

Lesson 5: Using the Updated Stabilizer

But instead of acting to prevent the unwanted movement of a body part they act to pull against and cancel out an unwanted line of pull from the agonist or prime mover. Many muscles can produce a pulling force in more than one direction so that an undesired joint action may occur simultaneously with the desired one. For example, the biceps brachii can do more than flex the elbow. In order for biceps action to flex the elbow without the forearm also being supinated another muscle must cancel out the supination torque that the biceps also produces. The pronator teres, being the principal forearm pronator, is responsible for this.

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Test the action of the pronator teres for yourself. The pronator teres will start to contract. You can feel it with your opposite fingers inside the middle of your forearm. Now, relax your forearm and bring your hand up toward the ceiling.

You will feel the pronator teres relax and lengthen. On the other hand, if forearm supination were desired without elbow flexion, the triceps would act isometrically to resist the flexion, making it a neutralizer. An antagonist is a muscle that is capable of opposing the movement of a joint by producing torque that is opposite to a certain joint action. This is usually a muscle that is located on the opposite side of the joint from the agonist. The triceps, an extensor of the elbow joint, is the antagonist for elbow flexion, and it would also be correct to say that the tricep is an antagonist to the biceps, and vice versa.

In order for an agonist to shorten as it contracts the antagonist must relax and passively lengthen. This occurs through reciprocal inhibition, which is necessary for the designated joint movement to occur unimpeded. Reciprocal inhibition is a neural inhibition of the motor units of the antagonist muscle. When the agonist muscle contracts, this causes the antagonist muscle to stretch. However, antagonists are not always inactive or passive during agonist movements. Antagonists also produce eccentric actions in order to stabilize a limp or decelerate a movement at the end of a motion.

For instance, during running the hip extensors are antagonists to the hip flexors, which act to bring the femur forward during the running stride. So, the hip extensor muscles must relax to some degree to allow this forward motion of the thigh to take place. When both the agonist and antagonist simultaneously contract this is called coactivation. It can be advantageous for coactivation to occur for several reasons. For instance, when movements require a sudden change in direction, when heavy loads are carried, and to make a joint stiffer and more difficult to destabilize. The purported reason that co-contraction may occur during changes in direction is that modulating the level of activity in one set of muscles is more economical than alternately turning them on and off. For heavy loads, increased joint stiffness is desirably for lifting heavier loads and co-contraction of the core muscles of the torso routinely occurs during these activities.

For fine motor activities of the fingers, as well, complex co-contraction activity is needed. Muscles can also be described as being spurt or shunt muscles. These roles are largely unknown in the strength training world but are described in the orthopedic and physical therapy fields. Again, we will consider the elbow joint. When a muscle acts on a bone it actually produces a force that, if one were to do a vector analysis, could be resolved into two component forces. These components are an angular component and a transarticular component.