The body has over 600 skeletal muscles. The force of skeletal muscular contraction applied to the bones of the body produces movement. In addition to producing movement, skeletal muscles contribute to providing protection for the body, posture and support, and are involved in thermoregulatory functions. 

Figure 1

Figure 2

Muscle Nomenclature – Inherent in the name of a muscle is information about that muscle. Muscles are usually named because of one or more distinctive characteristics as listed below. As such, learning how they are named, according to these various characteristics, is helpful in learning about the muscle and its functions. Often there is overlapping among the various nomenclature characteristics.

Individual Muscle named by:                                        Muscle Group

• Shape ___________________________________        _____________________
• Size _____________________________________       
• Number of divisions _____________________         ______________________
• Direction of fibers _______________________
• Location ________________________________          ______________________
• Points of attachment ____________________
• Action __________________________________          ______________________
• Action & shape _________________________
• Action & size ___________________________
• Location & attachment __________________          ______________________

Muscle Shape and Fiber Arrangements

Muscle shape and fiber arrangement play a role in the muscle’s ability to exert force and the ROM through which it can effectively exert force. Muscle fibers are arranged within the muscle in a direction that is either parallel or oblique to the muscle’s long axis. Muscles with parallel muscle fiber arrangement tend to be longer and thus allow for greater range of motion. Muscles with obliquely (pennate) arranged muscle fibers tend to be shorter, but contain more fibers per given area thereby increasing the cross-sectional area of the muscle and consequently its force production potential.

Figures 3,4,5,6,7,8,9,10

Muscle Tissue Characteristics

Below are five (5) important characteristics or properties of skeletal tissues. 

• Excitability – the ability of a muscle to respond to a stimulus, whether that is a natural stimulus from a motor neuron or an artificial stimulus from an electrical current.
• Contractility – the ability of a muscle to contract and develop tension, whether that results in the muscle shortening, staying the same length, or lengthening.
• Extensibility – the ability of a muscle to be stretched or lengthened beyond its normal resting length.
• Elasticity – the ability of a muscle to recoil and return to its original length after being stretched.
• Tone/Tonus/Tonicity – a state of firmness or slight tension (readiness for muscle response) of a muscle due to nerve stimulation and motor unit contraction, and its physical properties.  

In essence, stretch a muscle and it will lengthen (extensibility). Remove the force causing the stretch and it will return to its normal resting length (elasticity). Stimulate a muscle and it will respond (excitability) by shortening (contractility).

General Muscle Terminology

To assist in understanding a discussion of muscle function in the body, a knowledge of some basic terms is important. These include:

1. Intrinsic – a muscle that is located within or belongs solely to the body part on which it acts.

• Ex. The Adductor Pollicis, Obicularis Oris, Quadratus Plantae

2. Extrinsic – a muscle that originates outside the body part on which it acts.

• Ex. Biceps Brachii, Rectus Femoris

3. Actions – the specific movement(s) of a joint resulting from a muscle’s “concentric” contraction.

• Ex. Elbow flexion, forearm supination, and shoulder flexion due to contraction of the biceps Brachii

4. Innervation – the nerve responsible to provide stimulus to the muscle fibers. A particular muscle may be

innervated by more than one nerve, and a particular nerve may innervate more than one muscle or portion

of a muscle.

• Ex. The axillary nerve innervates the deltoid muscle

5. Excursion (Amplitude) – range of fiber length between maximum elongation and maximum shortening.

• Ex. Assume a muscle’s resting length is 8 inches. When it contracts maximally it can shorten to approximately 4 inches, and when stretched maximally it can lengthen to approximately 12 inches. Therefore, its amplitude is 8 inches, or the difference between 4 and 12 inches.

6. Origin – the muscle’s attachment to the least moveable or more fixed body part/bone, usually the

proximal attachment.

7. Insertion – the muscle’s attachment to the most moveable body part/bone, usually the distal attachment.

8. Reversal of Muscle Action – when the “origin” attachment point moves while the “insertion”

attachment point remains fixated, e.g., elbow flexion while performing a pull-up movement.  

Types of Muscle Contractions

When muscles contract they develop tension. That force is then applied to the bones to which they attach. The muscle force will either cause, control, or prevent movement. The muscle contractions that produce these movements (or control or prevent) are classified as either isometric or isotonic. 

Isometric: (same length) A type of muscle contraction in which:

• Tension develops, yet there is no change in length of muscle or angle of the joint = “preventing”
• Muscular Force __ Resistive Force and the tension is providing a ___________________ effect

Isokinetic: (same speed) A type of muscle contraction that is controlled by an isokinetic computerized-machine which varies the resistance to keep the angular velocity of the movement the same throughout the range.

• This equipment is used in research to measure force, torque, and power output of muscles
• Also used in rehab settings to control range of motion and speed for recovering athletes

Isotonic: (same tone or tension) Tension within the muscle actually does change as the joint is moved throughout its range of motion and therefore this term is somewhat a misnomer. It is sometimes referred to as dynamic contraction, a type of muscle contraction in which tension develops either “causing” or controlling” the movement. There are two types of dynamic contractions. They are concentric and eccentric.

 Figure 11   

Note: Eccentric muscle contractions are sometimes referred to as lengthening contractions. However, while the muscle is lengthening at a gross level, it is still developing active tension through cross-bridging. What the muscle is actually doing is returning to its normal resting position from a shortened position. Eccentric contractions can produce much greater forces than concentric contractions and cause the delayed onset of muscle soreness (DOMS) that you feel 24-48 hours after you work out.

Various Roles Muscles Can Play

A muscle attaches, via its tendon(s), from one bone to another bone, thereby crossing a joint or the union between the two bones. When a muscle contracts it creates a pulling force on its attachments and attempts to pull the bones toward each other (i.e., shortening the muscle toward its center). As this occurs, both of the bones to which the muscle attaches will have tension placed upon them. The “insertion” (mobile) attachment point will typically move toward the “origin” (fixed) attachment point.

Agonist: muscle/muscle group, that when contract concentrically, causes the desired joint movement.

Prime Mover: the muscle whose contribution to a specific joint movement is greater than the other agonists. It is the most powerful muscle in performing a particular action. Note: The determination of whether a muscle is a prime mover or a secondary/assistant mover can be dependent upon the muscle’s size, angle of pull, and force generation capacity. 

Secondary or Assistant Movers: agonists whose contribution to a specific movement is less than that of the prime mover.

Elbow Flexion

Prime Mover: ________________________

Secondary Movers:

1.

2.

Forearm Supination

Prime Mover: ________________________

Secondary Movers:

1.

2.

Hip Abduction

Prime Mover: ________________________

Secondary Movers:

1.

2.

3.

4.

Ankle Dorsiflexion

Prime Mover: ________________________

Secondary Movers:

1.

2.

3.

Functional Muscle Groups (e.g. shoulder flexors): muscles are grouped according to their specific concentric action.

How to Determine Which Muscle Group is the Agonist?

Note: The agonist or “involved” muscle group opposes the applied resistance. It is also referred to as the “active” or “working” muscle(s). 

Knowing how to determine what muscle group or groups (and therefore individual muscles) are “involved” in an activity is important for a variety of reasons. It allows a person to know what muscles to strengthen or stretch for a specific activity in the enhancement of performance, for muscle testing, for rehabilitation purposes, etc. The “involved” muscle group(s) can be determined via a simple two-step process.

Step #1: 

Identify what movement or movements (flexion, extension, abduction, adduction, internal rotation, external rotation, etc.) the resistance is “tending” to cause at each joint the resistance is acting on. 

For example, when performing curls, the dumbbell is tending to cause elbow extension. 🡪 🡪 🡪 🡪 🡪

Step #2: 

The “opposite” muscle group is the agonist or “involved” muscle group.

• Therefore, the elbow flexors are “involved.”
  

Because the resistive force is being applied against the “involved” muscle group, the involved muscle group can:

• Contract to either cause, control, or prevent movement. This will strengthen the muscle(s) concentrically, eccentrically, or isometrically.
• Relax, allowing the muscle to be lengthened or stretched by the resistance.

Practice:

Identify the agonist muscle group(s) in the illustrations below:

How to Determine Why a Muscle (Agonist) Performs its Particular Actions/Movements at a Joint?

Note: Five tools/principles are helpful to enable you to identify all the actions that a muscle will cause when it contracts.

Tool #1 - The Line of Muscle Pull: the relationship of a muscle/tendon as it crosses a joint or joints. The where (anteriorly, posteriorly, medially, laterally, superiorly, inferiorly) and the how (vertically, horizontally, obliquely).

a. What joint(s) does the muscle cross? Identified by knowing the muscle’s origin and insertion.

b. Where does the muscle cross the joint? Does it cross the joint anteriorly, posteriorly, medially, laterally, superiorly, inferiorly, or a combination of these?

c. How does the muscle cross the joint? Does it cross the joint vertically, horizontally, or obliquely?

Practice: 

Identify the line of muscle pull. Then specifically answer the what, where, and how of each muscle below.

Tool #2 - Vector Analysis: If a muscle’s line of pull is diagonal/oblique then the line of pull can be broken down into its horizontal and vertical components. A vector is drawn as an arrow representing the line of pull of a muscle. The vector has both magnitude and direction.

1. The direction of the arrowhead indicates the direction of the line of pull of the muscle. This arrow begins at the muscle’s “insertion” and points towards the muscle’s “origin”. The direction of the arrow indicates that the less stable bone moves toward the more fixed or stabilized bone.
2. The length of the arrow indicates the magnitude of the muscle’s pull. Assessing the length of each vector component can be helpful in determining which action the muscle does BEST.

Note: We will only focus on the direction of the muscle’s line of pull for the purpose of helping to determine what actions the muscle performs when contracting concentrically.

• Vectors are useful to help give us a visual image of the muscles action(s).
• If a muscle’s line of pull is diagonal or oblique, component vectors can be drawn to represent the muscle’s sagittal plane (vertical component) and frontal plane (horizontal component) movements.
• A diagonal vector can be “resolved” into its component vectors (vertical and horizontal) by drawing in those vectors beginning at the tail of the vector arrow and ending at the vector’s arrowhead, forming a right angle at the intersection of the two component vectors.  Each component vector represents an action this muscle performs. Vertical component vectors are described as “up” or “down” and typically represent flexion and extension in the sagittal plane. Horizontal component vectors are described as “in” or “out” and typically represent the frontal plane movements of abduction and adduction.

Practice:

Draw in the vector representing the muscle’s line of pull, then “resolve” the vector into its vertical and horizontal components to identify the actions the muscle performs in the sagittal and frontal planes. Then, view the muscle from the standpoint of line of muscle pull to determine if it also performs movements in the transverse plane.

Tool #3 – Joint’s Functional Design: Diarthrodial joint classification (the # of planes of motion allowed by a joint).

• What plane(s) of motion does the joint allow? Is it uni-planar, bi-planar, or multi-planar? If a joint allows all three planes of motion (e.g., the shoulder joint) then you must consider whether a muscle that crosses the shoulder joint has a line of pull for motions in those planes.  As such, consider the actions of the following muscles:

        Gluteus Maximus: ______________________________________________________

        Extensor Carpi Radialis: ___________________________________________________

        Anterior Deltoid: _________________________________________________________

        Triceps Brachii: __________________________________________________________

Tool #4 – Palpation: palpation means to examine or explore by touching. By performing a movement against resistance the involved muscles contract and tense up. This state of tension/contraction indicates that the muscle is working to cause the movement (agonist).

Tool #5 – EMG: Electromyography is used in the clinical and research settings to record the electrically activity of a muscle. The magnitude of the activity correlates with the amount of force being produced by the muscle and therefore can be used to determine how active a muscle is during a specific movement and/or exercise.

Neutralization: Neutralization occurs when there are at least two agonists (Neutralizers) that also have antagonistic actions to each other in another plane(s) of movement. Because those antagonistic actions would be undesirable, they must neutralize each other’s actions so that only the common action is performed. Therefore, as they contract their antagonistic actions counteract or neutralize each other to prevent an undesirable movement.

Practice:

Identify the Neutralizers for the exercises below.

Antagonist: a muscle, or group of muscles, located on the opposite side of the joint from the agonist(s) and have opposite concentric actions from the agonists. During an open chain movement, the antagonist will typically be relaxed (reciprocal inhibition) as to not impede the movement of the agonist. Because the antagonist is relaxed, the action performed by the agonist will cause the origin and insertion of the antagonistic muscle to move farther apart thus creating a lengthening or stretching of the antagonist muscle.

Note: Keep in mind that the role of a muscle (agonist or antagonist) is specific to a particular joint action. In the case of performing elbow flexion (Illustration A), the biceps muscle is the agonist and the triceps is the antagonist. However, in performing the movement of elbow extension (Illustration B), the triceps muscle is the agonist and the biceps muscle is the antagonist.  

Figure 28 

Illustration A                                                                    

Figure 29     

Illustration B  

Stabilizer: a muscle(s) that contracts to fixate, hold firm, or stabilize the origin of the agonist so that the agonist muscle can perform its intended function/movement more efficiently. The process is referred to as Stabilization.

• A stabilizer muscle will always insert on the bone from which the agonist originates and works to prevent the action of the pull of the agonist muscle on that bone (the proximal segment).
• The “best stabilizer” muscle is the one that performs the opposite movements of what the pull of the agonist would cause on its origin, if it were not stabilized.
• The process to determine the stabilizer is 1) determine the agonist, 2) identify the origin of the agonist, 3) determine what movements (undesirable movements) the agonist would cause on its origin if the bone was not stabilized, and 4) identify a muscle(s) that performs the opposite actions = “best stabilizer”.

Activity
Figure 30	Agonist: _____________________________ Stabilized Body Part/Bone: _______________________ Undesirable Movement: _________________________ Best Stabilizer: ____________________________
Figure 31	Agonist: _____________________________ Stabilized Body Part/Bone: ______________________ Undesirable Movement: _________________________ Best Stabilizer: ____________________________
Figure 32	Agonist: _____________________________ Stabilized Body Part/Bone: _______________________ Undesirable Movement: _________________________ Best Stabilizer: ____________________________
Figure 33	Agonist: _____________________________ Stabilized Body Part/Bone: ________________________ Undesirable Movement: __________________________ Best Stabilizer: ____________________________
Figure 34	Agonist: _____________________________ Stabilized Body Part/Bone: _______________________ Undesirable Movement: __________________________ Best Stabilizer: ____________________________
Figure 35	Agonist: _____________________________ Stabilized Body Part/Bone: _______________________ Undesirable Movement: _________________________ Best Stabilizer: ____________________________

Synergist: a muscle(s) that is a partial antagonist and contracts to prevent an undesired movement of the agonist from occurring. This is known as synergism.

• Synergists are only needed when the agonist(s) are biarticular or multiarticular.
• When determining when synergism is involved remember the principle, “a muscle tends to perform all its actions when it contracts unless something (some force) prevents those movements from occurring.

Note: Sometimes the term synergist is used to describe an “assistant mover” or sometimes to encompass all the roles of agonists. This can be confusing. For this text, we will assign the term synergist as described above. In the examples below identify agonist, synergist, and undesired movement that the synergist prevents from occurring.

Table

Support Muscles: Muscles that contract to hold the body upright, or in proper position, while the action(s) being performed by the agonist(s) is occurring. These muscles usually help to maintain normal posture that might be compromised without their support function. Therefore, they primarily work to hold the spine stable, upright, plank, firm.

What muscles are working as support muscles during these exercises?

Figure 36

Figure 37

Figure 38

Considering that all muscles are involved somewhat in providing a “stabilizing or preventing” effect, their specific roles are further described according to additional characteristics involved. As such:

If the muscle providing a “stabilizing/preventing effect” is a partial antagonist, it is called a “__________________________”. 

If the muscle providing a “stabilizing/preventing effect” fixates the origin of the agonist, then it is called a “__________________________”. 

If the muscles providing a “stabilizing/preventing effect have antagonistic action(s) to each other, as well as a common action, they are called “__________________________”.

If the muscles providing a “stabilizing/preventing effect” are working to hold the body up right, stable, plank, or firm, they are functioning as “___________________________”.