Muscle Actions, Naming, and Arrangements: Study Notes for ANP College Students

Muscle Actions and Their Anatomical Location

Relationship Between Muscle Location and Action

Muscle actions are closely related to their anatomical position in the body. Understanding these relationships helps predict the function of a muscle based on its location.

• Anterior (front) muscles: Typically act as flexors, bending joints (e.g., biceps brachii flexes the elbow).

• Posterior (back) muscles: Usually serve as extensors, straightening joints (e.g., triceps brachii extends the elbow).

• Medial muscles: Function as adductors, moving limbs toward the body's midline (e.g., adductor longus in the thigh).

• Lateral muscles: Act as abductors, moving limbs away from the midline (e.g., deltoid abducts the arm).

Functional Groups of Muscles in Movement

Roles of Muscles During Movement

Muscles work in groups to produce coordinated movement. Each muscle in a group has a specific role:

• Prime mover (agonist): The main muscle responsible for a specific movement. Example: Biceps brachii is the prime mover for elbow flexion.

• Antagonist: The muscle that opposes the action of the prime mover. It is usually located on the opposite side of the joint. Example: Triceps brachii is the antagonist during elbow flexion, but becomes the prime mover during elbow extension.

• Synergist: Assists the prime mover by adding extra force or reducing unnecessary movement. Example: Brachialis and brachioradialis assist the biceps brachii in elbow flexion. If a synergist is strained, the movement can still occur but with reduced efficiency (about 15-20% less force).

• Fixator: Stabilizes the origin of the prime mover so that it can act more efficiently. Example: The deltoid stabilizes the shoulder during biceps curls.

Muscle Naming Conventions

How Muscles Get Their Names

Muscle names often describe their characteristics, including location, shape, size, direction of fibers, number of origins, attachment points, and action.

• Location: Indicates where the muscle is found (e.g., rectus abdominis in the abdomen, biceps femoris in the thigh).

• Shape: Describes the muscle's form (e.g., deltoid is triangular, trapezius is trapezoid-shaped).

• Size: Compares relative size (e.g., gluteus maximus is larger than gluteus minimus).

• Direction of fibers: Refers to the orientation of muscle fibers (e.g., rectus means straight, oblique means diagonal).

• Number of origins: Indicates how many tendons of origin a muscle has (e.g., biceps = 2, triceps = 3, quadriceps = 4).

• Location of attachments: Names the points where the muscle attaches (e.g., sternocleidomastoid attaches to the sternum, clavicle, and mastoid process).

• Action: Describes the movement produced (e.g., flexor digitorum flexes the fingers).

Examples of Muscle Names and Their Origins

• Extensor digitorum: Named for its action (extension) and the body part it acts on (digits/fingers). It extends the fingers.

• External oblique: Named for its location (external, on the outer side of the abdomen) and the direction of its fibers (oblique/diagonal).

• Pectoralis major: Named for its location (pectoral/chest region) and size (major = larger muscle).

• Biceps femoris: Named for its number of origins (biceps = two) and location (femoris = thigh region).

• Orbicularis oris: Named for its circular shape (orbicularis) and location (oris = mouth).

• Trapezius: Named for its trapezoid shape.

Muscle Shapes and Arrangements

Types of Muscle Fiber Arrangements

The arrangement of muscle fibers affects the muscle's function and strength. Common arrangements include:

• Circular: Fibers are arranged in concentric rings. These muscles act as sphincters to open and close body openings. Example: Orbicularis oris (mouth), orbicularis oculi (eye).

• Convergent: Broad origin with fibers converging to a single insertion point. Example: Pectoralis major.

• Fusiform: Spindle-shaped with a thick belly and tapered ends. Example: Biceps brachii.

• Parallel: Fibers run parallel to the long axis of the muscle, resulting in uniform thickness. Example: Sartorius.

• Pennate: Short fibers attach obliquely to a central tendon, resembling a feather. Types include:

  • Unipennate: Fibers insert into one side of the tendon (e.g., extensor digitorum longus).

  • Bipennate: Fibers insert into both sides of the tendon (e.g., rectus femoris).

  • Multipennate: Multiple bipennate arrangements (e.g., deltoid).

Table: Comparison of Muscle Fiber Arrangements

Arrangement	Description	Example
Circular	Fibers arranged in concentric rings	Orbicularis oris
Convergent	Broad origin, fibers converge to a single tendon	Pectoralis major
Fusiform	Spindle-shaped, thick in the middle	Biceps brachii
Parallel	Fibers run parallel to the long axis	Sartorius
Unipennate	Fibers insert into one side of the tendon	Extensor digitorum longus
Bipennate	Fibers insert into both sides of the tendon	Rectus femoris
Multipennate	Multiple bipennate arrangements	Deltoid

Application: Elbow Extension

Muscle Roles in Elbow Extension

During elbow extension, specific muscles act as the prime mover and antagonist:

• Prime mover: Triceps brachii (extends the elbow)

• Antagonist: Biceps brachii (opposes extension, flexes the elbow)

Summary Table: Muscle Roles in Movement

Role	Description	Example (Elbow Flexion)
Prime mover (Agonist)	Main muscle causing movement	Biceps brachii
Antagonist	Opposes the prime mover	Triceps brachii
Synergist	Assists the prime mover	Brachialis, brachioradialis
Fixator	Stabilizes the origin of the prime mover	Deltoid (shoulder stabilization)

Key Definitions

• Flexion: Decreasing the angle between two bones (bending a joint).

• Extension: Increasing the angle between two bones (straightening a joint).

• Adduction: Movement toward the midline of the body.

• Abduction: Movement away from the midline of the body.

• Origin: The fixed attachment point of a muscle.

• Insertion: The movable attachment point of a muscle.

Additional Info

• Unipennate muscle arrangement: Resembles a single feather, with muscle fibers attaching obliquely to one side of the tendon.

• If a synergist muscle is strained, the movement can still be performed, but with reduced strength and stability (synergists contribute about 15-20% of the force).