The Muscular System: Functional Groups, Actions, and Naming of Skeletal Muscles

The Muscular System

Overview

The muscular system is essential for movement, posture, and stability in the human body. Muscles work in groups to produce coordinated actions, and their structure and position relative to joints determine their function.

Functional Groups of Muscles

Prime Movers (Agonists), Antagonists, Synergists, and Stabilizers

Muscles are organized into functional groups based on their roles in movement:

• Prime mover (agonist): The muscle primarily responsible for producing a specific movement at a joint.

• Antagonist: The muscle that opposes or reverses the action of the prime mover.

• Synergist: A muscle that assists the prime mover by adding extra force or reducing undesirable movements.

• Stabilizer (fixator): A type of synergist that stabilizes the origin of the prime mover, providing a stable base for movement.

Key Point: Prime movers and antagonists are typically located on opposite sides of the joint they act upon.

Example: During elbow flexion, the biceps brachii acts as the prime mover, while the triceps brachii serves as the antagonist.

Muscle Actions Relative to Joints

Flexion and Extension

The position of a muscle relative to a joint determines its action:

• Flexion: Muscles crossing on the anterior side of a joint typically produce flexion.

• Extension: Muscles crossing on the posterior side of a joint typically produce extension.

Example: The pectoralis major (anterior view) produces flexion at the shoulder, while the latissimus dorsi (posterior view) produces extension.

Additional info: There are exceptions to these general rules depending on the specific joint and muscle orientation.

Muscle Actions: Abduction and Adduction

Movement Away From or Toward the Midline

• Abduction: Movement of a limb away from the midline of the body, typically produced by muscles crossing on the lateral side of a joint.

• Adduction: Movement of a limb toward the midline, produced by muscles crossing on the medial side of a joint.

Example: The deltoid muscle abducts the arm at the shoulder, while the pectoralis major adducts it.

Muscle Roles Can Change

Functional Versatility

The same muscle may act as a prime mover, antagonist, or synergist depending on the movement being performed.

• Example: The deltoid is a prime mover for shoulder abduction, but may act as a synergist or antagonist in other shoulder movements.

Naming Skeletal Muscles

Criteria for Muscle Names

Skeletal muscles are named based on several criteria:

• Location: Indicates the body region (e.g., temporalis over the temporal bone).

• Shape: Describes the muscle’s form (e.g., deltoid = triangle).

• Size: Terms such as maximus (largest), medius (medium), minimus (smallest), longus (long), brevis (short).

• Direction of fibers: rectus (straight), transversus (transverse), oblique (angled).

• Number of origins: biceps (two origins), triceps (three origins).

• Location of attachments: Named for sites of origin and insertion (e.g., sternocleidomastoid attaches to sternum, clavicle, and mastoid process).

• Action: Describes the movement produced (e.g., flexor, extensor, abductor, adductor).

Muscle Fiber Arrangement and Power Generation

Patterns of Muscle Fiber Arrangement

The arrangement of muscle fibers affects the muscle’s range of motion and power:

Pattern	Description	Example
Parallel	Fibers run parallel to the long axis of the muscle	Sartorius
Pennate	Fibers are short and attach obliquely to a central tendon	Rectus femoris
Convergent	Broad origin, fibers converge toward a single tendon	Pectoralis major
Circular	Fibers arranged in concentric rings	Orbicularis oris

• Parallel muscles can shorten the most, allowing for greater range of motion.

• Pennate muscles are generally more powerful due to the higher number of fibers packed into a given muscle volume.

Additional info: Fusiform muscles are a subtype of parallel muscles with a spindle-shaped belly.

Key Principles

• Muscles can only pull; never push. Movement is produced by contraction, not by extension of muscle tissue.

Example: The biceps brachii pulls the forearm upward during flexion, but cannot push it downward.