Muscular System

Actions and Interactions of Muscles

The muscular system enables movement by the coordinated action of skeletal muscles. Muscles interact in specific ways to produce, assist, or oppose movements at joints.

• Muscles can only pull, never push: Muscles contract to create movement by pulling on bones; they cannot push bones away.

• Agonist (Prime Mover): The muscle primarily responsible for a specific movement. Example: The deltoid is the agonist for abduction of the arm.

• Synergist: A muscle that assists the agonist in performing its action. Example: The supraspinatus assists the deltoid in arm abduction.

• Antagonist: A muscle that opposes the action of the agonist. Example: For arm abduction, the teres major and latissimus dorsi act as antagonists.

Muscle Attachments: Origin and Insertion

Muscles attach to bones at two main points, which determine their function during movement.

• Origin: The fixed or immovable attachment point of a muscle, usually on a bone.

• Insertion: The movable attachment point, typically on the bone that moves during contraction.

• Movement occurs when the insertion is pulled toward the origin.

Muscle-Lever Systems

Basic Principles of Levers in the Body

Muscles act on bones using lever systems to produce movement. The arrangement of bones and muscles determines the efficiency and type of movement.

• Lever: A rigid bar (bone) that moves on a fixed point called the fulcrum (joint).

• Effort: The force applied by muscle contraction to move the lever.

• Load: The resistance (weight of bone, tissues, or external objects) moved by the lever.

Mechanical Advantage: If the effort is farther from the fulcrum than the load, the lever operates at a mechanical advantage (less force needed). Mechanical Disadvantage: If the effort is nearer to the fulcrum than the load, more force is needed (mechanical disadvantage).

Classes of Levers

There are three classes of levers in the human body, each with distinct arrangements of fulcrum, effort, and load.

Major Skeletal Muscles: Naming Criteria

Muscles are named based on several criteria that describe their location, function, and structure.

• Location: Indicates the body region where the muscle is found (e.g., brachialis in the arm).

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

• Shape: Descriptive terms like deltoid (triangle-shaped).

• Direction of Muscle Fibers: Rectus (straight), transversus (right angles), oblique (angled).

• Number of Origins: Biceps (2 origins), triceps (3 origins).

• Location of Attachments: Named according to origin and insertion (origin named first).

• Action: Named for the movement produced (e.g., flexor, extensor).

Example: Extensor carpi radialis longus combines action (extensor), location (carpi/radialis), and size (longus).

Learning Muscle Names

To effectively learn muscle names, students should:

• Be aware of information learned from muscle name

• Read descriptions and relate location and description

• Relate attachments to actions

• Act out movements on yourself

Example: Sternocleidomastoid muscle name indicates its attachments: sternum, clavicle, and mastoid process.

Muscle Injection Sites

Some muscles are preferred for injections due to their size and location.

• Injected substance cannot survive the oral route.

• Less painful because there are less pain receptors in deep muscle.

• Absorption is faster than other routes.

Common injection sites are discussed in class and shown on diagrams.

Key Equations and Concepts

• Lever Law:

• Mechanical Advantage:

Additional info: The notes provide foundational concepts for understanding muscle function, lever mechanics, and muscle identification, which are essential for Anatomy & Physiology students.