Chapter 10: The Muscular System – Structure, Function, and Mechanics

Muscle Actions and Interactions

Overview of Muscle Tissue

Muscle tissue is essential for movement and consists of three types: skeletal, cardiac, and smooth muscle. This chapter focuses on skeletal muscle, which is responsible for voluntary movements and is attached to bones.

• Origin: The fixed, immobile attachment point of the muscle.

• Insertion: The movable attachment point where the muscle exerts force.

• Body (Belly): The thick, central part of the muscle.

Functional Groups of Muscles

Muscles work in groups to produce coordinated movements. Each muscle can play different roles depending on the movement:

• Prime Mover (Agonist): The main muscle responsible for a specific movement.

• Antagonist: Opposes or reverses the action of the prime mover.

• Synergist: Assists the prime mover by adding force or reducing unwanted movements.

• Fixator: A type of synergist that stabilizes the origin of the prime mover, providing a stable base.

Key Principle: Muscles can only pull; they never push. When one muscle group contracts to produce movement, another group relaxes or contracts to reverse it.

Examples of Muscle Actions

• Flexion: A muscle crossing the anterior side of a joint produces flexion (e.g., pectoralis major).

• Extension: A muscle crossing the posterior side of a joint produces extension (e.g., latissimus dorsi).

• Abduction: A muscle crossing the lateral side of a joint produces abduction (e.g., deltoid middle fibers).

Naming Skeletal Muscles

Criteria for Naming Muscles

Skeletal muscles are named based on several criteria, often combining more than one:

• Location: Named for the bone or region they are associated with (e.g., temporalis over the temporal bone).

• Shape: Named for their distinctive shape (e.g., deltoid means triangle-shaped).

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

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

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

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

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

Example: Extensor carpi radialis longus – extends the wrist (carpi), is located near the radius, and is long.

Fascicle Arrangements and Muscle Shape

Patterns of Fascicle Arrangement

The arrangement of fascicles (bundles of muscle fibers) determines the shape and functional capabilities of muscles. Common patterns include:

• Circular: Fascicles arranged in concentric rings (e.g., orbicularis oris).

• Convergent: Fascicles converge toward a single tendon (e.g., pectoralis major).

• Parallel: Fascicles run parallel to the long axis of the muscle (e.g., sartorius).

• Fusiform: Spindle-shaped with expanded belly (e.g., biceps brachii).

• Pennate: Fascicles attach obliquely to a central tendon. Types include:

  • Unipennate: Fascicles attach to one side of the tendon (e.g., extensor digitorum longus).

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

  • Multipennate: Multiple tendons with fascicles arranged like feathers (e.g., deltoid).

Functional Implications: Parallel fascicles allow greater range of motion but less power. Pennate muscles have more fibers and generate more power but less range of motion.

Lever Systems in Muscle Action

Principles of Leverage

Skeletal muscles move bones using lever systems. A lever is a rigid bar (bone) that moves on a fixed point called the fulcrum (joint). The effort (muscle contraction) is applied to move a load (resistance).

• Lever: Bone

• Fulcrum: Joint

• Effort: Force applied by muscle

• Load: Resistance (bone, tissues, added weight)

Mechanical Advantage vs. Disadvantage

• Mechanical Advantage (Power Lever): Load is close to fulcrum, effort is far. Small effort moves large load. Used where strength is needed.

• Mechanical Disadvantage (Speed Lever): Load is far from fulcrum, effort is close. Load moves rapidly over a large distance; greater range of motion but less force.

Principle:

• If effort is farther from fulcrum than load, lever operates at mechanical advantage.

• If effort is nearer to fulcrum than load, lever operates at mechanical disadvantage.

Classes of Lever Systems

Class	Arrangement	Example	Advantage
First-Class	Fulcrum between load and effort	Seesaw, scissors, neck extension	Can be advantage or disadvantage
Second-Class	Load between fulcrum and effort	Wheelbarrow, standing on toes	Mechanical advantage (power)
Third-Class	Effort between fulcrum and load	Biceps curl, tweezers	Mechanical disadvantage (speed)

Summary: Power levers are slower but stronger; speed levers are faster but weaker. The arrangement of bones, joints, and muscles determines the type of lever and the movement produced.

Superficial Muscles of the Body

Anterior and Posterior Views

Superficial muscles are those located near the surface of the body and are visible in anatomical diagrams. They are important for identifying muscle groups and understanding their functions in movement.

• Anterior View: Shows muscles on the front of the body (e.g., pectoralis major, deltoid).

• Posterior View: Shows muscles on the back of the body (e.g., latissimus dorsi, trapezius).

Application: Knowledge of superficial muscles aids in clinical assessment, physical training, and understanding movement mechanics.