Muscles of the Body: Structure, Function, and Organization - Chapter 11

Mechanics of Muscle Function

Overview of Skeletal Muscle Function

Skeletal muscles are responsible for producing movement by attaching to the bony skeleton. The way a muscle acts on the skeleton is determined by several physical parameters.

• Fascicle Arrangement: The organization of muscle fibers (fascicles) within a muscle influences its range of motion and power.

• Lever Mechanics: Muscles act on bones as levers, and the principles of lever mechanics determine the efficiency and type of movement produced.

• Position Relative to Joints: The location where a muscle crosses a joint affects the movement it produces.

Patterns of Fascicle Arrangement in Muscles

Types of Fascicle Arrangements

Muscle fascicles can be arranged in several patterns, each conferring different functional properties.

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

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

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

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

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

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

• Multipennate: Fascicles attach obliquely from many directions to several tendons (e.g., deltoid).

Example: The deltoid muscle is multipennate, allowing for powerful abduction of the arm.

Muscle Actions and Interactions

Principles of Muscle Action

Muscles work in groups to produce and control movement. A single muscle cannot reverse its own action; instead, opposing muscles are required.

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

• Antagonist: A muscle that opposes or reverses the action of the agonist.

• Synergist: Assists the agonist by adding extra force or reducing unwanted movements.

• Fixator: A type of synergist that stabilizes a bone or joint, providing a firm base for movement.

Example: During elbow flexion, the biceps brachii is the agonist, the triceps brachii is the antagonist, and the brachialis acts as a synergist.

Muscle Position and Movement

• Muscles with opposite actions are typically located on opposite sides of a joint.

• The action of a muscle can often be inferred from its position relative to the joint it crosses.

Example: Flexors are usually found on the anterior side of limbs, while extensors are on the posterior side.

Additional info:

• Lever mechanics in the body follow the basic principles of physics, where bones act as levers, joints as fulcrums, and muscles provide the force. The efficiency of movement depends on the type of lever system (first, second, or third class).

• Understanding muscle actions and interactions is crucial for diagnosing movement disorders and planning rehabilitation strategies.