Chapter 10: Muscular System

Muscle Actions and Interactions

The muscular system enables movement by acting on the skeleton. Muscles rarely act alone; instead, they work in groups to produce, control, or oppose movement. Understanding the roles of different muscles is essential for analyzing body motion.

• Pull, never push: Muscles can only contract (pull), not push. Movement is produced by pulling on bones.

• Agonist (Prime Mover): The muscle primarily responsible for a specific movement. Example: Pectoralis major is the prime mover for arm flexion.

• Antagonist: Muscle that opposes the action of the agonist. Example: Trapezius opposes the action of the pectoralis major in bench press.

• Synergist: Muscle that assists the agonist by adding extra force or reducing undesirable movement. Example: A muscle that stabilizes the shoulder during arm movement.

• Fixator: A type of synergist that immobilizes a bone or muscle's origin, providing a stable base for movement.

• Muscle group hierarchy:

  • Primary (agonist)

  • Secondary (synergist)

  • Tertiary (fixator)

Relative Position to Joints

Muscle actions are often described relative to the joints they move. Understanding these terms is crucial for describing movement accurately.

• Flexion: Decreases the angle between two bones at a joint. Example: Bending the elbow.

• Extension: Increases the angle between two bones at a joint. Example: Straightening the knee.

• Abduction: Movement away from the midline of the body. Example: Raising the arm sideways.

• Adduction: Movement toward the midline of the body. Example: Bringing the arm back to the side.

Functional Groups of Muscles

Muscles may play different roles depending on the movement and context. Their function can be predicted based on their position relative to the joint.

• Anterior side of a joint: Usually produces flexion. Example: Pectoralis major crosses the anterior side of the shoulder joint and flexes the arm.

• Posterior side of a joint: Usually produces extension. Example: Latissimus dorsi crosses the posterior side of the shoulder joint and extends the arm.

• Lateral side of a joint: Usually produces abduction. Example: Deltoid middle fibers abduct the arm.

• Medial side of a joint: Usually produces adduction. Example: Teres major adducts the arm.

Naming Skeletal Muscles

Muscle names often describe their characteristics, such as location, size, shape, direction of fibers, number of origins, and action.

• Location: Indicates the bone or region. Example: Temporalis (over the temporal bone).

• Number of origins: Prefixes like biceps (two), triceps (three), quadriceps (four). Example: Biceps brachii.

• Shape: Deltoid (triangle-shaped).

• Size: Maximus (largest), minimus (smallest), longus (long), brevis (short), magnus, medius.

• Direction of fibers: Rectus (straight), transversus (across), oblique (at an angle).

• Location of attachments: Named for points of origin and insertion. Example: Sternocleidomastoid (originates at sternum and clavicle, inserts on mastoid process).

• Action: Flexor, extensor, adductor, etc.

Fascicle Arrangement

The arrangement of muscle fibers (fascicles) affects muscle power and range of motion. There are several main types:

• Circular: Fascicles arranged in concentric rings. Example: Orbicularis oris (mouth).

• Convergent: Broad origin with fascicles converging toward a single tendon. Example: Pectoralis major.

• Fusiform: Spindle-shaped with a thick middle. Example: Biceps brachii.

• Parallel: Fascicles run parallel to the long axis. Example: Sartorius.

• Pennate: Fascicles attach obliquely to a central tendon.

  • Unipennate: Fascicles insert into one side of the tendon. Example: Extensor digitorum longus.

  • Bipennate: Fascicles insert into both sides of the tendon. Example: Rectus femoris.

  • Multipennate: Fascicles attach from many directions to several tendons. Example: Deltoid.

Fascicle Arrangement Table

Lever Systems in the Body

Muscles and bones act together as lever systems to produce movement. Understanding the components and classes of levers helps explain how force and speed are generated in the body.

• Lever: A rigid bar (bone) that moves on a fixed point (fulcrum) when a force is applied.

• Fulcrum: The fixed point around which the lever moves (usually a joint).

• Effort: The force applied by muscle contraction.

• Load: The resistance or weight that is moved.

Mechanical Advantage and Disadvantage

• Mechanical Advantage (Power Lever): Load is close to the fulcrum, effort is farther away. Allows a small effort to move a large load.

• Mechanical Disadvantage (Speed Lever): Effort is close to the fulcrum, load is farther away. Allows greater speed and range of motion, but less force.

Lever Classes Table

Key Equations

• Law of the Lever:

• Mechanical Advantage:

Summary

• Muscles work in groups as agonists, antagonists, and synergists to produce movement.

• Muscle actions are described relative to joints (flexion, extension, abduction, adduction).

• Muscle names reflect their characteristics and functions.

• Fascicle arrangement determines muscle power and range of motion.

• Lever systems in the body explain how muscles move bones efficiently.