Skeletal Muscle Moves Bones at Joints

Overview

Skeletal muscles are responsible for moving bones at joints, enabling voluntary movement and stability. This chapter explores how muscles act, are named, and how their structure affects their function.

Actions & Interactions of Skeletal Muscles

Functional Groups

• 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.

• Fixator: A type of synergist that stabilizes the origin of the prime mover, allowing efficient movement.

Prime movers and antagonists are typically located on opposite sides of the joint they act upon, ensuring coordinated movement.

Naming Skeletal Muscles

Criteria for Naming

• Number of origins: Indicates how many tendons of origin a muscle has (e.g., biceps = two origins).

• Location of attachments: Names may reflect the bones to which the muscle is attached (e.g., sternocleidomastoid).

• Muscle action: Describes the primary movement produced (e.g., flexor, extensor).

• Several criteria can be combined for more descriptive names (e.g., extensor carpi radialis longus).

Understanding muscle names helps identify their location, function, and structure.

Muscle Mechanics: Fascicle Arrangement and Lever Systems

Factors Impacting Muscle Force and Speed

• Fascicle arrangement: The organization of muscle fibers affects the range of motion and power.

• Lever systems: Bones and joints act as levers and fulcrums, influencing movement efficiency.

Fascicle Arrangements in Muscles

Types of Fascicle Arrangements

The arrangement of fascicles determines a muscle's range of motion and power. Common types include:

Each arrangement affects the muscle's ability to generate force and the range of movement it can produce.

Lever Systems in Muscle Mechanics

Components of a Lever System

• Lever: A rigid structure (usually a bone).

• Fulcrum: The pivot point (a joint).

• Effort: The force applied by a muscle.

• Load: The resistance or weight moved (body part or external object).

Lever systems allow muscles to move loads efficiently, with the position of the fulcrum relative to the effort and load determining mechanical advantage or disadvantage.

Mechanical Advantage vs. Disadvantage

• Mechanical advantage (power lever): The effort arm is longer than the load arm, allowing a small effort to move a large load. Example: calf raises.

• Mechanical disadvantage (speed lever): The effort arm is shorter than the load arm, allowing rapid movement but requiring more force. Example: bicep curls.

Equation for lever mechanics:

Classes of Lever Systems

• First-class lever: Fulcrum is between the effort and load (e.g., seesaw, neck extension).

• Second-class lever: Load is between the fulcrum and effort (e.g., wheelbarrow, standing on tiptoe).

• Third-class lever: Effort is between the fulcrum and load (e.g., tweezers, biceps curl).

Each class of lever provides different advantages in terms of speed, range of motion, and force.

Summary Table: Fascicle Arrangements and Examples

Conclusion

Understanding the actions, naming conventions, fascicle arrangements, and lever systems of skeletal muscles is essential for comprehending how the body produces movement and maintains posture. These principles form the foundation for further study of muscle anatomy and physiology.