Chapter 11: Skeletal Muscle Anatomy

How Skeletal Muscles Produce Movement

Skeletal muscles are responsible for voluntary movements by contracting and pulling on bones. This process is fundamental to locomotion and manipulation of the environment.

• Muscle Contraction: Muscles contract, shortening their fibers and generating force.

• Attachment to Bones: Muscles are attached to bones via tendons, allowing them to move the skeleton.

• Movement: The contraction pulls on the bone, causing movement at a joint.

• Example: The biceps brachii contracts to flex the elbow.

Relationship Between Bones and Skeletal Muscles in Producing Body Movements

Bones act as levers, and joints serve as fulcrums. Muscles provide the force (effort) needed to move the bones (load).

• Lever System: Bones function as levers, joints as fulcrums, and muscles as the source of effort.

• Movement: The interaction between bones and muscles enables complex movements.

• Example: The forearm acts as a lever when the biceps brachii contracts to lift an object.

Origin and Insertion

The origin is the fixed attachment point of a muscle, while the insertion is the movable attachment.

• Origin: The stationary bone where the muscle begins.

• Insertion: The bone that moves when the muscle contracts.

• Action: The specific movement produced by muscle contraction.

Levers and Fulcrums

A lever is a rigid structure (bone) that moves around a fixed point called a fulcrum (joint). Muscles provide the effort to move a load.

• Lever: A bone that moves about a fulcrum.

• Fulcrum: The joint where movement occurs.

• Effort: The force applied by the muscle.

• Load: The weight or resistance moved.

Types of Levers

• First-Class Lever: Fulcrum is between the effort and load (e.g., neck muscles).

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

• Third-Class Lever: Effort is between the fulcrum and load (e.g., biceps brachii at the elbow).

Most common lever in the human body: Third-class lever.

Fascicle Arrangements in Skeletal Muscle

Fascicles are bundles of muscle fibers. Their arrangement affects muscle strength and range of motion.

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

• Pennate: Fascicles attach obliquely to a tendon (e.g., rectus femoris).

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

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

• Strength vs. Range: Pennate muscles are stronger; parallel muscles have greater range of motion.

Muscle Group Roles: Prime Mover, Antagonist, Synergist, Fixator

Muscles work together to produce coordinated movements.

• Prime Mover (Agonist): Main muscle responsible for movement.

• Antagonist: Opposes the prime mover.

• Synergist: Assists the prime mover.

• Fixator: Stabilizes the origin of the prime mover.

• Example: During elbow flexion, biceps brachii is the prime mover, triceps brachii is the antagonist, brachialis is the synergist, and muscles of the shoulder act as fixators.

Muscle Classification and Naming

Muscles are classified and named based on various features.

• Size: (e.g., gluteus maximus)

• Shape: (e.g., deltoid)

• Action: (e.g., flexor carpi radialis)

• Location: (e.g., pectoralis major)

• Direction of fibers: (e.g., rectus abdominis)

• Number of origins: (e.g., biceps brachii)

• Attachment points: (e.g., sternocleidomastoid)

• Relative position: (e.g., external oblique)

Eight features used in naming muscles: Size, shape, action, location, direction of fibers, number of origins, attachment points, relative position.

Principal Skeletal Muscles: Origin, Insertion, and Action

Selected muscles from each region of the body, with their origin, insertion, and action.

Additional info: Table entries are representative and may be expanded for more detailed study.