Muscles and Their Actions

Intrinsic and Extrinsic Muscles

• Intrinsic muscles: small muscles located within the same body region, e.g. muscles in the hands and feet.
• Extrinsic muscles: muscles that originate in one body part and end in another, often crossing multiple joints and performing multiple actions.

Muscle Actions and Joint Actions

• Muscle action: the result of muscle contraction at a joint, e.g. the bicep brachii flexes the elbow.
• Joint actions: movements such as flexion, extension, abduction, adduction, internal rotation, and external rotation.
• Aggregate muscle action: multiple muscles working together to produce movement, including stabilization by antagonistic muscles, e.g. wrist flexion.

Muscle Innervation and Amplitude

• Innervation: the nerve supply to a muscle, crucial for muscle function, especially during injuries or diseases.
• Amplitude: the range of muscle fiber length from fully stretched to fully contracted.

Muscle Structure and Tendons

• Gaster (belly/body): the part of the muscle containing contractile fibers.
• Tendons: dense, fibrous connective tissue that connects muscle to bone, non-contractile, providing attachment points.

Aponeurosis and Fascia

• Aponeurosis: a flattened tendon, e.g. plantar fascia, IT band, binding, wrapping, and connecting muscles to other structures.
• Fascia: connective tissue that organizes, separates, or groups muscles and other structures, forming retinaculum around joints.

Origin and Insertion

• Origin: the proximal attachment of a muscle (closest to the midline), least movable part or attachment of the muscle.
• Insertion: the distal attachment of a muscle (farthest from the midline), most movable part.

Contraction Mechanics

• Contraction: muscles contract toward the center, pulling both ends.
• Stabilization: stabilizing muscles prevent unwanted movement, allowing specific joint actions.
• Electrical Stimulation: muscles contract toward the center when stimulated electrically without voluntary control.
• Stabilization Importance: other muscles stabilize parts of the body to direct movement efficiently.

When a Muscle Contracts

• Pulls both ends toward the gaster.
• If neither bone is stabilized, both bones move toward each other.
• Typically, one bone is more stabilized, and the less stabilized bone moves toward the more stabilized bone.