Muscles: Structure, Function, and Movement in Human Anatomy

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Muscle Actions and Functional Groups

Overview of Muscle Actions

Muscles are responsible for producing movement by pulling on bones. They always pull a movable bone (insertion) toward a fixed bone (origin) when they contract. Muscles never push on bones. For every action performed by a muscle, there is an opposing muscle that reverses the action, known as an antagonist.

Prime Mover (Agonist): The main muscle responsible for a specific movement.
Antagonist: Muscle that opposes or reverses a particular movement.
Synergist: Assists the prime mover by adding force or reducing unnecessary movement.
Fixator: A type of synergist that stabilizes the origin of a muscle.

Example: The biceps brachii is the prime mover for forearm flexion, while the triceps brachii is the antagonist. For forearm extension, their roles reverse.

Naming Skeletal Muscles

Criteria for Muscle Naming

Skeletal muscles are named based on several criteria that describe their characteristics or actions:

Location: Indicates the bone or region (e.g., sternocleidomastoid).
Shape: Describes the muscle's form (e.g., deltoid means triangular).
Number of Origins: Indicates the number of attachment points (e.g., biceps has two origins, triceps has three).
Action: Describes the movement produced (e.g., flexor, extensor).
Relative Size: Terms like maximus (largest), minimus (smallest), longus (long), brevis (short).
Direction of Fibers: Rectus (straight), transversus (horizontal), oblique (diagonal).
Location of Attachments: Named for origin and insertion points.

Deltoid muscle, triangular shape Biceps and triceps muscle origins

Muscle Movements and Body Planes

Understanding Movements

Muscle movements are described relative to anatomical position and occur in specific planes of the body:

Sagittal Plane: Divides the body into left and right parts.
Frontal (Coronal) Plane: Divides the body into anterior and posterior parts.
Transverse Plane: Divides the body into superior and inferior parts.

Body planes: sagittal, frontal, transverse

Angular Movements

Angular movements change the angle between bones at a joint:

Flexion: Decreases the angle of the joint (e.g., bending the elbow).
Extension: Increases the angle of the joint (e.g., straightening the elbow).
Hyperextension: Extension beyond the normal range of motion.

Flexion, extension, and hyperextension of the neck Flexion and extension at the shoulder and knee

Movements in the Frontal Plane

Abduction: Movement away from the midline.
Adduction: Movement toward the midline.
Circumduction: Circular movement combining flexion, abduction, extension, and adduction.

Body planes Abduction, adduction, and circumduction of the upper limb

Rotational Movements

Rotation is the turning of a bone around its own long axis. Examples include shaking the head "no" (between C1 and C2 vertebrae) and rotation of the humerus or femur.

Rotation of the head, neck, and lower limb

Special Movements of the Foot

Dorsiflexion: Lifting the foot so the superior surface approaches the shin.
Plantar Flexion: Pointing the toes downward.
Inversion: Turning the sole of the foot medially.
Eversion: Turning the sole of the foot laterally.

Dorsiflexion and plantar flexion Foot inversion and eversion

Special Movements of the Mandible and Thumb

Protraction: Moving a body part anteriorly (e.g., jutting the jaw forward).
Retraction: Moving a body part posteriorly (e.g., pulling the jaw back).
Elevation: Lifting a body part superiorly (e.g., closing the mouth).
Depression: Moving a body part inferiorly (e.g., opening the mouth).
Opposition: Movement of the thumb to touch the tips of other fingers, enabling grasping.

Protraction and retraction of the mandible Elevation and depression of the mandible Opposition of the thumb

Learning Muscle Locations

Three-Dimensional Structure of Muscles

Muscles are three-dimensional organs, and many regions of the body have multiple layers of skeletal muscle. Some muscles may appear in both anterior and posterior views due to their position and orientation.

Anterior view of superficial muscles of the body Posterior view of superficial muscles of the body

Muscles of the Head

Groups of Head Muscles

Muscles of the head are divided into two main groups:

Muscles of facial expression
Muscles of mastication (chewing) and tongue movement

Muscles of facial expression Muscles of mastication and tongue movement

Muscles of the Neck and Vertebral Column

Head and Neck Movement

Sternocleidomastoid: Major head flexor; when both sides contract, the neck flexes. When one side contracts, the head rotates toward the opposite shoulder.
Erector Spinae Group: Prime movers of back extension and lateral bending.

Sternocleidomastoid muscle Erector spinae group

Muscles of the Thorax, Shoulder, and Abdominal Wall

Major Muscles and Their Actions

Muscle	Origin	Insertion	Action
Pectoralis major	Clavicle, sternum, cartilage of ribs 1-6 (or 7), aponeurosis of external oblique muscle	Fibers converge to insert by short tendon into intertubercular sulcus of humerus	Prime mover of arm flexion; adducts, medially rotates arm
Serratus anterior	By a series of muscle slips from ribs 1-8 (or 9)	Entire anterior surface of vertebral border of scapula	Rotates scapula so its inferior angle moves laterally and upward
Deltoid	Lateral 1/3 of clavicle; acromion and spine of scapula	Deltoid tuberosity of humerus	Prime mover of arm abduction

Table of thorax, shoulder, and abdominal wall muscles Muscles of the thorax and shoulder

Summary Table: Muscle Actions Relative to Joint Position

Muscle Position	Movement Produced	Example
Anterior side of joint	Flexion	Pectoralis major
Posterior side of joint	Extension	Latissimus dorsi
Lateral side of joint	Abduction	Deltoid middle fibers
Medial side of joint	Adduction	Teres major

Muscle actions relative to joint position Planes of the body

Additional info: The above tables and images provide a comprehensive overview of muscle actions, naming conventions, and anatomical relationships, which are essential for understanding human movement and anatomy at the college level.