Skeletal Muscle Movements, Roles, and Anatomy: Study Guide

Muscle Movements, Roles, and Names

The Five Golden Rules of Skeletal Muscle Activity

Understanding skeletal muscle activity is fundamental to anatomy and physiology. The following five rules summarize the essential principles governing muscle function:

• Rule 1: With few exceptions, all skeletal muscles cross at least one joint.

• Rule 2: The bulk of a skeletal muscle typically lies proximal to the joint it crosses.

• Rule 3: All skeletal muscles have at least two attachments: the origin (fixed point) and the insertion (movable point).

• Rule 4: Skeletal muscles can only pull; they never push.

• Rule 5: During contraction, the muscle insertion moves toward the origin.

Example: The biceps brachii crosses the elbow joint, with its bulk proximal to the joint, and pulls the forearm toward the shoulder during contraction.

Types of Body Movements

Muscle Attachments: Origin and Insertion

Muscles are attached to bones at two points: the origin (immovable or less movable bone) and the insertion (movable bone). When a muscle contracts, the insertion moves toward the origin, resulting in body movement across joints.

Flexion, Extension, and Hyperextension

These are fundamental movements at hinge and ball-and-socket joints:

• Flexion: Decreases the angle of the joint, bringing two bones closer together (e.g., bending the elbow or knee).

• Extension: Increases the angle between two bones, straightening the joint (e.g., straightening the elbow or knee).

• Hyperextension: Extension beyond the normal range (greater than 180°).

Rotation

Rotation is the movement of a bone around its longitudinal axis, common in ball-and-socket joints. For example, shaking your head "no" involves rotation of the atlas around the dens of the axis.

Abduction, Adduction, and Circumduction

• Abduction: Movement of a limb away from the midline.

• Adduction: Movement of a limb toward the midline.

• Circumduction: Combination of flexion, extension, abduction, and adduction, resulting in a circular motion at ball-and-socket joints.

Special Movements

• Dorsiflexion: Lifting the foot so the superior surface approaches the shin.

• Plantar flexion: Pointing the toes away from the head.

• Inversion: Turning the sole of the foot medially.

• Eversion: Turning the sole of the foot laterally.

• Supination: Forearm rotates laterally so the palm faces anteriorly; radius and ulna are parallel.

• Pronation: Forearm rotates medially so the palm faces posteriorly; radius and ulna cross.

• Opposition: Moving the thumb to touch the tips of other fingers on the same hand.

Interactions of Skeletal Muscles in the Body

Muscle Action and Group Roles

Skeletal muscles work in groups to produce coordinated movements. Muscles can only pull as they contract, not push. Groups of muscles that produce opposite actions are located on opposite sides of a joint.

• Prime mover: Muscle with the major responsibility for a certain movement.

• Antagonist: Muscle that opposes or reverses a prime mover.

• Synergist: Muscle that aids a prime mover or reduces undesirable movements.

• Fixator: Specialized synergists that stabilize the origin of a prime mover.

Naming Skeletal Muscles

Criteria for Muscle Naming

Skeletal muscles are named based on several criteria:

• Direction of muscle fibers: e.g., rectus (straight), oblique (angled), transverse (horizontal).

• Relative size: e.g., maximus (largest).

• Location: e.g., temporalis (temporal bone).

• Number of origins: e.g., triceps (three heads).

• Origin and insertion: e.g., sternocleidomastoid (originates at sternum and clavicle, inserts at mastoid process).

• Shape: e.g., deltoid (triangular).

• Action: e.g., flexor and extensor (flexes or extends a bone).

Fascicle Arrangement and Muscle Structure

The arrangement of muscle fascicles determines muscle shape and function. Common arrangements include circular, convergent, parallel, fusiform, unipennate, bipennate, and multipennate.

Superficial Muscles of the Body

Superficial Anterior Muscles

Major superficial muscles of the anterior body include muscles of the head, neck, trunk, arm, and leg. Their origins, insertions, and primary actions are summarized in the following tables:

Name	Origin	Insertion	Primary action(s)
Frontalis	Cranial aponeurosis	Skin of eyebrows	Raises eyebrows
Orbicularis oculi	Frontal bone and maxilla	Tissue around eyes	Blinks and closes eyes
Orbicularis oris	Mandible and maxilla	Skin and muscle around mouth	Closes and protrudes lips
Temporalis	Temporal bone	Mandible	Closes jaw
Zygomaticus	Zygomatic bone	Skin and muscle at corner of mouth	Raises corners of mouth
Masseter	Temporal bone	Mandible	Closes jaw
Buccinator	Maxilla and mandible near molars	Orbicularis oris	Compresses cheek (as in sucking); holds food between teeth during chewing
Sternocleidomastoid	Sternum and clavicle	Temporal bone (mastoid process)	Flexes neck; laterally rotates head
Platysma	Connective tissue covering of superior chest muscles	Tissue around mouth	Pulls corners of mouth inferiorly

Name	Origin	Insertion	Primary action(s)
Pectoralis major	Sternum, clavicle, and first to sixth ribs	Proximal humerus	Adducts and flexes humerus
Rectus abdominis	Pubis	Sternum and fifth to seventh ribs	Flexes vertebral column
External oblique	Lower eight ribs	Iliac crest	Flexes and rotates vertebral column
Biceps brachii	Scapula of shoulder girdle	Proximal radius	Flexes elbow and supinates forearm
Brachialis	Distal humerus	Proximal ulna	Flexes elbow
Deltoid	See Table 6.4	Abducts arm

Name	Origin	Insertion	Primary action(s)
Iliopsoas	Ilium and lumbar vertebrae	Femur (lesser trochanter)	Flexes hip
Adductor muscles	Pelvis	Proximal femur	Adduct and medially rotate thigh
Sartorius	Ilium	Proximal tibia	Flexes thigh on hip
Quadriceps group	Vastus: femur; rectus femoris: pelvis	Tibial tuberosity via patellar ligament	All extend knee; rectus femoris also flexes hip on thigh
Tibialis anterior	Proximal tibia	First cuneiform (tarsal) and first metatarsal of foot	Dorsiflexes and inverts foot
Extensor digitorum longus	Proximal tibia and fibula	Distal toes 2–5	Extends toes
Fibularis muscles	Fibula	Metatarsals of foot	Plantar flex and evert foot

Superficial Posterior Muscles

Major superficial muscles of the posterior body include muscles of the head, neck, trunk, arm, and leg. Their origins, insertions, and primary actions are summarized in the following tables:

Name	Origin	Insertion	Primary action(s)
Occipitalis	Occipital and temporal bones	Cranial aponeurosis	Pulls scalp posteriorly
Trapezius	Occipital bone and all cervical and thoracic vertebrae	Scapular spine and clavicle	Raises, retracts, and rotates scapula
Latissimus dorsi	Lower spine and iliac crest	Proximal humerus	Extends and adducts humerus
Erector spinae	Iliac crests, sacrum, and lumbar vertebrae	Ribs, thoracic and cervical vertebrae	Extends and laterally flexes spine
Quadratus lumborum	Iliac crest, lumbar fascia	Transverse processes of lumbar vertebrae	Flexes spine laterally
Deltoid	Scapular spine and clavicle	Humerus (deltoid tuberosity)	Abducts humerus

Name	Origin	Insertion	Primary action(s)
Triceps brachii	Shoulder girdle and proximal humerus	Olecranon process of ulna	Extends elbow
Flexor carpi radialis	Distal humerus	Second and third metacarpals	Flexes wrist and abducts hand
Flexor carpi ulnaris	Distal humerus and posterior ulna	Carpals of wrist and fifth metacarpal	Flexes wrist and adducts hand
Extensor carpi radialis	Humerus	Second and third metacarpals	Extends wrist and abducts hand
Extensor digitorum	Distal humerus	Base of second and third phalanges of fingers 2–5	Extends fingers

Name	Origin	Insertion	Primary action(s)
Gluteus maximus	Sacrum and ilium	Proximal femur (gluteal tuberosity)	Extends hip (when forceful extension is required)
Gluteus medius	Ilium	Proximal femur	Abducts thigh; steadies pelvis during walking
Hamstring muscles (biceps femoris, semimembranosus, semitendinosus)	Ischial tuberosity	Proximal tibia (head of fibula in case of biceps femoris)	Flexes knee and extends hip
Gastrocnemius	Distal femur	Calcaneus (heel via calcaneal tendon)	Plantar flexes foot and flexes knee
Soleus	Proximal tibia and fibula	Calcaneus	Plantar flexes foot

Major Superficial Muscles: Anterior and Posterior Views

Comprehensive diagrams of the major superficial muscles of the body are essential for memorization and identification in anatomy courses.

Developmental Aspects of the Muscular System

Muscle Control and Maturation

Increasing muscular control reflects the maturation of the nervous system. Muscle control is achieved in a superior/inferior and proximal/distal direction.

Exercise and Muscle Health

Regular exercise is essential for muscle health. Without exercise, muscles atrophy (decrease in size and strength). With vigorous exercise, muscles hypertrophy (increase in size).

Aging and Muscle Mass

As humans age, muscle mass decreases and muscles become more sinewy. Exercise helps retain muscle mass and strength throughout life.