Study Guide: Muscles of the Human Body – Anatomy & Physiology

Muscle Naming Criteria

Overview of Muscle Naming

Muscles are named based on several anatomical and functional criteria. Understanding these criteria helps in identifying and classifying muscles throughout the body.

• Action of the muscle: Describes the primary movement produced (e.g., flexor, extensor).

• Shape of the muscle: Refers to the muscle's geometric form (e.g., deltoid = triangular).

• Location of the origin and insertion: Indicates where the muscle starts and ends.

• Number of origins (heads): Muscles may have multiple points of origin (e.g., biceps = two heads).

• Location relative to a bone or body region: Specifies anatomical position (e.g., femoris = thigh).

• Direction of muscle fibers: Orientation of fibers (e.g., rectus = straight).

• Relative size of the muscle: Compares size to other muscles (e.g., maximus = largest).

Example: The gluteus maximus is named for its location (gluteal region) and size (maximus = largest).

Muscles of the Head

Major Muscles and Their Functions

The muscles of the head are responsible for facial expressions, mastication (chewing), and movements of the jaw.

• Buccinator: Used in smiling and compresses the cheek.

• Frontalis: Raises the eyebrows and wrinkles the forehead.

• Orbicularis oculi: Used in winking; closes the eyelids.

• Masseter: Prime mover of jaw closure; elevates the mandible.

• Temporalis: Synergist muscle for jaw closure.

• Sternocleidomastoid: Prime mover of head flexion, lateral neck movement.

• Zygomaticus: Raises the corners of the mouth (smiling).

Example: The orbicularis oris is known as the "kissing muscle" because it purses the lips.

Muscles of the Trunk

Abdominal and Thoracic Muscles

These muscles support the trunk, aid in breathing, and protect internal organs.

• Rectus abdominis: Flexes the vertebral column and compresses abdominal contents.

• External oblique: Part of the abdominal girdle; forms the external lateral wall.

• Internal oblique: Paired muscle forming the abdominal wall.

• Transversus abdominis: Compresses abdominal contents.

• Intercostal muscles: Aid in breathing by moving the rib cage.

• Diaphragm: Main muscle of inspiration.

• Pectoralis major: Adducts and flexes the humerus.

Example: The rectus abdominis is commonly referred to as the "abs" and is visible in well-defined individuals.

Posterior Trunk Muscles

Muscles of the Back and Shoulder

These muscles stabilize and move the scapula, shoulder, and vertebral column.

• Trapezius: Allows you to shrug your shoulders; elevates and rotates the scapula.

• Rhomboids: Adduct the shoulder and cause extension.

• Deltoid: Prime mover for shoulder abduction.

• Latissimus dorsi: Extends, adducts, and medially rotates the arm.

• Erector spinae: Prime mover of back extension; maintains posture.

Example: The erector spinae group consists of three muscles that run along the spine and are key for maintaining upright posture.

Muscles of the Arm and Forearm

Flexors and Extensors

These muscles control movements of the elbow, wrist, and fingers.

• Biceps brachii: Flexes the elbow and supinates the forearm.

• Triceps brachii: Extends the elbow.

• Brachialis: Flexes the elbow.

• Flexor carpi ulnaris: Wrist flexor that follows the ulna.

• Extensor carpi radialis: Extends and abducts the wrist.

• Extensor digitorum: Extends the fingers.

Example: The biceps brachii is a two-headed muscle responsible for flexing the forearm.

Muscles of the Hip, Thigh, and Leg

Major Muscle Groups and Actions

These muscles are essential for locomotion, posture, and movement of the lower limb.

• Iliopsoas: Hip flexor, deep in the pelvis.

• Gluteus maximus: Extends the hip.

• Adductors: Adduct the thigh.

• Quadriceps femoris: Extends the knee; includes rectus femoris, vastus lateralis, vastus medialis, and vastus intermedius.

• Hamstrings: Flex the knee and extend the hip; includes biceps femoris, semitendinosus, and semimembranosus.

• Sartorius: Flexes, abducts, and laterally rotates the thigh.

• Tibialis anterior: Dorsiflexes the foot.

• Gastrocnemius: Plantar flexes the foot.

• Soleus: Plantar flexes the foot.

Example: The rectus femoris is part of the quadriceps group and is commonly used as an injection site.

General Body Muscles Review

Key Concepts and Classifications

Reviewing muscle groups and their functions helps reinforce understanding of muscular anatomy.

• Intramuscular injection sites: Deltoid, gluteus medius, and vastus lateralis are commonly used.

• Muscle groups: Quadriceps (knee extension), hamstrings (knee flexion), adductors (thigh adduction).

• Antagonists: Muscles that oppose the action of another muscle (e.g., triceps brachii is antagonist to biceps brachii).

• Synergists: Muscles that assist the prime mover (e.g., temporalis assists masseter in jaw closure).

• Prime movers: Muscles primarily responsible for a movement (e.g., biceps brachii for elbow flexion).

Example: The pectoralis major and deltoid act synergistically to flex the arm.

Key Muscle Terms and Definitions

Important Anatomical Structures

• Linea alba: A fibrous structure running down the midline of the abdomen, formed by the fusion of the aponeuroses of the abdominal muscles.

• Tendinous intersection: Transverse bands of connective tissue that segment the rectus abdominis muscle.

• Rectus sheath: A fibrous sheath enclosing the rectus abdominis muscle, formed by the aponeuroses of the abdominal muscles.

Example: The linea alba is a landmark for abdominal surgery and divides the left and right rectus abdominis muscles.

Muscle Group Comparison Table

Major Muscle Groups and Their Actions

Muscle Group	Location	Primary Action	Example Muscles
Quadriceps	Anterior thigh	Knee extension	Rectus femoris, vastus lateralis
Hamstrings	Posterior thigh	Knee flexion, hip extension	Biceps femoris, semitendinosus
Adductors	Medial thigh	Thigh adduction	Adductor magnus, adductor longus
Calf Muscles	Posterior lower leg	Plantar flexion	Gastrocnemius, soleus
Abdominal Muscles	Anterior trunk	Trunk flexion, compression	Rectus abdominis, external oblique

Important Equations and Concepts

Muscle Contraction Principle

• Sliding Filament Theory: Muscle contraction occurs when actin and myosin filaments slide past each other, shortening the sarcomere.

Equation:

$\text{Force} = \text{Cross-sectional area} \times \text{Muscle tension per unit area}$

Additional info: The above equation relates muscle force to its anatomical size and physiological properties.

Summary Table: Muscle Actions and Examples

Action	Prime Mover	Antagonist	Synergist
Elbow Flexion	Biceps brachii	Triceps brachii	Brachialis
Knee Extension	Quadriceps femoris	Hamstrings	Tensor fasciae latae
Hip Adduction	Adductor magnus	Gluteus medius	Adductor longus
Plantar Flexion	Gastrocnemius	Tibialis anterior	Soleus

Conclusion

Understanding the naming, location, and function of muscles is essential for mastering human anatomy and physiology. These study notes provide a comprehensive overview of major muscle groups, their actions, and key anatomical terms relevant for college-level exam preparation.