Muscle Attachment

Origin and Insertion

Muscles attach to bones at two main points: the origin and the insertion. Understanding these points is essential for describing muscle movement and function.

• Origin: The less movable bony attachment of the muscle, typically proximal.

• Insertion: The more movable bony attachment, usually distal. When a muscle contracts, the insertion moves toward the origin.

• Example: In the arm, the biceps brachii originates on the scapula and inserts on the radius. When the muscle contracts, the forearm moves toward the shoulder.

Additional info: The insertion is usually the more distal point of muscle attachment.

Head and Neck Muscles

Facial Muscles

Facial muscles control expressions and movements of the face. They are innervated by the cranial VII nerve and insert into the skin.

• Orbicularis oris: Kissing/puckering lips

• Buccinator: Compresses cheeks (whistling)

• Orbicularis oculi: Closes eyelids

• Zygomaticus muscle: Smiling

• Paralysis: Facial muscle paralysis on one side is called Bell's palsy.

Sternocleidomastoid

• Located on the front and side of the neck.

• Contraction causes neck flexion (pulls head forward).

• When only one side contracts, the head rotates toward the opposite shoulder.

Scalene Muscles

• Assist in neck flexion.

• If the head is bent sideways, scalenes help in tilting and rotation.

Trapezius

• Located on the back of the neck and upper back.

• Contraction can cause neck hyperextension.

Muscles That Move the Shoulder and Arm

Pectoralis Major

• Inserts into the humerus.

• Medially rotates, flexes, and adducts the arm/shoulder.

• Prime mover for shoulder flexion.

Latissimus Dorsi

• Extends, adducts, and medially rotates the shoulder.

• Prime mover for shoulder extension.

Teres Major

• Extends, adducts, and medially rotates the shoulder.

Rotator Cuff Muscles

• Supraspinatus, infraspinatus, teres minor, subscapularis

• Stabilize and rotate the shoulder.

• Synergistic and antagonistic actions for shoulder movement.

Deltoid Muscle

• Prime mover for shoulder abduction.

• Anterior fibers flex/medially rotate; posterior fibers extend/laterally rotate.

• Synergistic to supraspinatus for abduction.

Biceps Brachii

• Prime mover for elbow flexion and supination.

• Crosses both shoulder and elbow joints.

Brachialis

• Prime mover for elbow flexion.

Triceps Brachii

• Extends the elbow and shoulder.

• Antagonist to biceps brachii at the elbow.

Muscles That Move the Trunk

Abdominal Muscles

• Rectus abdominis, external oblique, internal oblique, transverse abdominis

• Flex the spinal column and compress the abdomen.

• External oblique: Most superficial.

• Transverse abdominis: Deepest muscle.

Erector Spinae

• Collective name for three major muscle groups of the back.

• Extend the trunk.

Diaphragm

• Main muscle for breathing.

• Made of skeletal muscle.

Muscles That Move the Hip and Leg

Iliopsoas (Iliacus and Psoas Major)

• Prime mover for hip flexion.

• Acts synergistically with rectus femoris.

Quadriceps Muscles

• Rectus femoris, vastus lateralis, vastus medialis, vastus intermedius

• Prime movers for knee extension.

• Rectus femoris also flexes the hip.

Sartorius, Gracilis, Tensor Fasciae Latae

• Cause movement at both hip and knee.

• Sartorius: Flexes hip and knee.

• Gracilis: Adducts/medially rotates hip, stabilizes knee via IT band.

• Tensor fasciae latae: Abducts/medially rotates hip, stabilizes knee.

Gluteus Maximus and Gluteus Medius

• Gluteus maximus: Extends/laterally rotates hip.

• Gluteus medius: Abducts/medially rotates hip.

Hamstrings

• Semimembranosus, semitendinosus, biceps femoris

• Prime movers for hip extension and knee flexion.

• Antagonistic to quadriceps for knee extension.

Triceps Surae: Soleus and Gastrocnemius

• Cause plantar flexion at the ankle.

• Gastrocnemius also flexes the knee.

• Both insert into the calcaneus via the Achilles tendon.

Tibialis Anterior

• Dorsiflexes the ankle.

Naming Muscles

Muscle Naming Criteria

Muscles are named based on several criteria:

• Location: e.g., sternocleidomastoid, infraspinatus

• Shape: e.g., trapezius, deltoid

• Direction of fibers: e.g., rectus abdominis, transverse abdominis

• Number of origins: e.g., biceps femoris (two), triceps brachii (three)

• Size: e.g., gluteus maximus, gluteus medius

• Origin and insertion: e.g., sternocleidomastoid

• Action: e.g., adductor femoris, pronator teres

Group Action of Muscles

Prime Mover, Antagonist, Synergist

Muscles work together to produce movement. "Group action" describes the roles muscles play during movement.

• Prime mover (agonist): Main muscle producing a desired action.

• Antagonist: Muscle that opposes the action of the prime mover.

• Synergist: Muscle that assists the prime mover, but is less powerful.

Example: During elbow flexion, the biceps brachii is the prime mover, the triceps brachii is the antagonist, and the brachialis is a synergist.

Muscle Function Questions

Prime Movers and Synergists

• Shoulder flexion: Prime mover is pectoralis major; synergists include anterior deltoid and biceps brachii (short head).

• Shoulder extension: Prime mover is latissimus dorsi; synergists include posterior deltoid.

• Shoulder abduction: Prime mover is deltoid; supraspinatus is a synergist.

• Elbow flexion: Prime mover is biceps brachii; brachialis and brachioradialis are synergists.

• Elbow extension: Prime mover is triceps brachii.

• Hip extension: Prime mover is gluteus maximus; antagonists are hip flexors.

• Knee extension: Prime mover is quadriceps femoris; antagonists are hamstrings.

• Knee flexion: Prime movers are hamstrings; antagonists are quadriceps femoris.

• Plantar flexion (ankle): Prime mover is gastrocnemius (with soleus); antagonists are tibialis anterior.

• Dorsiflexion (ankle): Prime mover is tibialis anterior; antagonists are gastrocnemius and soleus.

Muscle Function Table

Prime Movers and Antagonists

The following table summarizes major muscle functions, their prime movers, and antagonists.

Key Terms and Definitions

• Flexion: Decreasing the angle between two body parts.

• Extension: Increasing the angle between two body parts.

• Abduction: Movement away from the midline.

• Adduction: Movement toward the midline.

• Synergist: Muscle that assists the prime mover.

• Antagonist: Muscle that opposes the prime mover.

• Prime mover (agonist): Main muscle responsible for a movement.

Formulas and Equations

• Muscle Force Equation:

• Torque produced by muscle:

Additional info: Where is force, is mass, is acceleration, is torque, and is the lever arm distance.