Groups of Muscles

Comprehensive anatomical study of the major muscle groups of the human body, detailing their origin, insertion, nerve supply, and action. This guide is organized by anatomical region to facilitate clinical reasoning and surgical planning.

1. Introduction

The human body contains over 600 skeletal muscles. These are organized into functional groups based on their anatomical location and the movements they produce. Muscles are named according to specific criteria:

Size: e.g., gluteus maximus.
Shape: e.g., deltoid.
Location: e.g., tibialis anterior.
Number of origins: e.g., biceps (two heads), triceps (three heads).
Direction of fibers: e.g., rectus abdominis (straight).
Action: e.g., flexor digitorum.

2. Muscles of the Head and Neck

2.1 Muscles of Facial Expression

These muscles are unique because they insert into the skin rather than onto bone. They are all innervated by the Facial nerve (Cranial Nerve VII).

Muscle	Origin	Insertion	Action
Frontalis	Epicranial aponeurosis	Skin of eyebrows and forehead	Raises eyebrows, wrinkles forehead
Orbicularis oculi	Medial orbital margin, lacrimal sac	Skin around orbit	Closes eyelids
Orbicularis oris	Maxilla, mandible, surrounding muscles	Lips	Closes and protrudes lips
Buccinator	Maxilla, mandible, pterygomandibular raphe	Orbicularis oris	Compresses cheek (blowing, sucking)
Platysma	Fascia of deltoid and pectoralis major	Mandible, skin of lower face	Depresses mandible, tenses neck skin

Clinical Correlation

Bell's Palsy

Facial nerve palsy results in the paralysis of the muscles of facial expression on the affected side. Patients typically present with an inability to close the eye, a drooping corner of the mouth, and an inability to wrinkle the forehead.

2.2 Muscles of Mastication

These muscles move the mandible and are all innervated by the Mandibular division of the Trigeminal nerve (CN V3).

Muscle	Origin	Insertion	Action
Masseter	Zygomatic arch	Lateral surface of mandibular ramus and angle	Elevates mandible (closes jaw)
Temporalis	Temporal fossa	Coronoid process of mandible	Elevates and retracts mandible
Medial pterygoid	Medial surface of lateral pterygoid plate	Medial surface of mandibular ramus and angle	Elevates and protrudes mandible
Lateral pterygoid	Lateral surface of lateral pterygoid plate, greater wing of sphenoid	Neck of mandibular condyle, articular disc of TMJ	Protrudes and depresses mandible (opens mouth)

3. Muscles of the Trunk

3.1 Anterior Abdominal Wall Muscles

This group consists of three flat muscles and one vertical strap-like muscle. They function to protect viscera and increase intra-abdominal pressure. All are innervated by the Thoracoabdominal nerves (T7-T12); the internal oblique and transversus also receive supply from L1.

Muscle	Origin	Insertion	Action
Rectus abdominis	Pubic symphysis, pubic crest	Xiphoid process, costal cartilages 5-7	Flexes trunk, compresses abdomen
External oblique	External surfaces of ribs 5-12	Iliac crest, pubic tubercle, linea alba	Compresses abdomen, flexes and rotates trunk
Internal oblique	Iliac crest, inguinal ligament, thoracolumbar fascia	Ribs 10-12, linea alba, pubis	Compresses abdomen, flexes and rotates trunk
Transversus abdominis	Iliac crest, inguinal ligament, ribs 7-12	Linea alba, pubic crest	Compresses abdomen ("corset" muscle)

3.2 Intercostal Muscles

Essential for respiration, located in the intercostal spaces and innervated by the Intercostal nerves.

External intercostal: Elevates ribs (Inspiration).
Internal intercostal: Depresses ribs (Expiration).
Innermost intercostal: Assists in expiration.

4. Muscles of the Upper Limb

4.1 Shoulder (Glenohumeral Joint) Muscles

These muscles provide stability to the most mobile joint in the body.

Muscle	Origin	Insertion	Nerve Supply	Action
Deltoid	Clavicle, acromion, spine of scapula	Deltoid tuberosity of humerus	Axillary n. (C5-C6)	Abducts, flexes, and extends arm
Supraspinatus	Supraspinous fossa	Greater tubercle of humerus	Suprascapular n. (C5-C6)	Initiates abduction (first 15°)
Infraspinatus	Infraspinous fossa	Greater tubercle of humerus	Suprascapular n. (C5-C6)	Laterally rotates arm
Teres minor	Lateral border of scapula	Greater tubercle of humerus	Axillary n. (C5-C6)	Laterally rotates arm
Subscapularis	Subscapular fossa	Lesser tubercle of humerus	Subscapular nn. (C5-C7)	Medially rotates arm

4.2 Anterior Arm Muscles

These muscles act primarily as flexors and are all supplied by the Musculocutaneous nerve.

Biceps brachii: Flexes elbow, supinates forearm, weakly flexes shoulder.
Brachialis: The primary flexor of the elbow.
Coracobrachialis: Flexes and adducts the arm.

5. Muscles of the Lower Limb

5.1 Anterior Thigh Muscles (Quadriceps Femoris)

Four muscles sharing a common insertion on the tibial tuberosity via the patellar ligament. All are supplied by the Femoral nerve (L2-L4).

Rectus femoris: Extends knee and flexes hip.
Vastus lateralis: Extends knee.
Vastus medialis: Extends knee, stabilizes patella.
Vastus intermedius: Extends knee.

5.2 Posterior Thigh Muscles (Hamstrings)

Responsible for hip extension and knee flexion, innervated by the Sciatic nerve.

Muscle	Origin	Insertion	Action
Biceps femoris	Ischial tuberosity (long), Linea aspera (short)	Head of fibula, lateral tibial condyle	Extends hip, flexes knee, laterally rotates leg
Semitendinosus	Ischial tuberosity	Proximal medial tibia (Pes anserinus)	Extends hip, flexes knee, medially rotates leg
Semimembranosus	Ischial tuberosity	Posterior medial condyle of tibia	Extends hip, flexes knee, medially rotates leg

Figure 1: Muscle anatomy reference chart showing major muscles of the hip and thigh

6. Clinical Relevance

Rotator Cuff Tears

The SITS muscles (Supraspinatus, Infraspinatus, Teres minor, Subscapularis) are common sites of injury. Supraspinatus tears are the most frequent, often occurring due to impingement under the acromion.

Peripheral Nerve Palsies

Radial Nerve Injury: Results in wrist drop due to paralysis of extensors.
Femoral Nerve Palsy: Results in an inability to extend the knee and a loss of the patellar reflex.
Sciatic Nerve Injury: Can result in foot drop if the common fibular division is affected.

Facial Nerve Palsy vs. UMN Lesion

Injury to CN VII paralyzes all facial muscles on the affected side (Bell's Palsy). This must be distinguished from a stroke (Upper Motor Neuron lesion), which spares the forehead because the forehead receives bilateral cortical innervation.

Summary Key Points

Facial expression muscles are unique for inserting into skin (CN VII).
Mastication muscles move the mandible (CN V3).
The Rotator cuff provides essential stability to the glenohumeral joint.
The Quadriceps extend the knee; Hamstrings flex the knee and extend the hip.
Understanding nerve supply is required to predict motor deficits after trauma.

Doctors Revision