6.2 Muscles of the Upper & Lower Extremities

6.2 Muscles of the Upper & Lower Extremities

The appendicular musculature comprises the muscle groups that stabilize the pectoral and pelvic girdles and mobilize the upper and lower extremities. The upper limb is adapted for exquisite spatial mobility, reach, and fine manual dexterity, whereas the lower limb is specialized for weight-bearing, postural stability, and bipedal locomotion. Understanding the functional compartments, major nerve innervations, and clinical intramuscular landmarks of these muscles is critical for clinical healthcare training.

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Muscles Crossing the Shoulder & Brachium

The glenohumeral (shoulder) joint is a freely movable multiaxial ball-and-socket synovial joint characterized by a shallow glenoid cavity and an expansive range of motion. Because skeletal articulation provides minimal intrinsic stability, the joint relies on large crossing prime movers and dynamic musculotendinous cuffs.

1. Deltoid: Multipennate Shoulder Cap

The deltoid is a thick, powerful, multipennate muscle that forms the rounded muscular contour of the shoulder.

• Attachments: Originates broadly from the lateral third of the clavicle, the acromion process, and the spine of the scapula; converges into a thick tendon that inserts into the deltoid tuberosity on the lateral mid-shaft of the humerus.
• Innervation: Axillary nerve (originating from posterior cord of brachial plexus, roots C5-C6).
• Actions:
  • All fibers contracting together: Acts as the prime mover of arm abduction along the frontal plane, taking over once the arm is elevated past the initial 15 degrees.
  • Anterior fibers: Flex and medially rotate the humerus.
  • Posterior fibers: Extend and laterally rotate the humerus.
• Clinical IM Landmark: The deltoid is a standard site for administering low-volume intramuscular (IM) injections in adults and older children (e.g., hepatitis B and influenza vaccines). The injection site is located 2 to 3 fingerbreadths (approximately 1 to 2 inches) below the acromion process along the midline lateral arm. This landmark avoids injury to the axillary nerve and the posterior circumflex humeral artery traversing the surgical neck of the humerus. Maximum injection volume is typically restricted to 1.0 to 2.0 mL.

2. Pectoralis Major: Anterior Chest Prime Mover

The pectoralis major is a massive, fan-shaped muscle covering the upper anterior chest.

• Attachments: Originates via a clavicular head (medial half of clavicle) and a sternocostal head (sternum and costal cartilages of ribs 1-6); inserts into the crest of the greater tubercle of the humerus.
• Innervation: Medial and lateral pectoral nerves (C5-T1).
• Actions: Acts as the prime mover of arm flexion; powerfully adducts and medially rotates the humerus against resistance (e.g., bench press, climbing, hugging).

3. Latissimus Dorsi: Posterior Arm Prime Mover

The latissimus dorsi is a broad, triangular muscle covering the lower posterior trunk. Often termed the "swimmer's muscle," it works in direct opposition to the pectoralis major in the sagittal plane:

• Attachments: Originates from the spinous processes of T7–T12, the lumbar and sacral vertebrae (via the thoracolumbar fascia), the iliac crest, and the lower ribs; winds around the teres major to insert into the floor of the intertubercular (bicipital) sulcus of the humerus.
• Innervation: Thoracodorsal nerve (C6-C8).
• Actions: Acts as the prime mover of arm extension; powerfully adducts and medially rotates the arm, driving the propulsive stroke in swimming and rowing.

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Dynamic Shoulder Stabilization: The Rotator Cuff (SITS)

The rotator cuff consists of four deep muscles whose tendons wrap around the glenohumeral joint, blending intimately with the fibrous articular capsule. These muscles dynamic-stabilize the humeral head, pulling it firmly into the shallow glenoid fossa during all arm movements to prevent subluxation and dislocation. They are recalled via the clinical acronym SITS:

+--------------------------------------------------------------------------+
|                       ROTATOR CUFF MUSCULATURE (SITS)                    |
|                                                                          |
|  S - SUPRASPINATUS:                                                      |
|      Origin: Supraspinous fossa of scapula                               |
|      Insertion: Greater tubercle of humerus (superior facet)             |
|      Action: Initiates first 15° of arm abduction; stabilizes joint      |
|      Innervation: Suprascapular nerve (C5-C6)                            |
|                                                                          |
|  I - INFRASPINATUS:                                                      |
|      Origin: Infraspinous fossa of scapula                               |
|      Insertion: Greater tubercle of humerus (middle facet)               |
|      Action: Laterally rotates arm; stabilizes joint                     |
|      Innervation: Suprascapular nerve (C5-C6)                            |
|                                                                          |
|  T - TERES MINOR:                                                        |
|      Origin: Lateral (axillary) border of scapula                        |
|      Insertion: Greater tubercle of humerus (inferior facet)             |
|      Action: Laterally rotates arm and assists adduction                 |
|      Innervation: Axillary nerve (C5-C6)                                 |
|                                                                          |
|  S - SUBSCAPULARIS:                                                      |
|      Origin: Subscapular fossa (anterior scapular surface)               |
|      Insertion: Lesser tubercle of humerus                               |
|      Action: Medially rotates arm; anterior joint stabilizer             |
|      Innervation: Upper and lower subscapular nerves (C5-C6)             |
+--------------------------------------------------------------------------+

Clinical Pathology: Rotator Cuff Tears

The supraspinatus tendon passes through a narrow subacromial space beneath the acromion process and coracoacromial ligament. Repetitive overhead arm motions (e.g., throwing, pitching, painting) cause chronic friction, inflammation (subacromial bursitis), and degenerative fraying termed rotator cuff impingement syndrome. Complete tendon rupture leaves the patient unable to smoothly initiate arm abduction, requiring a characteristic shrugging or lateral body tilt to swing the arm outward.

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Muscles Crossing the Elbow & Radioulnar Joints

The arm (brachium) is partitioned by medial and lateral intermuscular septa into an anterior flexor compartment and a posterior extensor compartment.

Anterior Compartment: Forearm Flexors

1. Brachialis: Situated deep to the biceps brachii on the distal anterior humerus. Originates from the lower anterior humeral shaft and inserts into the coronoid process of the ulna. Innervated by the musculocutaneous nerve. The brachialis is the true prime mover of elbow flexion. Because it inserts onto the ulna (which does not rotate during pronation or supination), the brachialis generates maximal flexion force equally well whether the forearm is supinated, pronated, or in neutral position.
2. Biceps Brachii: A prominent, two-headed superficial muscle of the anterior arm. The long head originates from the supraglenoid tubercle of the scapula (gliding through the intertubercular groove), and the short head originates from the coracoid process. Both heads unite into a common tendon inserting into the radial tuberosity of the radius, with a medial aponeurotic expansion (bicipital aponeurosis) bracing into deep forearm fascia. Innervated by the musculocutaneous nerve. The biceps brachii flexes the elbow, but its most dramatic mechanical action is as the most powerful supinator of the forearm. It works with maximal mechanical efficiency when the elbow is flexed at 90 degrees (e.g., turning a corkscrew or driving a screw into hardwood).
3. Brachioradialis: A superficial muscle residing along the lateral border of the forearm. Originates from the lateral supracondylar ridge of the humerus and inserts into the base of the radial styloid process. Uniquely among elbow flexors, it resides in the forearm and is innervated by the radial nerve. It acts as a powerful synergist in elbow flexion, especially when the forearm is in a semi-pronated ("thumbs up" or neutral) position.

Posterior Compartment: Forearm Extensor

• Triceps Brachii: The massive three-headed muscle occupying the entire posterior compartment of the arm.
  • Long head: Originates from the infraglenoid tubercle of the scapula (crosses shoulder joint, assists arm extension and adduction).
  • Lateral head: Originates from the posterior humeral shaft superior to the radial groove.
  • Medial head: Deep, broad head originating from the posterior humeral shaft inferior to the radial groove.
  • Insertion: All three heads coalesce into a thick aponeurotic tendon inserting into the olecranon process of the ulna.
  • Innervation: Radial nerve (C6-C8).
  • Action: Acts as the prime mover of forearm (elbow) extension. Essential for pushing motions and resisting gravity during transfers.

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Forearm Compartments & Epicondylitis

The forearm contains over twenty small muscles arranged in superficial and deep layers controlling the wrist, hand, and digits:

1. Anterior Forearm (Flexor-Pronator Compartment)

• Common Origin: The superficial flexors originate from a common flexor tendon attached to the medial epicondyle of the humerus.
• Key Muscles: Pronator teres (pronates forearm), flexor carpi radialis (flexes and abducts wrist), palmaris longus (weak wrist flexor; absent in ~14% of population), flexor carpi ulnaris (flexes and adducts wrist; ulnar nerve), and deep finger flexors (flexor digitorum superficialis and profundus).
• Innervation: Dominated by the median nerve (except flexor carpi ulnaris and medial half of flexor digitorum profundus, supplied by the ulnar nerve).
• Clinical Correlates:
  • Medial Epicondylitis ("Golfer's Elbow"): Repetitive forceful wrist flexion and pronation produces microtearing and pain at the common flexor tendon origin on the medial epicondyle.
  • Carpal Tunnel Syndrome: Compression of the median nerve as it passes deep to the flexor retinaculum, causing paresthesia and weakness in the thumb, index, middle, and radial half of the ring finger.

2. Posterior Forearm (Extensor-Supinator Compartment)

• Common Origin: The superficial extensors originate from a common extensor tendon on the lateral epicondyle of the humerus.
• Key Muscles: Extensor carpi radialis longus and brevis (extend and abduct wrist), extensor digitorum (prime mover of finger extension), and extensor carpi ulnaris (extends and adducts wrist).
• Innervation: Innervated entirely by the radial nerve and its deep motor branch (posterior interosseous nerve).
• Clinical Correlate: Lateral Epicondylitis ("Tennis Elbow"): Repetitive stress from backhand strokes or forceful wrist extension strains the extensor carpi radialis brevis tendon at the lateral epicondyle.

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Muscles Crossing the Pelvic Girdle & Hip Joint

The hip joint is a deep, stable multiaxial ball-and-socket articulation anchored within the acetabulum of the pelvis. Powerful muscles cross the joint to maintain upright balance and power bipedal propulsion.

1. Iliopsoas: Prime Hip Flexor

The iliopsoas is a composite muscle formed by the union of two distinct muscle bellies:

• Psoas Major: Originates from the transverse processes and bodies of lumbar vertebrae T12-L5.
• Iliacus: Originates from the concave iliac fossa of the pelvic bone.
• Insertion & Action: Both bellies merge to pass beneath the inguinal ligament, inserting via a common tendon into the lesser trochanter of the femur. Innervated by lumbar spinal nerves (L2-L3) and the femoral nerve. The iliopsoas is the prime mover of hip flexion, pulling the thigh anteriorly when walking and flexing the trunk forward when rising from a supine posture.

2. Gluteus Maximus: Prime Hip Extensor

The gluteus maximus is the largest, thickest, and most massive muscle in the human body, forming the predominant rounded bulk of the buttock.

• Attachments: Originates from the posterolateral ilium, sacrum, and coccyx; inserts into the gluteal tuberosity of the femur and the iliotibial (IT) tract.
• Innervation: Inferior gluteal nerve (L5, S1, S2).
• Actions: Acts as the prime mover of thigh (hip) extension. It is relatively inactive during relaxed standing and level walking, but generates explosive force during stair climbing, running, jumping, and rising from a deep seated posture.

3. Gluteus Medius & Minimus: Pelvic Stabilizers

Residing deep and lateral to the gluteus maximus, the gluteus medius and gluteus minimus originate from the external surface of the ilium and insert into the greater trochanter of the femur. Both are innervated by the superior gluteal nerve.

• Actions: They act as powerful abductors and medial rotators of the thigh. Crucially, during normal walking, when one foot is lifted off the ground (swing phase), the gluteus medius on the weight-bearing stance leg contracts forcefully to hold the pelvis level, preventing the unweighted contralateral side of the pelvis from sagging downward.
• Trendelenburg Sign: Injury or denervation of the superior gluteal nerve causes gluteus medius paralysis. When the patient stands on the affected leg, the pelvis tilts downward toward the opposite, unsupported side—a clinical finding designated a positive Trendelenburg sign.

Clinical IM Landmark: Ventrogluteal Injection Site

In modern nursing practice, the ventrogluteal site (targeting the gluteus medius and minimus) is the safest and preferred site for deep intramuscular injections in adults and children over 7 months. To locate the site, the healthcare provider places the palm of the hand over the patient's greater trochanter, points the index finger toward the anterior superior iliac spine (ASIS), and spreads the middle finger dorsally along the iliac crest. The injection is administered into the triangular center of the "V" formed between the index and middle fingers. This site is completely free of major systemic blood vessels and large peripheral nerve trunks, and possesses a thick, dense muscle mass with minimal overlying subcutaneous adipose tissue. In contrast, the traditional dorsogluteal site is strictly discouraged because misplaced needles risk direct chemical neurotoxicity or mechanical transection of the massive sciatic nerve.

4. Tensor Fasciae Latae (TFL)

A small muscle located on the anterolateral hip. Originates from the iliac crest and ASIS; inserts directly into the iliotibial (IT) tract (a dense lateral thickening of the fascia lata that extends down to insert on Gerdy's tubercle of the tibia). Innervated by the superior gluteal nerve. It flexes and abducts the thigh and tenses the lateral fascia to brace the knee.

5. Medial Compartment: Thigh Adductors

The medial compartment comprises the adductor group: adductor longus, adductor brevis, adductor magnus, and gracilis (the slender superficial medial strap). Originating from the pubic and ischial bones and inserting along the linea aspera of the femur, these muscles are innervated predominantly by the obturator nerve. They powerfully adduct, flex, and medially rotate the thigh (e.g., squeezing knees together, horseback riding).

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Muscles Crossing the Knee Joint: Quadriceps & Hamstrings

+--------------------------------------------------------------------------+
|                       ANTERIOR VS. POSTERIOR THIGH                       |
|                                                                          |
|  ANTERIOR THIGH: QUADRICEPS FEMORIS                                      |
|  - Rectus femoris (crosses hip & knee: flexes hip, extends knee)         |
|  - Vastus lateralis (extends knee; PREFERRED PEDIATRIC IM SITE)          |
|  - Vastus medialis (extends knee; stabilizes patella tracking)           |
|  - Vastus intermedius (deep to rectus femoris; extends knee)             |
|  - Insertion: Common patellar tendon -> Patellar ligament ->             |
|               Tibial tuberosity                                          |
|  - Innervation: FEMORAL NERVE (L2-L4)                                    |
|                                                                          |
|  POSTERIOR THIGH: HAMSTRINGS GROUP                                       |
|  - Biceps femoris (lateral hamstring: knee flexion, hip extension)      |
|  - Semitendinosus (medial hamstring: knee flexion, hip extension)        |
|  - Semimembranosus (deep medial hamstring: knee flexion, hip extension)  |
|  - Common Origin: Ischial tuberosity (except short head biceps femoris)  |
|  - Innervation: SCIATIC NERVE (Tibial division; common fibular for       |
|                 short head)                                              |
+--------------------------------------------------------------------------+

1. Quadriceps Femoris (Anterior Thigh)

The quadriceps femoris is a massive four-headed muscle group covering the anterior, lateral, and medial thigh. It is the prime mover of knee extension, vital for standing, walking, kicking, and stair climbing. All four heads converge distally into a thick, common patellar tendon, which envelops the patella (a large sesamoid bone) and continues inferiorly as the patellar ligament to insert securely into the tibial tuberosity. All four heads are innervated by the femoral nerve.

• Rectus Femoris: The only head that crosses two joints (the hip and the knee). Originates from the anterior inferior iliac spine (AIIS). It powerfully extends the knee and synergistically flexes the hip.
• Vastus Lateralis: Forms the large muscular mass on the anterolateral thigh. Originates from the greater trochanter and lateral lip of the linea aspera. It is the clinically preferred intramuscular injection site for infants and young children under 12 months of age because it is the most well-developed muscle mass at birth and is situated safely distant from major neurovascular structures. It is also routinely used for self-administration of medications (such as epinephrine auto-injectors) in adults.
• Vastus Medialis: Forms the teardrop bulge on the anteromedial distal thigh. Originates from the medial lip of the linea aspera. Essential for the terminal 15 degrees of knee extension and stabilizes patellar tracking within the femoral groove.
• Vastus Intermedius: Resides deep to the rectus femoris directly on the anterior femoral shaft. Acts purely to extend the knee.

2. Sartorius: The Tailor's Muscle

The sartorius is a long, narrow, ribbon-like muscle that courses diagonally across the anterior surface of the thigh from lateral to medial. It is the longest individual muscle in the human body.

• Attachments: Originates from the anterior superior iliac spine (ASIS); winds across the thigh to insert into the medial proximal shaft of the tibia at the pes anserinus (alongside gracilis and semitendinosus).
• Innervation: Femoral nerve.
• Actions: Flexes, abducts, and laterally rotates the thigh at the hip, while simultaneously flexing the leg at the knee joint. This combined mechanical movement places the lower limb into the cross-legged posture historically adopted by tailors sitting on the floor ("tailor's muscle").

3. Hamstrings Group (Posterior Thigh)

The hamstrings comprise three long muscles located in the posterior compartment of the thigh: the biceps femoris (lateral hamstring), the semitendinosus (medial hamstring with a prominent cord-like tendon), and the semimembranosus (deep medial hamstring with a broad membranous tendon).

• Attachments: All three muscles share a common origin on the ischial tuberosity of the pelvis (the "sitting bone"), with the short head of the biceps femoris originating from the linea aspera of the femur. Distally, the biceps femoris inserts on the head of the fibula and lateral tibial condyle, while the semitendinosus and semimembranosus insert onto the medial aspect of the proximal tibia.
• Innervation: Sciatic nerve (specifically the tibial nerve division; common fibular nerve supplies the short head of biceps femoris).
• Actions: The hamstrings act as the prime movers of knee flexion and are powerful synergists in thigh (hip) extension when walking or running.

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Muscles of the Leg, Ankle & Foot

The leg (crus) is divided by dense crural fascia into anterior, posterior, and lateral compartments controlling movements of the ankle joint (talocrural) and foot (subtalar).

1. Anterior Compartment: Dorsiflexors

• Tibialis Anterior: A thick superficial muscle paralleling the sharp anterior crest of the tibia. Originates from the upper lateral condyle and shaft of the tibia and interosseous membrane; inserts into the medial cuneiform and base of the first metatarsal.
  • Innervation: Deep fibular (peroneal) nerve.
  • Actions: Acts as the prime mover of dorsiflexion (pulling the foot upward toward the shin) and inverts the foot (turning the sole inward).
  • Clinical Correlate: Foot Drop: Compression or laceration of the common fibular nerve as it winds around the superficial neck of the fibula paralyzes the deep fibular nerve. The patient loses the ability to dorsiflex the foot (foot drop). During walking, the toes drag along the floor during the swing phase, forcing the patient to adopt a compensatory high-stepping steppage gait.
• Extensor Digitorum Longus & Extensor Hallucis Longus: Lie lateral and deep to the tibialis anterior; dorsiflex the foot and extend the toes.

2. Posterior Compartment: Plantar Flexors (Triceps Surae)

The superficial posterior calf muscles form the triceps surae, generating the primary propulsive force required for walking, running, and jumping. They insert together via the calcaneal (Achilles) tendon—the strongest and thickest tendon in the human body—onto the posterior surface of the calcaneus (heel bone). Both are innervated by the tibial nerve.

• Gastrocnemius: A prominent, two-headed superficial muscle forming the visual bulk of the calf. Its medial and lateral heads originate from the medial and lateral condyles of the femur. Because it crosses both the knee and ankle joints, it acts as a powerful prime mover of plantar flexion (pointing toes downward, elevating the body on tiptoes) when the knee is extended, and assists in flexing the knee.
• Soleus: A broad, flat, highly vascular muscle lying directly deep to the gastrocnemius. Originates from the proximal posterior tibia and fibula. Because it crosses only the ankle joint, it functions as a tireless postural plantar flexor active during sustained standing and walking.
• Deep Posterior Muscles: Include the tibialis posterior, flexor digitorum longus, and flexor hallucis longus. Their tendons course behind the medial malleolus into the sole of the foot, executing plantar flexion, inversion, and supporting the longitudinal arch.

3. Lateral Compartment: Foot Everters

• Fibularis (Peroneus) Longus & Fibularis Brevis: Reside on the lateral aspect of the leg, originating from the fibular shaft. The longus tendon wraps around the lateral malleolus and travels under the sole to insert on the first metatarsal, while the brevis inserts on the tuberosity of the fifth metatarsal.
  • Innervation: Superficial fibular (peroneal) nerve.
  • Actions: Execute plantar flexion and eversion of the foot (tilting the sole of the foot outward/laterally). Eversion is essential for dynamic ankle balance when traversing uneven terrain and prevents lateral ankle inversion sprains.

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Extremities Muscle Reference & Clinical Landmarks Table