Muscle Origin & Insertion

General Principles:

• Origin (O): Typically the proximal, less mobile attachment of a muscle.
• Insertion (I): Typically the distal, more mobile attachment of a muscle.
• Action (A): Contraction pulls the insertion towards the origin, producing movement (e.g., flexion, extension, rotation) at the joint(s) the muscle crosses.
• Innervation (N): Nerve supply crucial for muscle function; damage leads to weakness/paralysis.

Clinically Significant Examples (O, I, A, N, Clinical Relevance):

• Rotator Cuff (SITS muscles stabilize glenohumeral joint):

  • Supraspinatus: O: Supraspinous fossa. I: Greater tubercle (sup. facet). A: Initiates abduction (first ~15°). N: Suprascapular n. Clinical: Most common rotator cuff tear/tendinopathy; impingement syndrome. "Empty can" test.
  • Infraspinatus: O: Infraspinous fossa. I: Greater tubercle (mid. facet). A: External rotation. N: Suprascapular n. Clinical: Weakness in external rotation with tear.
  • Teres Minor: O: Lat. border of scapula. I: Greater tubercle (inf. facet). A: External rotation, adduction. N: Axillary n. Clinical: Test external rotation strength.
  • Subscapularis: O: Subscapular fossa. I: Lesser tubercle. A: Internal rotation, adduction. N: Upper/Lower Subscapular n. Clinical: Weakness in internal rotation (e.g., "lift-off" test).

• Biceps Brachii: O: Short head - coracoid process; Long head - supraglenoid tubercle. I: Radial tuberosity, bicipital aponeurosis. A: Forearm supination (powerful), elbow flexion. N: Musculocutaneous n. Clinical: Long head tendonitis; distal tendon rupture ("Popeye" sign).

• Triceps Brachii: O: Long head - infraglenoid tubercle; Lat/Med heads - post. humerus. I: Olecranon process of ulna. A: Elbow extension. N: Radial n. Clinical: Weakness/paralysis with radial n. injury (e.g., mid-shaft humerus fx, crutch palsy).

• Deltoid: O: Clavicle, acromion, scapular spine. I: Deltoid tuberosity of humerus. A: Arm abduction (>15°), flexion, extension. N: Axillary n. Clinical: Axillary n. injury (e.g., ant. shoulder dislocation, surgical neck humerus fx) -> deltoid atrophy, impaired abduction.

• Common Flexor Origin: O: Medial epicondyle of humerus (Muscles: Pronator teres, FCR, PL, FCU, FDS). Clinical: Medial epicondylitis ("Golfer's elbow") - pain at medial epicondyle, worse w/ resisted wrist flexion/pronation.

• Common Extensor Origin: O: Lateral epicondyle of humerus (Muscles: ECRB/L, ED, EDM, ECU). Clinical: Lateral epicondylitis ("Tennis elbow") - pain at lateral epicondyle, worse w/ resisted wrist extension/supination.

• Adductor Muscles of the Thigh (Medial Compartment): Generally adduct and flex thigh; primary N: Obturator n. (L2-L4). Clinical: Groin strains common, especially in athletes; obturator nerve injury can cause weakness in adduction and sensory loss to medial thigh.

  • Adductor Longus: O: Anterior pubis. I: Linea aspera (middle 1/3). A: Adducts, flexes thigh. N: Obturator n. (ant. division). Clinical: Forms medial border of femoral triangle.
  • Adductor Brevis: O: Inferior pubic ramus. I: Pectineal line & proximal linea aspera. A: Adducts, flexes thigh. N: Obturator n. (often separates ant/post divisions of nerve).
  • Adductor Magnus: Large muscle with two parts.
    • Adductor part: O: Inferior pubic ramus, ischial ramus. I: Gluteal tuberosity, linea aspera, medial supracondylar line. A: Adducts, flexes thigh. N: Obturator n. (post. division).
    • Hamstring (Ischiocondylar) part: O: Ischial tuberosity. I: Adductor tubercle of femur. A: Adducts, extends thigh. N: Tibial division of Sciatic n. (L4).
  • Gracilis: O: Inferior pubic ramus, body of pubis. I: Pes anserinus (medial tibia, with Sartorius & Semitendinosus). A: Adducts thigh, flexes knee, medially rotates tibia on femur. N: Obturator n. (ant. division). Clinical: Pes anserinus bursitis/tendinopathy.
  • Pectineus: O: Pectineal line of pubis (pecten pubis). I: Pectineal line of femur (inf. to lesser trochanter). A: Adducts, flexes thigh. N: Femoral n. (L2-L3); may receive branch from Obturator n. (dual innervation). Clinical: Forms part of floor of femoral triangle.

• Sartorius: O: Anterior Superior Iliac Spine (ASIS). I: Pes anserinus (anteromedial tibia, with Gracilis & Semitendinosus). A: Hip flexion, abduction, ext. rotation; knee flexion. N: Femoral n. Clinical: ASIS avulsion fx; Pes anserinus bursitis (medial knee pain).

• Iliopsoas (Psoas Major + Iliacus): O: T12-L5 vertebrae (psoas), iliac fossa (iliacus). I: Lesser trochanter of femur. A: Primary hip flexor. N: Ventral rami L1-L3 (psoas), Femoral n. (iliacus). Clinical: Psoas sign (retrocecal appendicitis, psoas abscess); hip flexor strain.

• Gluteus Maximus: O: Post. ilium, sacrum, coccyx. I: Gluteal tuberosity (femur), Iliotibial (IT) band. A: Major hip extensor, ext. rotation. N: Inferior gluteal n. Clinical: Difficulty rising from seat, climbing stairs if weak/damaged.

• Gluteus Medius & Minimus: O: Outer surface of ilium. I: Greater trochanter of femur. A: Hip abduction, int. rotation; stabilize pelvis during gait. N: Superior gluteal n. Clinical: Trendelenburg gait/sign (pelvic drop on contralateral swing side) due to sup. gluteal n. injury.

• Tibialis Anterior: O: Anterolateral tibia, interosseous membrane. I: Medial cuneiform, base of 1st metatarsal. A: Foot dorsiflexion, inversion. N: Deep peroneal (fibular) n. Clinical: Foot drop if deep peroneal n. injured (e.g., fibular neck fx); often involved in anterior compartment syndrome.

• Gastrocnemius & Soleus (Triceps Surae): O: Femoral condyles (gastroc), post. tibia/fibula (soleus). I: Calcaneus via Achilles tendon. A: Plantarflexion. N: Tibial n. Clinical: Achilles tendon rupture (+ Thompson test); gastroc strain ("tennis leg").