Muscle types: Skeletal (striated + voluntary + multinucleate) | Smooth (non-striated + involuntary + uninucleate) | Cardiac (striated + INVOLUNTARY + intercalated discs + never fatigues).

Sarcomere during contraction: A band = CONSTANT; I band = DECREASES; H zone = DECREASES (may vanish); neither actin nor myosin filaments change their own length.

Contraction key steps: Ca2+ from SR → troponin-C → tropomyosin shifts → cross-bridge → power stroke → ATP binds (detach) → ATP hydrolysis (re-energize). No ATP = rigor mortis.

Skeleton counts: Total = 206 (axial 80 + appendicular 126). Axial: skull 22 + vertebrae 26 + ribs 24 + sternum 1 + hyoid 1. Hyoid = only non-articulating bone.

Disorders: Myasthenia gravis = autoimmune ACh receptor | Muscular dystrophy = X-linked dystrophin | Tetany = hypocalcemia | Osteoarthritis = degenerative cartilage | Rheumatoid arthritis = autoimmune synovium | Osteoporosis = post-menopausal bone loss | Gout = uric acid crystals (big toe).