Movement is one of the defining characteristics of life, and in the human body it manifests at three distinct levels: ciliary movement, flagellar movement, and muscular movement. Ciliary movement is seen in the ciliated epithelium lining the respiratory tract (where cilia propel mucus and debris upward as part of the mucociliary escalator) and in the fallopian tubes (where cilia create a current that moves the ovum toward the uterus). Flagellar movement is responsible for the motility of spermatozoa, which use their single flagellum — a whip-like structure with a 9+2 microtubule axoneme powered by dynein ATPase — to swim toward the egg. Muscular movement is the most prominent and complex type, enabling not only locomotion (coordinated whole-body movement) but also the function of internal organs and the cardiovascular system. It depends on the integrated action of the skeletal and muscular systems working together.

The human body contains three types of muscle tissue, each with distinctive properties suited to its function. Skeletal muscle (also called voluntary or striated muscle) is characterized by alternating dark and light bands (striations) visible under the microscope, voluntary control through the somatic nervous system, and a multinucleated syncytial structure arising from the fusion of many myoblasts during development. Its cells are long, cylindrical, and unbranched, with nuclei pushed peripherally. Skeletal muscle fatigues readily during intense activity. Smooth muscle (visceral muscle) lacks striations (hence the name "smooth"), is involuntary, and has a single centrally placed nucleus per cell. It is spindle-shaped (fusiform) and is found in the walls of hollow visceral organs including the gastrointestinal tract, blood vessels, urinary bladder, uterus, and respiratory airways. It has no T-tubules but uses caveolae (flask-shaped membrane invaginations) for calcium handling. Smooth muscle is resistant to fatigue and can maintain sustained contractions efficiently. Cardiac muscle is unique in being both striated (like skeletal) and involuntary (like smooth) — the combination that makes it the most commonly tested NEET trap in this chapter. Cardiac cells are short, branched, and cylindrical, with a single centrally placed nucleus (occasionally binucleate). They are connected by intercalated discs containing desmosomes (mechanical coupling) and gap junctions (electrical coupling), enabling the entire heart to contract as a functional syncytium. Cardiac muscle has intrinsic rhythmicity (the SA node pacemaker), abundant mitochondria, and never fatigues throughout life.

At the molecular level, the sarcomere is the fundamental contractile unit of skeletal muscle, bounded by two successive Z-lines. Within each sarcomere, the A band (anisotropic, dark) spans the full length of thick myosin filaments plus overlapping thin actin filaments — critically, the A band does NOT change in width during contraction. The I band (isotropic, light) contains only actin filaments and decreases in width during contraction. The H zone is the central region of the A band containing only myosin (no actin overlap) and also decreases during contraction, potentially disappearing completely. The M-line at the centre of the H zone anchors myosin filaments. The Z-line anchors actin filaments and bisects the I band.

The sliding filament theory explains how the sarcomere shortens. A motor nerve impulse causes acetylcholine (ACh) to be released at the neuromuscular junction, generating an action potential on the sarcolemma that travels into T-tubules. This triggers calcium ion release from the sarcoplasmic reticulum. Ca2+ binds to troponin-C on the thin filament, causing a conformational change that shifts tropomyosin away from myosin-binding sites on actin. The myosin head, pre-energized by ATP hydrolysis, forms a cross-bridge with actin. The power stroke moves actin toward the sarcomere centre. A fresh ATP molecule then binds to the myosin head, causing detachment; ATP hydrolysis re-energizes the head for the next cycle. Relaxation occurs when Ca2+ is actively pumped back into the SR by SERCA. Neither actin nor myosin filaments change their own length — only their relative positions change as actin slides over stationary myosin.

The human skeletal system contains 206 bones in adults: 80 in the axial skeleton (skull 22, vertebral column 26, ribs 24, sternum 1, hyoid 1) and 126 in the appendicular skeleton. The hyoid bone is uniquely the only bone in the body that does not articulate with any other bone. Joints are classified as fibrous (immovable, e.g., skull sutures), cartilaginous (slightly movable, e.g., pubic symphysis, intervertebral discs), and synovial (freely movable). Six types of synovial joints exist: hinge (knee, elbow), pivot (atlas-axis), ball-and-socket (shoulder, hip), gliding (intercarpal), saddle (thumb CMC), and ellipsoid (wrist).

Seven major musculoskeletal disorders are tested in NEET: myasthenia gravis (autoimmune destruction of ACh receptors at NMJ, causing progressive skeletal muscle weakness), muscular dystrophy (genetic X-linked dystrophin deficiency causing progressive skeletal muscle degeneration), tetany (sustained involuntary muscle contraction due to hypocalcemia), osteoarthritis (degenerative cartilage wear-and-tear in weight-bearing joints), rheumatoid arthritis (autoimmune attack on the synovial membrane with symmetric joint involvement), osteoporosis (decreased bone mineral density due to post-menopausal estrogen deficiency), and gout (uric acid crystal deposition in joints, most commonly the big toe).