Locomotion is movement from one place to another. In humans, it depends on the coordinated action of bones, joints and muscles. CBSE Class 11 has a full chapter on Locomotion and Movement; NEET asks one to two questions a year.
Core Concepts
Types of movement
Amoeboid (by pseudopodia), ciliary (by cilia) and muscular (by contraction of muscle fibres). Humans use mostly muscular movement via the skeletal system.
Where each type operates in the human body:
- Amoeboid: WBCs (leucocytes) move by extending pseudopodia through blood vessel walls to reach infection sites. Macrophages also use amoeboid movement.
- Ciliary: epithelial cells lining the trachea have cilia that move mucus upward (mucociliary escalator). Fallopian tube cilia move the ovum toward the uterus.
- Muscular: skeletal muscles for locomotion, smooth muscles for peristalsis (intestine), cardiac muscle for heart pumping.
Skeletal system
206 bones in an adult, divided into axial (skull, vertebrae, ribs, sternum — 80 bones) and appendicular (limbs and girdles — 126 bones). Functions — support, protection, movement, blood cell production, mineral storage.
Types of joints
Fibrous (immovable, skull sutures), cartilaginous (slightly movable, between vertebrae) and synovial (freely movable). Synovial joints include ball-and-socket (hip, shoulder), hinge (elbow, knee), pivot (atlas-axis), saddle (thumb).
Synovial joint structure in detail: A synovial joint has: articular cartilage (smooth covering on bone ends), synovial membrane (secretes synovial fluid), synovial fluid (lubricant, reduces friction), joint capsule (tough outer covering), ligaments (connect bone to bone). Some joints also have menisci (shock-absorbing pads — knee has two menisci).
Muscle contraction — the sliding filament theory
This is the most NEET-tested concept from this chapter. Let us walk through it step by step.
Structure of a muscle fibre: Each muscle fibre (cell) contains many myofibrils. Each myofibril has repeating units called sarcomeres — the functional unit of contraction. A sarcomere runs from one Z-line to the next and contains:
- Thin filaments (actin): anchored to Z-lines, extend inward
- Thick filaments (myosin): in the center, with protruding heads
- A-band: region with thick filaments (dark band) — does NOT change length during contraction
- I-band: region with only thin filaments (light band) — SHORTENS during contraction
- H-zone: central region with only thick filaments — SHORTENS during contraction
A motor neuron releases acetylcholine at the neuromuscular junction. This triggers an action potential in the muscle fibre membrane (sarcolemma).
The action potential travels along T-tubules into the fibre, stimulating the sarcoplasmic reticulum to release Ca2+ ions into the sarcoplasm.
Ca2+ binds to troponin C on thin filaments. This shifts tropomyosin away from the myosin-binding sites on actin, exposing them.
Myosin heads (energised by ATP hydrolysis) bind to exposed actin sites, pull the thin filaments toward the centre (power stroke), release, and rebind. This is the “ratchet mechanism.” Each cycle shortens the sarcomere by about 10 nm.
When the neural signal stops, Ca2+ is actively pumped back into the sarcoplasmic reticulum. Troponin-tropomyosin complex re-covers the binding sites. Myosin cannot bind actin. Muscle relaxes.
- Filaments do NOT shorten — they slide past each other
- Sarcomere shortens — Z-lines move closer together
- A-band stays constant — length of thick filaments does not change
- I-band and H-zone shorten — as thin filaments slide inward
- ATP is needed for both contraction (power stroke) and relaxation (detaching myosin heads and pumping Ca2+ back)
Muscle types comparison
| Feature | Skeletal (Striated) | Smooth (Visceral) | Cardiac |
|---|---|---|---|
| Shape | Long, cylindrical | Spindle-shaped | Branched, cylindrical |
| Striations | Present | Absent | Present |
| Nuclei | Multinucleate, peripheral | Uninucleate, central | Uni/binucleate, central |
| Control | Voluntary | Involuntary | Involuntary |
| Contraction | Fast, powerful, fatigable | Slow, sustained | Rhythmic, never fatigues |
| Location | Attached to bones | Walls of hollow organs | Heart wall only |
| Special feature | Motor end plates | Gap junctions | Intercalated discs |
Muscle disorders
Myasthenia gravis — autoimmune, antibodies block acetylcholine receptors. Muscular dystrophy — genetic, progressive muscle weakness. Tetany — low calcium causes sustained contraction.
Additional disorders:
- Osteoarthritis: wear and tear of cartilage at joints. Common in elderly.
- Rheumatoid arthritis: autoimmune — immune system attacks synovial membrane.
- Gout: uric acid crystals deposit in joints, causing inflammation.
- Osteoporosis: bone density loss, common in post-menopausal women.
Worked Examples
After death, ATP production stops. Without ATP, myosin heads cannot release actin, so muscles stay contracted for several hours until enzymes break the bonds.
Calcium released from the sarcoplasmic reticulum binds troponin, shifting tropomyosin to expose myosin-binding sites on actin. Without calcium, contraction cannot begin.
Cardiac muscle has abundant mitochondria (about 25% of cell volume, compared to 2% in skeletal muscle) and relies almost entirely on aerobic respiration. It has a rich blood supply and never develops an oxygen debt. The rhythmic contraction-relaxation cycle allows continuous perfusion.
A sarcomere at rest is 2.5 micrometres long. After contraction, it is 2.0 micrometres. The A-band (1.6 micrometres) stays the same. The I-band has shortened from 0.9 to 0.4 micrometres (0.45 on each side). The H-zone has shortened from 0.5 to 0 micrometres (thin filaments now overlap in the centre).
Common Mistakes
Counting 207 bones. Adults have 206; children have more (some fuse with age).
Calling the knee a ball-and-socket joint. It is a hinge.
Saying muscles push. They only pull — pushing requires an opposing muscle group to pull the other way.
Writing that the A-band shortens during contraction. The A-band (region of thick filaments) stays the SAME length. Only the I-band and H-zone shorten.
Saying ATP is needed only for contraction. ATP is also needed for relaxation — to pump Ca2+ back into the sarcoplasmic reticulum and to detach myosin heads from actin. This is why rigor mortis occurs when ATP runs out.
Exam Weightage and Revision
Locomotion and Movement carries 2-3 NEET questions per year. The sliding filament theory is the single most important topic — understand the sequence of events from neural signal to contraction. CBSE Class 11 boards ask for comparison tables (muscle types, joint types) and diagrams (sarcomere structure).
| Question Type | NEET Frequency | Difficulty |
|---|---|---|
| Sliding filament mechanism | Every year | Medium |
| Muscle type comparison | Most years | Easy |
| Joint type identification | Most years | Easy |
| Sarcomere band changes | Every 2 years | Medium |
| Muscle disorders | Occasional | Easy |
The guaranteed NEET question: something about the sliding filament theory. Know what changes during contraction (I-band shortens, H-zone shortens, sarcomere shortens) and what does not change (A-band, filament lengths).
Practice Questions
Q1. What happens to the I-band and H-zone during muscle contraction?
Both shorten. The I-band (region with only thin filaments) shortens because the thin filaments slide inward over the thick filaments, reducing the zone of only thin filaments. The H-zone (region with only thick filaments) shortens because the thin filaments slide into this region. At maximum contraction, the H-zone may disappear entirely.
Q2. Why is calcium essential for muscle contraction?
Ca2+ binds to troponin C on thin filaments, causing a conformational change that moves tropomyosin away from the myosin-binding sites on actin. Without calcium, tropomyosin blocks the binding sites, and myosin heads cannot attach to actin — no cross-bridge formation means no contraction.
Q3. Differentiate between tendons and ligaments.
Tendons connect muscle to bone — they transmit the force of muscle contraction to the skeleton. They are made of dense regular connective tissue (parallel collagen fibres) and are inelastic. Ligaments connect bone to bone — they stabilise joints. They are also dense regular tissue but contain more elastin, making them slightly elastic.
Q4. A person has myasthenia gravis. Why do they experience muscle weakness?
Myasthenia gravis is an autoimmune disease where antibodies attack and destroy acetylcholine receptors at the neuromuscular junction. Fewer receptors mean the muscle fibre receives a weaker signal — the action potential may not be generated or may be too weak to trigger adequate calcium release. Result: muscle weakness and fatigue, especially with repeated use.
FAQs
Why do muscles work in antagonistic pairs?
Muscles can only pull (contract), never push. To move a joint in two directions, you need at least two muscles pulling in opposite directions. The biceps flexes the elbow (bending); the triceps extends it (straightening). When one contracts, the other relaxes.
What is the oxygen debt after exercise?
During intense exercise, anaerobic respiration in muscles produces lactic acid (when O2 supply is insufficient). After exercise, extra oxygen is consumed to: (1) convert lactic acid back to glucose in the liver, (2) replenish ATP and creatine phosphate reserves, (3) restore O2 bound to myoglobin. The extra O2 needed is the “oxygen debt.”
Why does stretching prevent muscle cramps?
Cramps often result from sustained involuntary contraction due to lactic acid accumulation or electrolyte imbalance. Stretching forcibly lengthens the muscle, breaking the cross-bridge cycle and allowing the muscle to relax. Staying hydrated and maintaining electrolyte balance (Na+, K+, Ca2+, Mg2+) also helps.
Learn the four synovial joint types with one example each. That covers most NEET locomotion questions.
Lever Systems in the Human Body
The musculoskeletal system works as a system of levers. Each lever has three components: fulcrum (joint), effort (muscle contraction), and load (weight of body part + any external weight).
Three classes of levers in the body:
| Class | Fulcrum Position | Example | Analogy |
|---|---|---|---|
| First class | Between effort and load | Head nodding (atlas-axis joint) | Seesaw |
| Second class | Load between fulcrum and effort | Standing on tiptoes (ankle joint) | Wheelbarrow |
| Third class | Effort between fulcrum and load | Biceps flexing forearm (elbow) | Tweezers |
Most joints in the body use third-class levers — the muscle inserts close to the joint (short effort arm), giving less mechanical advantage but allowing faster movement over a greater range. This is why our limbs can move quickly even though muscles are relatively close to the joints.
Muscle Fibre Types — Red vs White
Not all skeletal muscle fibres are the same. Two main types exist:
| Feature | Red (Slow-twitch, Type I) | White (Fast-twitch, Type II) |
|---|---|---|
| Colour | Red (high myoglobin) | Pale (low myoglobin) |
| Contraction speed | Slow | Fast |
| Fatigue resistance | High (aerobic) | Low (anaerobic, fatigues quickly) |
| Mitochondria | Many | Few |
| Energy source | Aerobic (fat oxidation) | Anaerobic (glycolysis, creatine phosphate) |
| Function | Endurance (marathon running, posture) | Power (sprinting, jumping, lifting) |
Most muscles contain a mix of both types. The proportion varies between individuals (genetics) and between muscles. The soleus (calf) is mostly red (postural), while the gastrocnemius (also calf) has more white fibres (for jumping).
Marathon runners tend to have more red fibres; sprinters have more white fibres. Training can increase the size and efficiency of existing fibre types but cannot convert one type to another.
Locomotion is the chapter where anatomy becomes mechanics. Levers, pulleys and hinges — your body is a machine, and bones are the frame.