Locomotion and Movement: The Mechanics of a Moving Body
Every time you pick up a pen, walk to the board, or blink — your body is executing a precisely coordinated sequence of muscle contractions, bone rotations, and nerve signals. Locomotion (movement of the whole organism from place to place) and movement (displacement of a body part) are two concepts students often blur, and that distinction alone has fetched marks in NEET.
Movement is broader — the beating of your heart, peristalsis in your gut, even ciliary movement of cells — all of these qualify. Locomotion is a subset: it specifically means self-propelled movement that changes location. A paramecium moving through water is locomoting. Your intestinal muscles pushing food forward are moving but not locomoting.
This chapter carries consistent 1-2 question weightage in NEET every year, mostly from joints, types of muscles, and the sliding filament theory. Board exams go heavier on diagrams — the sarcomere structure and joint types are the high-value diagrams to practice.
Key Terms and Definitions
Sarcomere — the functional unit of a myofibril, bounded by two Z-lines (Z-discs). Understanding what happens inside a sarcomere during contraction is the core concept of this chapter.
Myofibril — thread-like structures inside a muscle fibre made up of repeating sarcomeres. Each muscle fibre can contain hundreds of myofibrils arranged in parallel.
Actin (thin filament) — a protein that forms the I-band region of a sarcomere. Two actin strands twisted together, with tropomyosin and troponin as regulatory proteins on the surface.
Myosin (thick filament) — occupies the A-band. Has globular heads (cross-bridges) that project outward toward actin filaments. These heads are the molecular motors.
Sliding filament theory — the mechanism of muscle contraction: actin filaments slide over myosin filaments, shortening the sarcomere without the filaments themselves changing length. Proposed by Hugh Huxley and Jean Hanson (1954).
Neuromuscular junction (NMJ) — the synapse between a motor neuron and a muscle fibre. Acetylcholine released here triggers the action potential that initiates contraction.
Rigor mortis — post-death muscle stiffness caused by inability to break actin-myosin cross-bridges (no ATP available). Relevant for NEET MCQs on ATP’s role in relaxation.
Antagonistic muscles — pairs of muscles with opposing actions (e.g., biceps and triceps). Crucial for joint-based questions.
Types of Muscles
| Feature | Skeletal | Cardiac | Smooth |
|---|---|---|---|
| Striation | Yes | Yes | No |
| Nucleus | Multi-nucleated | Single, central | Single, central |
| Control | Voluntary | Involuntary | Involuntary |
| Location | Limbs, trunk | Heart wall | Visceral organs, blood vessels |
| Intercalated discs | Absent | Present | Absent |
| Speed | Fast | Moderate | Slow |
Skeletal muscle fibres are multinucleated (nuclei pushed to the periphery) because they form by fusion of many myoblasts during development. This is a favourite NEET trap — students often say the nucleus is central, confusing it with cardiac.
Cardiac muscle is unique: striated like skeletal, involuntary like smooth, and connected by intercalated discs that allow rapid electrical coupling between cells. This is why the heart contracts as a unit.
The Sliding Filament Theory — Step by Step
This is the highest-yield concept in the chapter. Let’s build it clearly.
Step 1: The Resting State
In a relaxed sarcomere, tropomyosin physically blocks the active sites on actin where myosin heads would bind. Troponin (a protein complex) holds tropomyosin in this blocking position.
Step 2: Nerve Signal Arrives
A motor neuron fires → acetylcholine is released at the NMJ → action potential spreads across the muscle fibre membrane (sarcolemma) → travels into the fibre via T-tubules → reaches the sarcoplasmic reticulum (SR).
Step 3: Calcium Release
The SR releases Ca²⁺ ions into the sarcoplasm. Ca²⁺ binds to troponin-C (the calcium-binding subunit of troponin). This causes a conformational change: troponin shifts tropomyosin away from actin’s active sites.
Remember: Ca²⁺ acts on troponin, which moves tropomyosin. The sequence is Ca²⁺ → troponin → tropomyosin → active site exposed. Get this chain right in one-word-answer questions.
Step 4: Cross-Bridge Formation
Myosin heads now bind to exposed actin active sites → actin-myosin cross-bridge forms.
Step 5: Power Stroke
Myosin head pivots (bends), pulling the actin filament toward the M-line. This is the power stroke. ADP + Pᵢ are released from the myosin head during this step.
Step 6: Cross-Bridge Detachment
A new ATP molecule binds to the myosin head → cross-bridge detaches from actin.
Step 7: Reset
ATP is hydrolysed (ATPase activity of myosin head) → myosin head returns to cocked position → cycle repeats if Ca²⁺ is still present.
Students write that ATP is needed for cross-bridge formation. Wrong direction — ATP is needed to break the cross-bridge (detachment step). Formation happens spontaneously once tropomyosin moves. This is why rigor mortis causes stiffness — no ATP means cross-bridges can’t detach.
What Changes During Contraction
| Band/Zone | Change |
|---|---|
| I-band | Decreases (shortens) |
| H-zone | Decreases (shortens) |
| A-band | No change (myosin length constant) |
| Z-lines | Move closer together |
| Sarcomere length | Decreases |
This table appears in NEET almost every alternate year. Memorise the A-band exception.
Joints: Classification and Examples
Joints (articulations) are the sites where two or more bones meet. Classification by movement:
Fibrous Joints (Synarthroses)
No movement. Bones held together by dense fibrous connective tissue.
- Example: sutures between skull bones
Cartilaginous Joints (Amphiarthroses)
Slight movement. Connected by cartilage.
- Example: intervertebral discs, symphysis pubis
Synovial Joints (Diarthroses)
Free movement. Have a synovial cavity filled with synovial fluid (lubrication + nutrient delivery to cartilage).
| Type | Movement | Example |
|---|---|---|
| Ball and socket | Multiaxial (all directions) | Shoulder (glenohumeral), hip |
| Hinge | Uniaxial (one plane) | Elbow, knee, interphalangeal |
| Pivot | Rotation only | Atlas-axis (head rotation) |
| Gliding | Sliding | Intercarpal joints |
| Saddle | Biaxial | Carpometacarpal of thumb |
| Condyloid | Biaxial, no rotation | Radiocarpal (wrist) |
NEET 2022 asked which joint allows rotation of the head — pivot joint (atlas rotating around the odontoid process of axis). For CBSE boards, the 5-mark question often asks to draw and label a synovial joint — include: articular cartilage, synovial membrane, synovial fluid, joint capsule, ligaments.
Skeletal System Overview
The adult human skeleton has 206 bones. For NEET, focus on the formula and distribution:
| Region | Bones |
|---|---|
| Skull (cranial + facial) | 22 (8 cranial + 14 facial) |
| Vertebral column | 26 (33 in foetus) |
| Ribs + Sternum | 25 (12 pairs ribs + 1 sternum) |
| Pectoral girdle | 4 (2 clavicle + 2 scapula) |
| Upper limbs | 60 (30 per limb) |
| Pelvic girdle | 2 (hip bones) |
| Lower limbs | 60 (30 per limb) |
The pectoral girdle connects the upper limb to the axial skeleton. It’s incomplete posteriorly (no dorsal bony connection). The pelvic girdle is complete (forms the bony birth canal).
Each hip bone forms by fusion of ilium, ischium, and pubis — the acetabulum is where these three meet and where the head of the femur articulates.
Solved Examples
Example 1 — Easy (CBSE Board Level)
Q: What is the role of calcium ions in muscle contraction?
Ca²⁺ is released from the sarcoplasmic reticulum when an action potential reaches the T-tubules. It binds to troponin-C, causing troponin to shift tropomyosin away from the active sites on actin. This exposes the sites for myosin head binding, initiating the cross-bridge cycle and contraction.
Without Ca²⁺, tropomyosin remains in blocking position — muscle stays relaxed.
Example 2 — Medium (NEET Level)
Q: During muscle contraction, which of the following does NOT change: A-band, I-band, H-zone, sarcomere length?
Answer: A-band.
The A-band corresponds to the full length of the myosin (thick) filament, which doesn’t shorten — only slides relative to actin. The I-band (only actin region) and H-zone (only myosin region) both decrease as actin slides inward. Sarcomere length decreases as Z-lines come closer.
Example 3 — Hard (NEET 2021 pattern)
Q: A person with a deficiency of acetylcholinesterase at the neuromuscular junction would show which of the following?
Answer: Sustained/prolonged muscle contraction.
Acetylcholinesterase breaks down acetylcholine in the synaptic cleft. Without it, ACh keeps stimulating the muscle fibre indefinitely → the muscle keeps receiving signals → tetanic (sustained) contraction. This is the mechanism of certain nerve agents and organophosphate pesticides — a favourite NEET application question.
Exam-Specific Tips
NEET Weightage: Locomotion and Movement typically contributes 1-2 questions per paper. Sliding filament theory steps, A-band/I-band changes, joint types, and muscle type comparisons are the consistent hit zones. NEET 2019 asked about intercalated discs; NEET 2021 about acetylcholinesterase; NEET 2022 about pivot joint.
CBSE Class 11 Board: 5-mark questions from this chapter usually ask (a) diagram of a sarcomere with labels, or (b) describe the sliding filament theory. For the theory, a 6-step answer covering Ca²⁺ release → troponin → active site exposure → power stroke → ATP-dependent detachment → relaxation scores full marks.
For NEET MCQs: The traps are always in the “which does NOT change” format for sarcomere bands, and in muscle type features (e.g., “which muscle has intercalated discs?”). Never mix up multinucleated (skeletal) with single-central nucleus (cardiac/smooth).
Common Mistakes to Avoid
Mistake 1 — Confusing ATP’s role: Students say ATP is needed for contraction (cross-bridge formation). Correct: ATP is needed for relaxation (cross-bridge detachment and Ca²⁺ reuptake into SR). Contraction without ATP leads to permanent rigidity — rigor mortis.
Mistake 2 — A-band changes during contraction: The most common wrong answer in band-change questions is “A-band shortens.” The A-band length = myosin length = constant. Write this 10 times if needed.
Mistake 3 — Pectoral vs pelvic girdle: Pectoral girdle is incomplete (no bony dorsal connection). Pelvic girdle is complete. Students flip this regularly.
Mistake 4 — Number of bones in skull: 22 total (8 cranial + 14 facial). Many students write 8 for the whole skull. The 14 facial bones include lacrimal, nasal, zygomatic, etc.
Mistake 5 — Cardiac muscle nucleus position: Cardiac muscle has a single, centrally placed nucleus (NOT peripheral like skeletal). When a question says “striated + single central nucleus + involuntary” — that’s cardiac, not smooth.
Practice Questions
Q1. Which zone disappears completely when a muscle is in maximum contraction?
The H-zone disappears. It represents the region containing only myosin (no overlap with actin). At maximum contraction, actin filaments from both sides overlap in the centre — the H-zone no longer exists.
Q2. The functional unit of a myofibril is called ____. Its boundaries are marked by ____.
Sarcomere. Bounded by Z-lines (Z-discs) on either side.
Q3. Name the joint that allows the thumb its wide range of motion, including opposition.
Saddle joint — carpometacarpal joint of the thumb. It allows biaxial movement and is the reason humans can oppose the thumb to all other fingers (key evolutionary feature for tool use).
Q4. What would happen to a muscle if the sarcoplasmic reticulum could not reabsorb Ca²⁺ ions?
The muscle would remain in a contracted state (tetany). Ca²⁺ would stay in the sarcoplasm, keeping troponin shifted, actin sites exposed, and cross-bridge cycling active. Relaxation requires Ca²⁺ to be pumped back into the SR by Ca²⁺-ATPase pumps (another ATP-consuming step).
Q5. A ball-and-socket joint is present at: (a) knee, (b) elbow, (c) shoulder, (d) wrist
(c) Shoulder (glenohumeral joint). Also present at the hip. Knee and elbow are hinge joints; wrist is a condyloid joint.
Q6. Why does rigor mortis occur? Which ATP-dependent step fails?
After death, ATP production stops. Without ATP, myosin heads cannot detach from actin filaments — cross-bridges remain locked. Additionally, Ca²⁺ cannot be pumped back into the SR. The result is permanent actin-myosin binding, causing muscle stiffness. The step that fails is cross-bridge detachment (requires ATP binding to myosin head).
Q7. How many bones form the pectoral girdle? Name them.
4 bones total — 2 clavicles (collar bones) and 2 scapulae (shoulder blades). The pectoral girdle is incomplete — there is no bony connection dorsally, only the sternum anteriorly via the clavicles.
Q8. Distinguish between red (slow-twitch) and white (fast-twitch) muscle fibres.
Red (Type I / slow-twitch): Rich in myoglobin (red colour), many mitochondria, aerobic metabolism, fatigue-resistant, used for sustained activity (e.g., postural muscles, marathon running).
White (Type II / fast-twitch): Less myoglobin, fewer mitochondria, anaerobic glycolysis, fast but fatigue quickly, used for explosive movements (e.g., sprinting, jumping). Most human muscles are a mix of both types.
FAQs
What is the difference between movement and locomotion?
Movement refers to displacement of a body part or internal structure — includes muscle contraction, ciliary beating, peristalsis, heartbeat. Locomotion is the movement of the entire organism from one place to another. All locomotion involves movement, but not all movement is locomotion.
Which protein directly blocks the actin active site in resting muscle?
Tropomyosin is the protein that physically covers the active sites on actin. Troponin is the regulatory complex that holds tropomyosin in place — Ca²⁺ displaces troponin-tropomyosin, not acts on actin directly.
Does the A-band change during muscle contraction?
No. The A-band represents the full length of the thick (myosin) filament, which does not shorten. Only the I-band and H-zone decrease. The A-band length is constant whether the muscle is relaxed or contracted.
How many bones are in the adult human body?
206 bones in adults. Newborns have approximately 270-300 bones (many fuse during growth). By age ~25, the count stabilises at 206.
What is the role of troponin vs tropomyosin?
Think of tropomyosin as the lock and troponin as the key-holder. Tropomyosin physically blocks actin. Troponin (specifically troponin-C) detects Ca²⁺ and causes the whole complex to shift, unlocking the active sites. In the absence of Ca²⁺, troponin keeps tropomyosin in blocking position.
Why can’t cartilaginous joints move as freely as synovial joints?
Synovial joints have a fluid-filled cavity (synovial fluid) that lubricates and absorbs shock, allowing free movement. Cartilaginous joints have fibrocartilage or hyaline cartilage directly connecting the bones — there’s no cavity, so movement is limited to slight gliding or compression.
What is the significance of intercalated discs in cardiac muscle?
Intercalated discs contain gap junctions that allow direct electrical coupling between cardiac muscle cells. This means an action potential spreads rapidly through the entire heart muscle — it contracts as a coordinated unit (functional syncytium). Skeletal muscle lacks this, which is why individual motor units can contract independently.