Locomotion Muscle Contraction

8 MCQs9-step worked example
Source: NCERT Human PhysiologyPYQ coverage: NEET 2021, 2023, 2024, 2026Official key: NTA-verifiedLast updated: 25 Sep 2026

Locomotion Muscle Contraction, explained for NEET

A high-frequency trap in this unit: students see a sarcomere before and after contraction and say the A-band shortens. It does not — the I-band gets reduced while the A-band retains its length (NCERT Class 11 Biology, Chapter 17, page 222); the sarcomere itself shortens because the thin filaments slide further into the fixed-length A-band.

The functional unit of contraction is the sarcomere: the stretch of myofibril between two successive Z-lines (page 220), not the myofibril or the whole fibre. Its central A-band (myosin, thick filaments) is flanked by two half I-bands (actin, thin filaments); the free centre of the A-band not yet overlapped by thin filaments is the H-zone (page 220).

Contraction begins at the neuromuscular junction: a motor neuron's signal releases acetylcholine, generating an action potential in the sarcolemma that triggers calcium release from the sarcoplasmic reticulum (page 222). Calcium binds a troponin subunit — not tropomyosin directly — unmasking myosin's binding sites on actin (page 221). The exposed myosin head, an ATPase, binds actin to form a cross bridge, and ATP hydrolysis powers the pulling stroke that slides actin toward the sarcomere centre, narrowing the I-band and the H-zone (page 222). Repeated cross-bridge cycling continues until calcium is pumped back into the sarcoplasmic reticulum, re-masking the sites and letting the Z-lines return to their resting spacing — relaxation (page 223).

Bridge to NEET: this A/I-band behaviour and the troponin (not tropomyosin) masking role are recurring one-liners across the biology paper's organ-system-pattern set. A second common confusion swaps red and white fibres: red fibres are myoglobin-rich, mitochondria-dense and aerobic; white fibres have little myoglobin, few mitochondria, more sarcoplasmic reticulum, and run anaerobically, which is why repeated stimulation drives lactic-acid build-up and fatigue (page 223).

Watch-out: never let "A-band shortens" or "tropomyosin masks the site" slip into an answer — both are the two most common wrong turns on this unit's questions.

Can you answer these Locomotion Muscle Contraction MCQs?

Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.

MCQ 1Easy RecallPractice

The portion of a myofibril between two successive Z-lines, considered the functional unit of muscle contraction, is called the:

Show answer and why every option is right or wrong

Answer: C. The region between two successive Z-lines is the sarcomere, defined as the functional unit of contraction (NCERT Class 11 Biology, Chapter 17, page 220).

Why A is wrong: A myofibril is the parallel bundle of many sarcomeres in series, not the single unit between two Z-lines (page 220).

Why B is wrong: A muscle fibre is the whole cell containing many myofibrils, a much larger structure than a single sarcomere (page 219).

Why D is wrong: A fascicle is a bundle of many muscle fibres held by fascia, far larger than a single sarcomere (page 219).

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MCQ 2Easy RecallPractice

In a myofibril, the light I-band and the dark A-band are distinguished mainly by which proteins they contain?

Show answer and why every option is right or wrong

Answer: A. The light I-band (isotropic) is made of thin actin filaments and the dark A-band (anisotropic) is made of thick myosin filaments (NCERT Class 11 Biology, Chapter 17, page 219–220).

Why B is wrong: This reverses the bands: the I-band holds actin and the A-band holds myosin, not the other way round (page 219–220).

Why C is wrong: The striped appearance exists precisely because actin and myosin are segregated into separate bands, not mixed evenly across both (page 219–220).

Why D is wrong: Troponin and tropomyosin are accessory proteins running along the thin (actin) filament; they do not define which band is which (page 221).

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MCQ 3Easy RecallPractice

In a resting skeletal muscle fibre, which protein masks the active myosin-binding sites on the actin filament?

Show answer and why every option is right or wrong

Answer: B. A subunit of troponin, distributed at intervals along the tropomyosin, masks the active binding sites for myosin on actin in the resting state (NCERT Class 11 Biology, Chapter 17, page 221).

Why A is wrong: Tropomyosin filaments run alongside the actin, but the masking itself is done by the troponin subunit sitting on tropomyosin, not by tropomyosin directly (page 221).

Why C is wrong: Meromyosin is the monomeric protein unit of the thick (myosin) filament, carrying the ATPase head; it plays no role in masking actin's binding sites (page 221).

Why D is wrong: Myoglobin is the oxygen-storing pigment that colours red fibres; it has no role in masking the actin-myosin binding site (page 223).

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MCQ 4Direct ApplicationPractice

A sarcomere is compared before and after a contraction. Which change is correctly reported?

Show answer and why every option is right or wrong

Answer: D. During shortening (contraction) the I-bands get reduced while the A-bands retain their length, because the thin filaments slide further into the fixed-length A-band (NCERT Class 11 Biology, Chapter 17, page 222).

Why A is wrong: This reverses the stated fact: it is the I-band, not the A-band, that shrinks during contraction (page 222).

Why B is wrong: The A-band's length is set by the fixed-length thick filaments and does not shorten at all during a normal contraction cycle (page 222).

Why C is wrong: The A-band does not lengthen during contraction; only the I-band is reduced as thin filaments slide inward (page 222).

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MCQ 5Direct ApplicationPractice

A muscle sample is rich in myoglobin and mitochondria and relies mainly on oxygen for ATP production. This describes:

Show answer and why every option is right or wrong

Answer: A. High myoglobin content and plentiful mitochondria that use stored oxygen for ATP production define red fibres, which are therefore called aerobic muscles (NCERT Class 11 Biology, Chapter 17, page 223).

Why B is wrong: White fibres have very little myoglobin, few mitochondria, more sarcoplasmic reticulum, and depend on anaerobic processes — the opposite profile (page 223).

Why C is wrong: Visceral muscle is a location-based category (smooth, involuntary muscle of hollow organs); myoglobin/mitochondria content is the separate red-vs-white fibre distinction (page 218).

Why D is wrong: Cardiac muscle is a location-based category for heart muscle; the aerobic, myoglobin-rich description in the stem is the red-fibre metabolic classification, not a location one (page 218).

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MCQ 6Direct ApplicationPractice

At the neuromuscular junction, a nerve impulse triggers release of acetylcholine, which generates an action potential in the sarcolemma. What is the immediate next event in the contraction sequence?

Show answer and why every option is right or wrong

Answer: C. The action potential spreading through the fibre causes release of calcium ions into the sarcoplasm, which then bind a troponin subunit and unmask actin's active sites (NCERT Class 11 Biology, Chapter 17, page 222).

Why A is wrong: ATP hydrolysis by the myosin head powers cross-bridge cycling only after the active sites are exposed by calcium binding, not before the calcium rise (page 222).

Why B is wrong: The Z-lines return to their resting position during relaxation, after calcium is pumped back out — the opposite end of the sequence from the one asked here (page 223).

Why D is wrong: Re-masking by troponin happens during relaxation, once calcium is resequestered, not right after the action potential (page 223).

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MCQ 7CalculationPractice

Arrange the following events of a single contraction cycle in the correct order: (1) myosin head binds the exposed actin site forming a cross bridge, (2) calcium binds troponin and unmasks the active site, (3) ATP hydrolysis powers the pulling stroke that slides actin toward the A-band centre, (4) action potential releases calcium from the sarcoplasmic reticulum.

Show answer and why every option is right or wrong

Answer: B. The action potential first releases calcium (event 4), calcium then unmasks the actin site by binding troponin (event 2), the myosin head binds the exposed site (event 1), and ATP hydrolysis powers the pulling stroke that slides the filament (event 3): the order is 4, 2, 1, 3 (NCERT Class 11 Biology, Chapter 17, page 222).

Why A is wrong: This places cross-bridge formation and the pulling stroke before calcium is even released, which is impossible since the binding site stays masked until calcium acts (page 222).

Why C is wrong: This releases calcium before the action potential that triggers it, reversing cause and effect at the start of the cycle (page 222).

Why D is wrong: This has the myosin head binding actin (1) before calcium unmasks the site (2), which cannot happen because the site is still masked at that point (page 222).

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MCQ 8Concept TrapPractice

During cross-bridge cycling, the thin filaments slide further into the central region of the A-band that was previously free of thin filaments. What happens to this central region, the H-zone, as contraction proceeds?

Show answer and why every option is right or wrong

Answer: D. The H-zone is the central part of the A-band not overlapped by thin filaments at rest; as thin filaments slide further toward the centre during contraction, this free central region narrows (NCERT Class 11 Biology, Chapter 17, page 220, 222; Figure 17.5).

Why A is wrong: The H-zone's width depends on how far the thin filaments have slid in; it is not fixed — the sliding-filament figure shows its size changing with contraction (page 222).

Why B is wrong: Sliding pulls the thin filaments further into, not away from, the H-zone, so the zone narrows rather than widens (page 222).

Why C is wrong: The H-zone narrows progressively as cross-bridge cycling continues; the text describes gradual sliding, not complete disappearance from a single cross-bridge event (page 222).

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How do you solve a Locomotion Muscle Contraction question? A worked example

  1. 1

    Given

    A motor neuron fires at the neuromuscular junction of a resting skeletal muscle fibre.

  2. 2

    Required

    Trace the full sequence of events from the nerve signal to sarcomere shortening and back to relaxation.

  3. 3

    Concept

    Sliding filament theory: contraction results from thin (actin) filaments sliding over thick (myosin) filaments, triggered by calcium-mediated exposure of actin's myosin-binding sites, and reversed when calcium is withdrawn (NCERT Class 11 Biology, Chapter 17, page 221–223).

  4. 4

    Formula

    No numerical formula applies here — the mechanism is a fixed causal chain: ACh release → action potential in sarcolemma → Ca++ release from sarcoplasmic reticulum → Ca++ binds troponin, unmasking actin → myosin head binds actin (cross bridge) → ATP hydrolysis powers the pulling stroke → Ca++ pumped back → troponin re-masks actin → relaxation.

  5. 5

    Substitution

    Apply the chain to this neuron's signal: the neurotransmitter released at this motor-end plate is acetylcholine (page 222), which generates an action potential specifically in this fibre's sarcolemma, spreading along it and triggering calcium release from this fibre's own sarcoplasmic reticulum.

  6. 6

    Calculation

    Calcium binds the troponin subunit on this fibre's actin filaments, unmasking the myosin-binding sites; the ATPase myosin head binds actin to form a cross bridge; ATP hydrolysis powers the head's pulling stroke, dragging the actin (and its attached Z-lines) toward the A-band centre; the cycle of cross-bridge formation, pulling and breaking repeats as long as calcium and ATP are available, progressively narrowing the I-band and H-zone.

  7. 7

    Final answer

    The sarcomere shortens (contraction) as the I-band and H-zone narrow while the A-band length is unchanged. Relaxation follows once calcium is pumped back into the sarcoplasmic reticulum, re-masking the actin sites, breaking the cross bridges, and letting the Z-lines return to their resting spacing (page 222–223).

  8. 8

    Common trap

    Do not say the A-band shortens (page 222) — the trap this lesson opened with — and do not say tropomyosin itself does the masking; it is a troponin subunit sitting on tropomyosin that masks the site (page 221).

  9. 9

    Similar NEET-style question

    A student observes that in a fatigued muscle, cross-bridge cycling slows and lactic acid accumulates, and this happens faster in some fibres than others. Which process causes the lactic acid, and why does it build up faster specifically in white fibres than red fibres? (Answer: repeated stimulation raises ATP demand faster than oxygen supply can meet it, so the muscle switches to anaerobic breakdown of glycogen, producing lactic acid and fatigue — NCERT Class 11 Biology, Chapter 17, page 222–223. The discriminating reason for the fibre-type difference is metabolic equipment, not the contraction mechanism: white fibres have few mitochondria and little myoglobin and depend on anaerobic metabolism by default, so they reach lactic-acid build-up sooner than mitochondria-dense, myoglobin-rich red fibres, which draw on stored oxygen instead — page 223.)

What to remember before solving Locomotion Muscle Contraction questions

Definition

The Sarcomere

The light band of a myofibril contains actin and is called the I-band (Isotropic band); the dark band, called the A-band (Anisotropic band), contains myosin. Actin filaments are thinner as compared to the myosin filaments, hence are commonly called thin and thick filaments respectively. An elastic Z-line bisects the centre of each I-band, and thin filaments are firmly attached to it. Thick filaments in the A-band are held together at the M-line. The portion of a myofibril between two successive Z-lines is the functional unit of contraction, called a sarcomere. In the resting state, the central part of the thick filament not overlapped by thin filaments is the H-zone.

-- NCERT Class 11 Biology, Chapter 17, p. 220
Definition

Structure of Contractile Proteins

Each thin actin filament is made of two F-actins helically wound together, each F-actin being a polymer of monomeric G-actin. Tropomyosin filaments run close to the F-actins along their length, and the complex protein troponin is distributed at regular intervals on tropomyosin. In the resting state, a troponin subunit masks the active myosin-binding sites on actin. Each thick myosin filament is a polymer of monomeric meromyosins; each meromyosin has a globular head with a short arm (heavy meromyosin, HMM) and a tail (light meromyosin, LMM). The globular head is an active ATPase enzyme and has binding sites for ATP and active sites for actin.

-- NCERT Class 11 Biology, Chapter 17, p. 221
Key Fact

Types of Movement

Cells of the human body show three main types of movement: amoeboid, ciliary and muscular. Amoeboid movement is shown by specialised cells such as macrophages and leucocytes in blood, effected by pseudopodia formed by streaming of protoplasm, with cytoskeletal microfilaments also involved. Ciliary movement occurs in most internal tubular organs lined by ciliated epithelium; coordinated ciliary movement in the trachea removes dust and foreign particles inhaled with air, and also moves ova through the female reproductive tract. Movement of limbs, jaws and tongue requires muscular movement, and locomotion needs coordinated activity of muscular, skeletal and neural systems.

-- NCERT Class 11 Biology, Chapter 17, p. 218
Key Fact

Muscle: Types by Location

Muscle is a mesodermal tissue contributing about 40-50% of adult body weight, with properties of excitability, contractility, extensibility and elasticity. Based on location, three types are recognised. Skeletal muscles are attached to the skeletal system, have a striped (striated) appearance, are under voluntary nervous control, and drive locomotory actions and posture changes. Visceral muscles line hollow visceral organs like the alimentary canal and reproductive tract, show no striation (smooth muscle), and are involuntary, assisting transport of food and gametes. Cardiac muscle, the muscle of the heart, is striated but involuntary, formed of branching cardiac muscle cells.

-- NCERT Class 11 Biology, Chapter 17, p. 218
Key Fact

Structure of the Muscle Fibre

A skeletal muscle is made of muscle bundles (fascicles) held together by fascia, each fascicle containing many muscle fibres. Each muscle fibre is lined by the plasma membrane called the sarcolemma, enclosing the sarcoplasm. The muscle fibre is a syncitium since the sarcoplasm contains many nuclei. The sarcoplasmic reticulum (the fibre's endoplasmic reticulum) is the store house of calcium ions. A characteristic feature is a large number of parallel filaments in the sarcoplasm called myofilaments or myofibrils, each showing alternate dark and light bands, giving the striated appearance from the distribution of the proteins actin and myosin.

-- NCERT Class 11 Biology, Chapter 17, p. 219
Key Fact

Mechanism of Muscle Contraction

The sliding filament theory states that muscle contraction occurs by the sliding of thin filaments over thick filaments. Muscle contraction is initiated by a signal sent by the central nervous system via a motor neuron; the neuromuscular junction (motor-end plate) releases acetylcholine, generating an action potential in the sarcolemma. This causes release of calcium ions into the sarcoplasm; Ca++ binds a troponin subunit, unmasking myosin-binding sites on actin. Using ATP hydrolysis energy, the myosin head binds exposed actin sites forming a cross bridge, pulling actin filaments and Z-lines inward, shortening the sarcomere. During contraction, I-bands reduce while A-bands retain their length.

-- NCERT Class 11 Biology, Chapter 17, p. 222
Key Fact

Fatigue and Red/White Fibres

Contraction continues via repeated cross-bridge formation and breakage until Ca++ ions are pumped back into the sarcoplasmic cisternae, masking actin filaments and causing Z-lines to return, i.e., relaxation. Repeated activation of the muscles can lead to accumulation of lactic acid from anaerobic breakdown of glycogen, causing fatigue. Muscle contains the red pigment myoglobin, which stores oxygen. Muscles high in myoglobin appear reddish and are called Red fibres; these contain plenty of mitochondria and use stored oxygen for ATP production, so are also called aerobic muscles. White fibres have little myoglobin, appear pale, have few mitochondria but plenty of sarcoplasmic reticulum, and depend on anaerobic processes for energy.

-- NCERT Class 11 Biology, Chapter 17, p. 223

Where do students lose marks on Locomotion Muscle Contraction?

These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.

Trap

A-band vs I-band shortening

Category: Similar Terms

Students confuse which band shortens during muscle contraction.

When it triggers

A NEET question shows a sarcomere before/after contraction and asks which band shortens, expecting students to wrongly say the A-band shortens.

How to avoid

During contraction the I-bands get reduced while the A-bands retain their length (C11_BIO_20_FACT_06); the sarcomere itself shortens because thin filaments slide over thick ones (C11_BIO_20_DEF_04).

Trap

Red vs white fibre metabolism

Category: Similar Terms

Students swap which fibre type is aerobic and which is anaerobic.

When it triggers

A question describes a muscle rich in mitochondria and myoglobin and asks whether it is a red or white fibre, or asks which fibre type depends on anaerobic metabolism.

How to avoid

Red fibres are high in myoglobin, contain plenty of mitochondria and are aerobic; white fibres have little myoglobin, few mitochondria, more sarcoplasmic reticulum, and depend on anaerobic processes (C11_BIO_20_FACT_07).

More in Human Physiology: 29 exam traps and mistakes · 2 formulas · 1 question pattern from its other lessons.

Locomotion Muscle Contraction questions from past NEET papers

4 questions from NEET 2021, 2023, 2024, 2026. Answers verified against NTA official keys.

NEET 2024Revised key

Three types of muscles are given as a, b and c. Identify the correct matching pair along with their location in human body: Name of muscle/location

1(a) Smooth - Toes (b) Skeletal – Legs (c) Cardiac – Heart
2(a) Skeletal - Triceps (b) Smooth – Stomach (c) Cardiac – Heart
3(a) Skeletal - Biceps (b) Involuntary – Intestine (c) Smooth – Heart
4(a) Involuntary – Nose tip (b) Skeletal – Bone (c) Cardiac – Heart
NTA Answer: Option 2(revised_final)

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NEET 2023

Which of the following statements are correct regarding skeletal muscle? A. Muscle bundles are held together by collagenous connective tissue layer called fascicle. B. Sarcoplasmic reticulum of muscle fibre is a store house of calcium ions. C. Striated appearance of skeletal muscle fibre is due to distribution pattern of actin and myosin proteins. D. M line is considered as functional unit of contraction called sarcomere. Choose the most appropriate answer from the options given below:

1B and C only
2A, C and D only
3C and D only
4A, B and C only
NTA Answer: Option 1(final)

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All 71 past-paper questions from Human Physiology →

Sources

NCERT refs: Class 11 Biology Chapter 17, p.222

Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.

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