It is known that red muscle fibres in animals can work for longer periods of time continuously. How is this po

NCERT Exemplar · Q30

Q. It is known that red muscle fibres in animals can work for longer periods of time continuously. How is this possible?

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Concept understanding — Red And White Muscle Fibres

Red and White Muscle Fibres — The First Meeting

Think about the meat you might have seen. Chicken breast is pale, almost white. Chicken leg or thigh is darker, reddish. That colour difference isn't random — it tells you something deep about how those muscles work. The red ones are built for endurance, the white ones for speed and power.

Now bring that to your own body. When you stand quietly, your postural muscles (back, neck, calves) are working all day without tiring. When you sprint for a bus, your leg muscles explode into action but fatigue within seconds. Different jobs, different muscle designs.

The Core Idea

The colour difference comes down to one protein: myoglobin. Myoglobin is an oxygen-storing protein, similar to haemoglobin in blood but found inside muscle cells. It gives muscle a reddish-brown colour. More myoglobin = redder muscle = more oxygen available inside the cell.

Red fibres have lots of myoglobin. They are designed for aerobic respiration — using oxygen to make ATP steadily. They contract slowly but can keep going for a long time. Think marathon runner.

White fibres have very little myoglobin. They rely on anaerobic glycolysis — making ATP without oxygen, fast but inefficient. They contract rapidly, generate huge force, but fatigue quickly. Think sprinter.

\text{Red fibres} \xleftrightarrow{\text{high myoglobin}} \text{aerobic, slow, fatigue-resistant}

The Precise Statement

Skeletal muscle fibres are classified into two broad types based on their myoglobin content and metabolic pathway:

Red Muscle Fibres (Type I / Slow-twitch)

Rich in myoglobin → red appearance
Many mitochondria → high oxidative capacity
Dense capillary network → good oxygen supply
Contract slowly, produce less force per fibre
Extremely resistant to fatigue
Used in posture, long-distance running, any sustained activity

White Muscle Fibres (Type II / Fast-twitch)

Poor in myoglobin → pale appearance
Few mitochondria → rely on glycolysis
Sparse capillary network
Contract rapidly, produce high force
Fatigue quickly (lactic acid buildup)
Used in sprinting, jumping, weightlifting — any explosive movement

Note

Most human muscles are mixed — they contain both types in varying proportions. The ratio is partly genetic and partly trainable. Endurance athletes develop more red fibres; sprinters develop more white fibres.

Why This Matters for Exams

You will be asked to differentiate red and white fibres. The table below is your go-to:

Feature	Red Fibres	White Fibres
Myoglobin content	High	Low

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Red muscle fibres sustain long, continuous activity because their abundant myoglobin and high mitochondrial density let them depend mainly on the far more efficient aerobic respiratory pathway, rather than low-yield anaerobic breakdown.

Muscle fibres are classed as red or white largely on the basis of their myoglobin content — an oxygen-binding, oxygen-storing pigment related to haemoglobin — and how many mitochondria they contain. Red fibres are rich in both: abundant myoglobin keeps a local oxygen supply readily available even during sustained contraction, and a high density of mitochondria provides the machinery needed to run aerobic respiration continuously.

The amount of ATP a working muscle fibre can generate, and how long it can keep generating it without fatigue, depends heavily on which respiratory route it relies on. Aerobic respiration, in which pyruvic acid is completely oxidised via the citric acid cycle and the electron transport system, yields a much larger amount of ATP per glucose molecule than fermentation-type anaerobic breakdown, which produces only a net gain of two ATP per glucose and leaves most of the substrate's energy locked in lactic acid.

Note

White fibres, by contrast, have comparatively little myoglobin and fewer mitochondria, so they depend more heavily on anaerobic glycolysis for quick, powerful bursts of contraction — this produces lactic acid via lactate dehydrogenase, and the build-up of this acid is a major factor in the rapid fatigue seen in intense, short efforts. …

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Q.It is known that red muscle fibres in animals can work for longer periods of time continuously. How is this possible?

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