Question

Assertion: The extensor muscles of the human back remain in sustained contraction to maintain erect posture against gravity.
Reason: The extensor muscles are rich in red muscle fibres with abundant mitochondria and myoglobin.

A) Both Assertion and reason are correct and reason is the correct explanation for Assertion.
B) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
C) Assertion is correct but Reason is incorrect.
D) Both Assertion and Reason are is incorrect

Answer 1

Hint: (i) In vertebrates skeletal muscles have two kinds of striated muscle fibres – tonic or red or slow and twitched or white or fast. A muscle may consist of only tonic fibres, or only twitch fibres, or a mixture of both.
(ii) Red muscle fibres are then, dark red and stone contorting muscle fibers.

Complete Answer:
(i) Extensor muscles of the back in man are examples of Red muscle fibres.
(ii) Red (Tonic or slow) muscle fibres are thin, dark, red and stone contracting muscle fibres. They contain high content of home – protein pigment called myoglobin, abundant mitochondria, low glycogen content and poorly formed sarcoplasmic reticulum.
(iii) Myoglobin imparts red corrosion and stores oxygen as oxymyoglobin. This oxygen by aerobic animation in mitochondria provides energy for muscle contraction.
(iv) Little lactic and accumulates in this respiration. This enables the red muscle fibres to carry on stone and sustained contractions for long periods without fatigue.
(v) They are not fatigued with work.
(vi) They are innervated by thin stone.
(vii)Conducting nerve fibres.
(viii)Therefore, extensor muscles of the human back are red in colour due to high amount of myoglobin and mitochondria. They can remain in sustained contraction to maintain erect posture against gravity due to high amounts of myoglobin. $O_2$ is present in large amounts in myoglobin. So in case of deficiency of $O_2$, these oxygen molecules are released into the cytoplasm of muscles and help the muscles to sustain contraction by supplying energy.

The correct option is A.

Note: Some flight muscles of birds are red muscles to permit prolonged slow flying as they are thin.