$A$ collagenous connective tissue layer holds the muscle bundles together.

Which of the following statements about the molecular arrangement of actin and myosin are correct?
$I.$ Each actin (thin filament) is made up of $2$ $F$ (filamentous) actins.
$II.$ $F$-actin is the polymer of $G$ (globular) actin.
$III.$ $2$ $F$-actins are twisted into a helix.
$IV.$ Two strands of tropomyosin (protein) lie in the grooves of $F$-actin.
The correct option is:

Rahul exercises regularly by visiting a gymnasium. Of late,he is gaining weight. What could be the reason? Choose the correct answer and elaborate.
$A$. Rahul has gained weight due to accumulation of fats in the body.
$B$. Rahul has gained weight due to increased muscle mass and loss of fat.
$C$. Rahul has gained weight because his muscle shape has improved.
$D$. Rahul has gained weight because he is accumulating water in the body.

Which of the following muscles are not under the voluntary control of the nervous system?

During muscular contraction,which of the following processes occurs?

Define the sliding filament theory of muscle contraction.