During muscle contraction in humans,the:

What happens when a sub-threshold stimulus is applied to a muscle?

Which of the following statements are false regarding the muscle structure?
$I.$ In the centre of each $I$-band is an elastic fibre ($Z$-line) which bisects it.
$II.$ Thin filaments are firmly attached to the $Z$-line.
$III.$ $M$-line is a fibrous membrane in the middle of $A$-band.
$IV.$ $A$ sarcomere comprises one full $A$-band and two half $I$-bands.

Give scientific reasons: Striated muscles get fatigued rapidly.

Select the correct option for the following statements:
$(i)$ The most important respiratory organ in frogs is the skin / lungs.
(ii) Skeletal muscle / Cardiac muscle exhibits rhythmic contraction.

Oxygen can be stored in