In which condition does relaxation occur in muscles?

Contraction of the muscles takes place by the sliding of

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.

The belly is the ...... of a muscle.

Name the ion responsible for unmasking of active sites for myosin for cross-bridge activity during muscle contraction.

Muscle fatigue occurs due to the accumulation of . . . . . . .