The dark bands ($A$-bands) of a skeletal muscle are known as

The largest muscle in the human body is . . . . . . .

$A -$ Calcium ions $(Ca^{2+})$ activate the interaction between myosin and actin filaments.
$R -$ The cross-bridge is formed in the presence of $ATP,$ and the breaking of the cross-bridge also requires the binding of a new $ATP$ molecule.

During muscle contraction,$(P)$ get reduced whereas $(Q)$ retain the length. Here $P$ and $Q$ are $-$

The head of a myosin monomer consists of:
$I.$ Actin binding sites
$II.$ $ATP$ binding sites
$III.$ $ADP$ binding sites
$IV.$ $AMP$ binding sites
Select the correct options:

During the mechanism of muscle contraction,$Ca^{2+}$ binds with which of the following?