$ATP_{ase}$ activity is found in:

Select the correct statement with reference to muscle structure.
$I.$ Each myosin is a polymerized protein.
$II.$ Many meromyosin constitute one thick filament (myosin).
$III.$ Each meromyosin's tail is called heavy meromyosin $(HMM)$ and head is called light meromyosin $(LMM)$.
$IV.$ The globular head is an active $ATPase$ enzyme and has binding sites for $ATP$ and active sites for actin.
Choose the option with correct statements.

Striated and voluntary muscle fibres are found in

The immediate energy source for muscle contraction is:

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

In a skeletal muscle fiber,what is the $H$-zone?