$A$ motor unit is a

Electron microscopic studies of the sarcomeres have revealed that during muscle contraction:

$A$ ...... muscle rotates the forearm to turn the palm downwards.

$A -$ 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.

Skeletal muscles show resemblance with visceral muscles in one aspect. It is in

In the myofibrils of muscle fibre: