Lack of relaxation between successive stimuli in sustained muscle contraction is known as

The largest smooth muscle is present in:

$A$: Application of stimuli repeatedly just at the start of relaxation of a muscle fibre shows an increase in the extent of contraction initially and results in treppe or staircase phenomenon.
$R$: It can be attributed to the summation effect of the sub-threshold stimuli being given again and again.

What is the source of energy for muscle contraction?

Muscle is attached to bone by

In the mechanism of muscle contraction or shortening of muscle,the . . . . . . get reduced whereas the . . . . . . retain the length.