Final answer:
The continuous smooth contraction resulting from rapid, multiple stimulations is known as tetanus, specifically complete tetanus when there is no relaxation phase, and incomplete tetanus when short relaxation phases still occur.
Step-by-step explanation:
The stage in the multiple stimuli graph when rapid, multiple stimulations cause the contractions to fuse into a smooth, continuous contraction is called tetanus. During this process, if the frequency of motor neuron signaling is so high that the relaxation phase disappears completely, the muscle contractions become continuous which is referred to as complete tetanus.
Conversely, if the stimulus frequency is not as high and there is still a short relaxation phase between contractions, this is known as incomplete tetanus. The tension during complete tetanus is significantly higher than that of a single muscle twitch.