If a second stimulus arrives before the relaxation phase has ended

a second, more powerful contraction occurs

Explanation:

A second and more powerful contraction occurs if the second stimulus arrives early before the relaxation phase has been ended. This is called wave summation where one twitch is added to another twitch in this way.

A twitch is a single stimulus contraction and relaxation sequence in the muscle fibers.

The excitation and contraction coupling effects of the successive motor neuron signaling when added together is called a wave summation. The bottom at each wave represents the stimulus at the end of relaxation phase.

Thus the answer is - -

a second, more powerful contraction occurs.