A drug can sometimes increase the strength of a contraction without changing the amplitude of the wave. In your own words, describe what kind of change in muscle contraction this might represent.

Refractory period of the cardiac action potential

Explanation:

Unlike the skeletal muscle which can be reactivated before it reaches a contraction, cardiac muscle must wait before it is almost completely relaxed to be activated again (because of its long refractory period), so one cannot make an increase in strength of contraction with a high frequency of stimulations.