What is the reason the amplitude of the wave does not change with increased stimulation frequency?

The amplitude of the cardiac action potential remains constant despite increased stimulation frequency primarily due to the long refractory period characteristic of cardiac muscle tissue. During this refractory period, the muscle cells do not respond to further stimulation, which prevents the summation of action potentials that is seen in other muscle types like skeletal muscle. As a result, even with increased stimulation frequency, the heart muscle cannot generate additional tension because it is still in the refractory phase of the previous contraction. This intrinsic property is vital for the heart to function properly, as it ensures that the heart has adequate time to fill with blood between contractions and to efficiently pump without risk of tetany or sustained contractions that could impair its function.

The other options do not accurately capture the physiological mechanisms at play in cardiac muscle. The cardiac muscle does respond to frequency changes, but its distinctive refractory period limits the ability to produce a stronger contraction at high frequencies.