Answer with Explanation:
"Cardiac muscle cells" are able to do this because they are "involuntary" in nature. They can continue pumping blood even without resting in the person's lifetime. This allows them to become even stronger over time and improve their strength, endurance and flexibility.
The rhythm of the heart's contraction largely relies on the "pacemaker cells." These are responsible for regulating the heart rate. This ability of the cardiac muscle cells is known as "autorhythmicity."