Respiration and the heartWatch
These contractions need a supply of energy.
During exercise, a higher rate of blood flow is needed to supply muscles with glucose and oxygen for respiration and to remove waste products such as carbon dioxide.
In order to increase the rate of blood flow, heart rate must increase.
Therefore, during exercise heart rate increases, so it contracts more.
So the heart needs more energy to power the increase in contractions.
tldr: during exercise, energy supply to heart muscle must go up