Source: http://www.todayifoundout.com/index.php/...

Cardiac muscle, like skeletal muscle, is also powered by mitochondria, but it has many, many more:

The total volume of the heart . . . is [comprised of] between 30 and 35% mitochondria. That massive amount of energy-generators means cardiac muscle, in a healthy state, need never rest: there is always some energy being transferred to the muscle at the same time that more energy is being derived from caloric intake.

However, this greater reliance on mitochondria means the heart also has a:

Greater dependence on cellular respiration for ATP . . .  has little glycogen and gets little benefit from glycolysis when the supply of oxygen is limited. Thus anything that interrupts the flow of oxygenated blood to the heart leads quickly to damage – even death – of the affected part. This is what happens in heart attacks.

Broken Hearts

Although it seems indefatigable, the strength of the human heart is not without limits. Recent research has shown that after extremely strenuous use, even the healthiest hearts can suffer damage.

Heart Fatigue

In 2001, scientists studied cardiac fatigue in endurance athletes:

Cardiologist Euan Ashley . . . set up a mobile heart lab at the finishing line of the ultra-endurance race “Adrenaline Rush” in the Scottish Highlands. . . . . The winning team . . . collapsed across the finishing line after 90 continuous hours of biking, climbing, swimming, paddling and rope work with virtually no sleep . . . After testing their hearts . . . before and after the 400 km race . . . the scientists determined that the hearts of athletes who finished the competition pumped 10 percent less blood at the end of the race compared with the amount pumped at the beginning.

Ashley was quick to note that “the athletes’ hearts that showed signs of cardiac fatigue did return to normal fairly quickly after the race and no permanent damage was done.”