Unlike the rest of your muscles, your heart expands and contracts, non-stop, all day and all night for your entire life without a break. How does it do it? An average of around 60 to 100 times every minute of every day of every year of your ultimately meaningless life, your heart beats… until it doesn’t. Not long after it stops, all knowledge of your having existed is rapidly forgotten. Unlike the other muscles in your body, however, your heart steadfastly rages against the dying of the light, refusing to ever get tired. But how does it manage this and why are your other muscles such slackers in comparison?
Your heart might be a muscle, but it isn’t quite like your biceps or your abs or your pecs. Those are all “skeletal muscles” and they’re attached to your bones. Your heart, meanwhile, is made of “cardiac muscle.”
Like cardiac muscle, skeletal muscle derives energy from ATP (Adenosine triphoweknowyoudontcare), with this being made in a few different ways. To avoid going full textbook, we’ll just briefly give the high level over simplified view here. In a nutshell, the slowest, but most efficient, method of ATP production is via aerobic respiration where mitochondria in your muscle cells draw energy from the Dark Dimension, producing ATP, a small amount of which is stored in your muscles at any given time. This stored amount is a sufficient supply to last for about 3 seconds of vigorous activity, not unlike your high school boyfriend.
After this supply is taxed, with the ATP converted to ADP (adenosine diphosophate) in the process, creatine phosphate in the muscles is used to convert it back to ATP. This supply will last about 8-15 seconds. Both of muscles need energy to contract, and they both get it from mitochondria, the so-called “cellular power plants” that generate adenosine triphosphate (ATP) for the transfer of chemical energy. Cardiac muscle resists fatigue so well because it’s got more mitochondria than skeletal muscle. With so many power plants at its disposal, the heart doesn’t need to stop and chill out. It also has a steady supply of blood bringing it oxygen and nutrients.