Cardiac muscleMyocardium

Cardiac muscle, or myocardium, is the specialised muscle of the heart. Like skeletal muscle it is striated, built from the same sliding filament machinery, but it is involuntary and it never gets a day off: roughly a hundred thousand contractions a day, every day, for a lifetime.

Two features make that possible. Its cells are joined by intercalated discs that let electrical signals pass directly between them, so the tissue contracts as a coordinated unit rather than as individual fibres. And it is extraordinarily rich in mitochondria, which can occupy around a third of the cell volume, because it must produce energy aerobically and continuously.

It generates its own rhythm. Pacemaker cells depolarise spontaneously, so a heart will beat without any input from the brain. The nervous system modulates the rate rather than setting it, which is why a transplanted heart still beats.

It adapts to training, and the adaptation is worth distinguishing from the pathological kind. Endurance work produces a larger chamber that fills more completely, raising stroke volume; that is athlete's heart, and it is benign and reversible. The thickening seen in untreated high blood pressure is a different process with a worse outlook.

It has almost no regenerative capacity, which is the hard fact underneath heart attacks. Muscle that dies from lack of blood supply is replaced by scar tissue that does not contract, so the damage is permanent and the priority is preventing it rather than repairing it afterwards.

That prevention is the ordinary list: blood pressure, lipids, glucose, not smoking, and regular aerobic exercise, which improves the tissue and the vessels supplying it at the same time.

In practice, our heart rate zone calculator sets the intensities that train it, and the heart age calculator estimates the risk to it from your own numbers.