Cardiac muscleMyocardium

Cardiac muscle is striated like skeletal muscle but involuntary like smooth muscle. Its cells are joined by intercalated discs that let electrical signals pass directly between them, so the whole chamber contracts as a coordinated unit, and it is exceptionally rich in mitochondria to fund non-stop work.

It matters because it adapts to training much as other muscle does. Endurance work enlarges the ventricular chamber, raising stroke volume; heavy resistance work thickens the wall somewhat. Both are physiological adaptations, distinct from the pathological thickening seen in untreated high blood pressure.

In practice, the low resting heart rate of a trained endurance athlete is this adaptation showing itself — a larger, better-filling heart moving the same blood in fewer beats.