Smooth muscle

Smooth muscle is the involuntary muscle lining blood vessels, the gut, airways, bladder and uterus. It is called smooth because it lacks the striped appearance of skeletal and cardiac muscle, its filaments being arranged less regularly.

It works slowly and tirelessly. It contracts and relaxes over seconds to minutes rather than milliseconds, uses far less energy to hold a contraction, and can maintain tone for hours without fatiguing, which is exactly what a blood vessel or a sphincter needs.

You cannot train it deliberately and you do not control it consciously. It responds to the autonomic nervous system, to circulating hormones, to local chemical signals and to stretch itself, which is how a blood vessel adjusts to pressure without instruction.

It matters for fitness indirectly but substantially. Smooth muscle tone in arteries is a major determinant of blood pressure, and the vasodilation that redirects blood to working muscle during exercise is a smooth muscle event. Regular aerobic exercise improves the function of the endothelium that signals to it.

It also appears in several common conditions. Asthma involves airway smooth muscle constriction; many blood pressure medicines act on vascular smooth muscle; and gut motility problems are smooth muscle problems.

The one training relevant caution involves it too: holding your breath while straining produces a sharp rise in blood pressure, which matters most for people whose vascular system is already under strain.

In practice, this is background biology, and the actionable version is that aerobic exercise improves vascular function, which is a smooth muscle outcome.