Plank

What it is

Forearms on the floor, elbows under your shoulders, feet together or slightly apart, body in a straight line from head to heels. Hold.

That is the whole thing, and the difficulty is entirely in the details: ribs down rather than flared, backside not raised, lower back not sagging, neck in line with the spine rather than craned up.

A hands-on-the-floor version is a straight-arm plank, which is the top of a push-up.

What it demands

Endurance of the trunk in one position, not strength through a range. This is an anti-extension movement: gravity is trying to extend your lower back, and your job is to stop it. Rectus abdominis, the obliques and transversus abdominis do that together.

Shoulder stability, with serratus anterior holding the shoulder blades flat against the ribs.

Nothing that requires equipment or space beyond your own length.

What usually goes wrong

The hips sag. The lower back extends, and the movement stops training the thing it exists to train. This is the point at which the set is over, regardless of the clock.

The hips rise. A raised backside makes it much easier and is the most common way plank times get long.

Holding the breath. A plank is a position, not a breath-hold, and being unable to breathe comfortably in it means it is too hard right now.

Chasing duration. Very long planks train tolerance of a position rather than anything transferable. Once you can hold a genuinely good one for a reasonable time, making it harder beats making it longer — lifting one foot, or moving to a side plank.

Where it sits

The entry point to trunk work, and the most reliable one because it has no coordination requirement.

It is worth knowing what it is for: the trunk's job in most real movement is resisting unwanted motion while the limbs work. That is exactly what a push-up asks of it, which is why a push-up is often described as a moving plank. The side plank trains the same idea sideways, and the dead bug trains it while the limbs move.