Definition
Producing force without the muscle changing length: holding a position still, or pushing against something that does not move.
Two kinds. Yielding isometrics hold a load still: a plank, a paused squat at the bottom. Overcoming isometrics push against something immovable: a bar in the pins.
They relieve tendon pain in rehab, strengthen a specific sticking point (hold two seconds at the weak angle), build the trunk, and teach beginners what a position feels like. A common dose is 3 to 5 holds of 10 to 30 seconds at 70 to 80% effort.
Strength gains are angle specific, about 15 degrees either side of the hold, so pick the angle that matters.