Leg Extension.

Knee extension with no way out.

You may think the quads are covered by squats, until you sit down here and take that one joint on its own. The back pad is set so the knee's pivot meets the machine's axis, and then the load goes where it should instead of settling on the joint. 100 kg in the stack means you can get strong here, not just warm, and 250 kg of dead weight holds the frame firm while you do it. The machine takes 1206 × 1121 mm. A hundred kilos for one muscle.