The old-school quad burner with extra load.
Hold something for balance if needed, keep hips and torso in one line, the stretch should hit the quads, not the knees painfully.
The sissy squat isolates the quads by doing what every other squat avoids, sending the knees far forward while the hips stay locked in line with the torso. Leaning back on the balls of your feet stretches the quads from hip to knee, and the plate at your chest adds load to a stretch position that bodyweight alone eventually stops challenging. It earns its place as a quad finisher that needs almost no equipment and delivers a stretch a leg extension machine cannot match.
They load the knees through a big range, which is demanding rather than dangerous. Own the bodyweight version first, build depth gradually, and add the plate only when the movement feels smooth.
Yes, grab an upright or frame with your free hand. Balance support does not reduce the quad work, it just lets you focus the effort where it belongs instead of on staying upright.
Both isolate the quads, but the sissy squat loads them in a deep stretch with the hips extended, a position the machine never reaches. The trade is that you supply the stability yourself.
