It's "Quad" day (actually, 'Lbi, Ab, Quad, Cardio' according to my pre-planned calendar schedule), and I always want to avoid stress to my knee joints but really be able to focus maximum flexing of my thigh muscles.   This is not always easy to do, so the stairway is my best option.  There, I can control to angles and use as much or little of my body weight to gain and maintain the most useful stress.   Here's what I do.

Facing the stairway as if getting ready to climb it, I keep one foot flat on the floor and place the other foot one or two steps up, flat on that tread.  I'll grab the railing for balance.

Slowly, I flex the thigh muscles of my higher leg as hard as I can and continue to increase the intensity, then hold it in that position while pretending to push myself up the stair.

The key is to change the angle of your flexed thigh while maintaining, and even increasing the intensity of the flex without hurting your knee joint.   If/when you feel any stress at all at the joint, BACK OFF!   Connective tissue located between the femur, tibia, patella assembly is not subject to the applied stress and recovery equation.   It can only hurt you, not lead to positive physical change.   Go back to the point where only the thigh muscles are stressed.   Focus on nothing else.

Now, experiment with the different stair levels and be sure to do the same for both legs until the thighs are exhausted.