Static quads are very early knee rehabilitation exercises to activate the quadriceps muscle of the lap.
With the back supported and the legs straight, push the kneecap down onto the surface - this tightens the quads muscle of the lap. Hold for 10 seconds. Then relax and repeat.
It may be necessary to put a rolled-up towel under the knee to help patients with weak extension to trigger their quads to tighten.
Quadriceps Strength, Muscle Activation Failure, and Patient-Reported Function at the Time of Return to Activity in Patients Following Anterior Cruciate Ligament Reconstruction: A Cross-sectional Study. Lepley LK and Palmieri-Smith RM. J Orthop Sports Phys Ther. 2015 Dec; 45(12): 1017–1025.