Supine isometric knee extension in 20 degrees flexion ("Quad sets" Knee terminal extension strengthening, towel under knee, supine)

Lie on your back with your legs straight.
Place a rolled towel underneath your knee on the leg to be exercised.
Your heel should rest on the ground with your toes pointing directly up towards the ceiling.
Tighten your thigh muscle and push the back of your knee into the towel.
Your leg should remain still as you tense your thigh.
Hold this position.
Relax, and repeat.