Heel slides are early exercises to help trigger the quads into action, and also to maintain early range of motion.
If the muscles are too weak to do a heel slide unassisted, you can use your hands to just start the heel sliding and control the movement.
In case you cannot see the video, here is a still sequence -
|Sit on the bed with your legs out straight and your back supported. Wearing socks and placing an inverted tea-tray under the lower leg helps to make this exercise easier.|
|Slide the heel up the tea-tray, using your hands to help bend the knee.|
|Hold the bent position for a few seconds, and then slowly slide the heel down again. Repeat 5 times and then repeat the exercise with the other leg.|