What with one thing and another, I haven't been getting to the weight room lately. And since flexibility and strength are both parts of my knee rehab, I have been doing exercises at home. I do have a couple of barbells here, a lightweight one, about eighty pounds, and one somewhat heavier, but in the pouring rain, going out back to work the iron -- and plastic, since the lightweight one is a cheapo set -- isn't much fun.
So I'm doing bodyweight stuff, and one of them is the invisible chair.
Simple to do. Find a patch of wall that isn't being used and put your back against it. Put your feet out eighteen or so inches from the wall -- more or less depending on your height -- and slide your back down so that your thighs are parallel with the floor.
Start out with thirty or so seconds, work up to two minutes, and when you can do that without falling over, you can begin adding weight -- small barbells in each hand, a pot of concrete on your lap.
Burns the quads, oh, yes, it does.