I think squats are a great exercise if you can do them. I never considered myself able to do them very well and always though that they were dangerous for the knees.
Then in 2009, a former co-worker, Michele (he's from Italy), took up crossfit and proceeded to show me how they do squats...they are very different from what I was used to before and my knees didn't mind them at all... in fact it seemed to strengthen lots of muscles that stabilize things.
I try to do 2 X 15-20 at least twice a week...you can do them anywhere. I often do them in the office in the middle of the day.
Jill has me also doing 'goblet squats' where you hold a weight in front of you..these are easier in that you don't have to keep the forward center of gravity with muscle power (i.e. you cheat on that part) , but they are harder in that you are lifting more weight (e.g. 20lbs more). Both are good.
WARNING: these exercises can potentially permanently injure your back, knees and god-knows-what-else. Use your noggin' and don't attempt them unless you know your limits. Nelly can tell you it's a tricky business (I believe he's recovering from a knee injury brought on by doing squats).
The key points are:
1) do not lean forward (which is why I face a wall so I can't), you can hold your hands out to help a bit (but don't touch the wall)
2) go ALL the way down by arching your lower back and tucking/rolling your butt BACK and OUT (I really don't do this very well.. I tend to roll inward rather than back)
3) careful! don't over do it, especially if you haven't done them in a while
4) keep your shoes FLAT on the floor, feet at shoulder width.
5) harder: go barefoot (or zero drop shoe) and still keep feet flat on floor.
6) advanced: stay down at the bottom and try to go deeper by opening your hips a bit more.
[Update: Nelly adds these IMPORTANT pointers:
Here's a video of me showing you how I do it..not pretty but it works for me, I cracked a good sweat just doing these few:
It's a great way to strengthen your legs and keep a full range of motion to boot. I find my knees and back have no problem with them...YMMV!
If you've ever been to Asia you'll know that people over there often rest in a squat position.
It also comes in handy when the toilet facilities are
somewhat, erm, primitive. (oblig feeble attempt at SUAR type humor ;)
Believe it or not, for them it's a very low energy position...for me, not so much! 8)