Here's a great leg workout for the day, and one that you can do pretty much anywhere. Add weights if you want, or just do it with your body weight. Spread your knees shoulder width apart. Bend your knees and squat half-way down - like you're sitting on a chair. Squat all the way down, and quickly come back up to your original half-squat position. Repeat the squat as many times as you can. Here's a video to show you how to do it correctly: It's a nice rapid-fire exercise, so you can do up to 50 or so in a minute. It won't put as much strain on your knees as a full squat, as you aren't forcing your knees to "crack the 90" (go past the 90 degree angle). If you've got knee problems but can't skip leg day, this is an exercise that deserves its place on your workout list. Do anywhere between 50 and 100 of these, and your thighs will be on fire!