This isometric move (pictured left and above) is great for everyone (especially those with knee problems) because it is highly effective!
Wall-sits involve leaning against a wall like you are sitting in a chair (hence the name) with your legs at roughly a 90-degree angle. Then all you have to do is hold this for a predetermined amount of time (try starting with 30 seconds). Trust me, it is a killer.
If you want to increase the difficulty you can always increase the duration of the exercise, but you can also hold a dumbbell in your lap to increase the resistance. One of my favorite ways to make this move harder is to perform bicep curls throughout the duration of the exercise, doing as many reps as possible with proper form. Doing this will torch the arms (taking your mind off your legs) and increase the difficulty as well!
Josh Anderson is an AFAA Certified Personal Fitness Trainer and the CEO of Always Active Athletics.