Have you hit a plateau with your back squat or your front squat? It is very possible that there is something about your (air) squat that needs fixing. If you need a lift that will help you troubleshoot what part of your squat needs fixing, then the overhead squat is that lift. Find out the finer points of performing a good overhead squat; how to improve your mobility for an overhead squat; and how the overhead squat makes you a better athlete.
As Greg Glassman describes,