Reverse Lunges, due to the angle the muscles have to work in, increase the pressure on the quads by a factor of at least two. They're a great warm-up exercise but they are also good for strengthening the legs for explosive sports activity. 

What's involved: Quads, calves, glutes, lower back (minimally).  

Powered by: Quads, calves. 

Tip: You can pay attention to form and try to get your knee to touch the floor each time you step back (for a deep reverse lunge) or you can go for numbers and try and do as many as you can in a set amount of time.