I was looking for a good, all-around leg exercise, and lunges are it! I'm going to start using dumbbells soon, but 21kg (roughly 45 pounds) is a bit much for me. I like the cardio of lunges and I'm up to about 25 per leg. As a variation, I hold the lunge (or down) position of the lunge for a second or 2. That really gets the burn and heart rate going for me.