It seems natural to work out at a level that's not too easy and not too hard. This is the infamous "gray zone," and it's infamous for good reason.
Medium-effort workouts will help you keep the fitness level you already have, but they won't stimulate much adaptation. In short, you won't see significant progress.
To escape the gray zone, think of the difficulty of your workouts on a scale of one to 10, with one being easy and 10 being full throttle. Consistent workouts at level five will give you lackluster results but exercising at level eight or above every day will leave you exhausted and prone to injury. The best course of action is to alternate easy workouts (level three or below) with strenuous ones (level eight or above). Your body will be stimulated to improve, and it will have enough downtime to mend and adapt [source: Fitzgerald].
When should you call in an expert? Read on to learn about personal trainers.