There's a myth that's been held for years and is still repeated around the water cooler: High rep workouts tone muscles and limited repetitions (6 to 12) with heavy weight create bulk. Science only backs half of that notion. It's true that heavy weight and a small number of repetitions stimulates the most muscle growth. As for toning, you need to discard the view that high reps are where it's at [sources: Birmingham, Norton].
A toned muscle has a solid appearance. In order for it to look firm, it has to be seen -- it can't be hidden by a layer of fat. So, your goal is twofold: eliminate fat and develop a hard physique. The best way to eliminate fat is with a cardio program that burns a large number of calories [source: Birmingham]. There are far more effective cardio workouts (think cycling, running or cross-country skiing) than lifting a relatively light weight repeatedly. You will see some muscle growth with high rep workouts, but limited rep exercises will help you reach your goal quicker.
Afraid of bulking up too much? Many would-be bodybuilders wish it were that easy. It takes an intense focus and months, if not years, of commitment to get truly big. A small integration of low rep/high weight workouts into your routine won't turn you into a hulking mass [source: Valluzzi].
High rep workouts have many benefits, but toning isn't one of them. Discarding that myth will help you make the best use of your time.