One of the most persistent and baseless myths in the world of muscle physiology is the "high reps/low weight builds defined and 'toned' muscles." Everyone from the gurus in Muscle and Fiction Magazine to the Jane Fonda aerobics types, to the Pilates peeps continue to spew out this myth. All the Pilates in the world won't build a long, sinewy muscle. Muscles have an origin and an insertion. They're attached to tendons that are attached to bones. You can't change that. Muscles aren't really that smart. They know how to do one thing, and one thing only – grow. If you work them hard enough, they get bigger. So the woman in the corner of the gym doing outer thigh exercises all day is either completely wasting her time, or making the muscles in her outer thighs a little bigger. In most cases, not a whole heck of a lot bigger, but bigger nonetheless. You don't suppose that the guy doing biceps curls is hoping his guns shrink, do you? Remember – five muscles enough stimulus and they grow. Otherwise they don't. End of story.
The main difference
between defined and not is body composition. Take a fat dude and have him do crunches all day and he'll have a fat dude with strong abs obscured by a big layer of fat. If you want a six-pack you need to have both muscle, and a lack of fat obscuring it. Think of it this way – if you could burn fat around a muscle or make it more defined by exercising it, you could chew gum all day and have a chiseled jaw.