In the muscles.

Period.

So let's do the logic. If you don't challenge your muscles, they atrophy at a rate of about a half-pound a year. Your weight may stay the same, but your body composition will change. More fat, less muscle, same weight -- and that's the best scenario, as we all know too well.

Sure, you can lose weight with aerobics alone. But all too often, you'll wind up being a "smaller" version of a flabby person. Significant muscle tone, definition, vitality and strength are rarely --- if ever -- achievable from cardio work alone.

The take-home point here is do your weights. You need 'em. They're going to give you the tools to burn calories while you're resting, watching TV, working at your desk, etc. And, let's face it -- that's where you spend most of your day. You need to train your body to be efficient at calorie burning during the other 23 hours a day when you're not in the gym. Remember that a person with muscle sitting in front of the TV is burning more calories at that moment than a flabby person.

Am I saying you shouldn't start an aerobic program until you're all set up with a weight-training program to balance it out? Of course not. Start right now, with whatever you can do. A few weeks of walking (or biking, or anything else like that) never hurt anyone and never will. I just want to plant one thing in your consciousness: You can't do it without weights. Or its equivalent, as veterans of the Shape Up Community Challenge can tell you.