Lean muscle burns 12 times more calories than fat, and for every pound of muscle added, experts say your body increases its basal metabolic rate by an additional 50 calories a day — when you're sitting still. If you can't get to the gym, make muscles while you're seated. Though you're welcome to pump water bottles at your desk, these core and lower body ideas are way less conspicuous.

Abs: Lean slightly back in your chair and lift your feet off the floor with straight legs at a 45 degree angle, then bend your knees toward your chest and shoot them back out to the starting position; do this for 3 sets and 20 reps, to burn up to 75 calories, says Kristin McGee, a celebrity fitness trainer. Another idea is to fully exhale, then squeeze in the stomach as much as possible, and hold the position for 5 to 8 seconds to feel your ab muscles "glued" to the spine; two sets of 8 reps will burn up to 100 calories, says Amir Pozderac, founder of Executive Fitness Consulting. Beyond the burn, a strong core also supports posture, promotes better breathing, and makes you look leaner.

Legs: For inner thighs, squeeze a mini exercise ball for 10 seconds in 3 sets of 20 reps, three to five times a week, to burn up to 75 calories a day, says McGee. For quads, Linda Cheek, MD, a Virginia physician, suggests "marching in place," in a seated position, by raising and lowering each bent leg. Work 5 minutes on each leg, and you'll burn 35 calories for 10 minutes total.

Rear: Sammie Kennedy, certified personal trainer and creator of the DVD "Booty Camp Fitness Ultimate Home Edition 2," suggests holding glutes tight for 5 seconds, then releasing, for 10 reps; do one set per office hour, to burn up to 35 calories. Squeezing glutes on alternating sides -- left for five seconds, right for five, for 10 to 20 reps each, will create circulation on your backside, too. Strong glutes do more than make you look good: studies often find a correlation between weak glutes and bulging or herniated disks.

Tap your feet. Wiggle in your seat. Tap that pen on your desk. According to an Iowa State University study, researchers found that those who frequently fidgeted, stood up, and walked around could burn an additional 300 calories per day, compared to those who remain stationary. In fact, Annette Cain, certified personal trainer and author of "Get In Shape the Lazy Way," says the impulse to fidget may be hardwired, so you might not even need to make a concerted effort to do this. If you do, Cain says to fidget for 3 to 5 minutes at a time, 8 to 10 times a day.

The reason this works? "Movement requires energy, thus calories get burned," she says. "Think about when you shiver from being cold — your body is trying to generate heat with involuntary muscle contractions that require energy and calories." Since engaging more muscles and using resistance will burn even more calories, try upping the ante the next time you fidget: if you shake your leg, Cain says to come to the balls of your feet to work the calves; if you tap your toes, alternate feet and keep your abs pulled in. Want a fun fidget fact? "The first two fidgets always burn the most calories," she says.

We don't care if it adjusts or has amazing lumbar support -- lose the office chair, and invest in alternatives. The first option is a stability ball, which requires you to engage your core muscles just to sit, helping burn calories and tone your abs and back, says Michelle Ross, certified fitness trainer at E.W. Training. Just don't sit on the ball to work for more than a total of 60 minutes a day, says Amir Pozderac founder of Executive Fitness Consulting LLC. "Your core muscles will get tired and relax, so your body position will alter and cause your back to bend forward." To rest your core, work while standing or use another chair in between. Health expert Susan Schenck suggests propping a rocking chair at your desk as a respite, or use one on its own. "Rocking burns 150 calories an hour," she says.

Your body burns more calories trying to stay warm, says Debbie Johnson, author of "Think Yourself Thin." In fact, one form of "brown fat" tissue — which is turned on when people get cold — sucks fat out of the rest of the body to fuel itself. Until docs have further studied this phenom for safe clinical use (nobody should lock themselves in a meat locker just to shed some lbs), Johnson likes to keep the temp a little lower in her office than what feels comfortable.

Break out your headphones or crank up the radio, and get grooving. "Listening to fun music encourages you to keep a lively beat while sitting down," says Lavina Rodriguez, PhD, clinical psychologist and author of "Mind Over Fat Matters: Conquering Psychological Barriers to Weight Management."

"Music has a universal positive psychological effect, and can serve to reduce stress, which is also known to help with weight control. It makes us feel happier, which makes us want to move more in general and reduces cravings," she says. Beyond a simple head bob and shoulder shrug, music also motivates you to move around faster and for long periods of time, adds Len Glassman, owner of Personal Best Training and Pilates Center. Depending on your current weight, you can burn up to 132 calories during your next little desk-dance party. Just switch to soothing music when you eat, he says, to help you relax and chow slower, improving metabolism and digestion.