1. Core Leg Lift

Office work often resembles Sir Isaac Newton's First Law of Motion: An object at rest will remain at rest until acted upon by an external force. Though her work as a columnist requires her to spend hours at the computer, Yeager employs a number of subtle exercises to keep her muscles, and particularly her abdominals, engaged. "We need those little bursts of activity (throughout the day)," she said [source:Yeager].

One of Yeager's favorite office exercises is the core leg lift, which works the deep abdominal muscles, the quadriceps and the hip flexors. To try it:

• Sit up tall in your seat. Contract your abs and lift one foot off the floor about six inches, so the knee comes straight up with the foot directly underneath (maintaining a 90-degree bend in the knee).
• Hold for 10 seconds and slowly lower it while relaxing your abs.
• Repeat with the opposite leg.
• Alternate throughout the exercise.

2. Chair Suitcase

April Bowling, a Level 1 certified coach with USA Triathlon and owner of TriLife Coaching, knows a strong core improves performance in all three disciplines of swimming, cycling andrunning, in addition to overall fitness. The chair suitcase, she says, targets all the main abdominal muscles except the obliques. You'll need to:

• Sit on the edge of the chair seat and lean back until your upper back touches the back of the chair. Tuck your tailbone under, and hold onto the arms of the chair for support.
• Bring your knees up -- with your shins parallel to the floor -- so that your torso and thighs make an "open suitcase."
• Close the "suitcase" by bringing your chest and knees toward one another.
• Open and close for 10 to 20 repetitions, two to three sets.

The sole caveat, she says, is to maintain good posture and keep your back straight, supporting your upper body on the arm rests.

"You never want to arch your back," Bowling said. "As soon as your back begins to arch, it means you need to take a break and rest"

3. The Plank

According to Boston-based strength and conditioning coach Mike Boyle, who has worked with athletes ranging from national collegiate champions to Hollywood celebrities, the key to safe abdominal exercises is minimizing flexion.

"Sit-ups and crunches are not something you should be doing," he said [source: Boyle]. "They're probably contributing to more back issues than helping."

Instead, Boyle recommends doing planks, which require holding a static position on your elbows:

• On the floor, get into a position where you're on your toes and on your elbows, as if you're going to do a push-up from your elbows.
• Simply hold that position.
• Start with 10 seconds, and then gradually increase duration, being mindful of maintaining good form (primarily a straight back and straight legs).

To work your obliques, do a side plank. Start on your right side, on your right elbow, with your feet on the ground, holding steady, keeping the spine straight. Then switch to the left side.

Ready to ride your bike? Don't worry -- you're still not leaving your desk.

4. Seated Bicycle Pedal

The late, great Jack LaLanne, who passed away in January 2011 at the age of 96, was the original television fitness guru, and he enjoyed simple exercises that got big results. "You've got 640 muscles," LaLanne said in a 2008 interview [source:LaLanne]. "They all need a share of work."

For LaLanne, core strength set the tone for the rest of the body. Though a tireless advocate of weight training and swimming, LaLanne also was a big believer in isometrics, or using the body's own weight to provide resistance. One of his favorite exercises was the seated bicycle pedal:

• Sitting in your chair, scoot down to the edge of the seat.
• Support your upper body on the armrests.
• Pretend you're riding a bicycle, bringing each knee near the chest, keeping the abdominal muscles contracted.
• Concentrate on "pedaling" in smooth circles.

Variety, LaLanne said, was the key to making this, or any exercise, enjoyable. "Every 30 days, I do something different," he said. "So, for 30 days, I'll do everything real fast. The next 30 days, I'll do everything real slow."

According to Bowling, the most important element to doing the Seated Bicycle Pedal safely is to keep your back straight. "Tuck the tailbone to prevent arching," she said

5. Desk Russian Twist

To incorporate more of your office furniture, Bowling likes thedesk Russian twist, which also targets all the major coremuscle groups, but primarily the obliques.

You'll need to:

• Sit on your desk with your knees over the edge.
• Lean back to a 45-degree angle, or as far as you can while maintaining a neutral spine (no arch).
• Rotate your torso and touch the desk beside your right hip with both hands.
• Repeat to the left.
• Begin with 8 to 10 repetitions to each side, two to three sets.

Proper technique, like most core exercises, is essential in the Russian Twist. If you can get to that 45-degree angle without arching your back, great. If not, don't. The moment your back begins to arch, you need to come forward to a point where you can hold that angle.

Want more than these five ways to work your abs? Keep reading for bonus coverage.