It's one of the fitness industry's best-kept secrets. Anytime, anywhere, just suck in that gut to strengthen your transverse abdominis and help tighten your abs and waistline!

Since I'm from Southern California, I know what it's like to spend two hours a day on the road. I'm convinced all the light signals are coordinated to ensure I miss every single one. And that amounts to a lot of wasted time idling.

But this doesn't have to be purely lost time from a strength-and-fitness perspective. There's actually one abdominal exercise I can do just about anywhere: at home while watching TV, standing in the supermarket check-out line, or even sitting in my car. It's called a vacuum, or stomach vacuum, and it was a favorite exercise of classic bodybuilders, including Arnold, who raved about it in "The Encyclopedia of Modern Bodybuilding."

Wondering how to do it? Just suck in your stomach muscles and hold.

"Holding that 'sucked-in' position for time strengthens the transverse abdominis (TVA), so you're not seeing a contraction like you would during an abdominal crunch," says Thomas DeLauer, trainer and founder of OptimizeCEO.com. "That makes this particular movement easy to do just about anywhere."

Training the TVA offers a number of benefits, including compressing the abdomen and increasing spinal and core stability. Aesthetically, many people also find that it makes their stomach protrude less over time. In the gym, it might actually make you stronger at big lifts.

"Big multijoint exercises that require strong core stabilization, like squats and deadlifts, also strengthen the TVA," DeLauer says. "But if you're always wearing a weight belt in the gym, you may be unknowingly weakening the muscle. The TVA is oftentimes referred to as 'nature's weight belt' because it stabilizes the spine and pelvis. Make it stronger, and you will be stronger, period."

So why not just do this movement for 3 sets of 10 in the gym? It's not quite that kind of movement, DeLauer says.

"The endurance-oriented fibers of the TVA aren't trained as you do most other skeletal muscle groups. They're best done for time. Strengthening the muscle means holding your gut in for a few seconds, and then building up to longer periods." This is why practicing it regularly, as Arnold also recommended, can be a game-changer.

How To Do A Stomach Vacuum

Practice the move standing, kneeling, on all fours, or seated—it's great in all positions.

To do it, blow out all the air in your lungs and diaphragm, then pull your belly button as far back as possible, emphasizing the compression of your abdomen. Do between 5 and 15 repetitions with a 3-second hold for 2-3 sets. Over time, build up to holds of 30 seconds or more. You may be tempted to hold your breath, but as the reps get longer, you'll find this likely isn't an option.

If you're seated in a car, do a set of a few reps at a long stoplight. Like texting, it's hard to drive and complete this move at the same time, so don't try. Either standing or seated, you'll strengthen your core, improve your posture, and may even help lower-back pain.