What is an ab exercise that can be done any time?

Ab Squeeze: Very simple but effective! You can do this at your desk, watching T.V., or driving. To perform this exercise, sit straight and keep your feet flat on the floor. Make sure back remains straight and slowly inhale and contract your abdominal muscles. Hold on the position for about 10 to 20 seconds, and relax. Repeat 10 times. 
Dr. Mike Clark, DPT

A great ab exercise you can do any time with nothing but your body weight is the All Fours Opposite Arm/Leg Raise. This move is slightly more intense and considered a progression from the drawing-in maneuver mentioned in some of the other responses, not only do you have to maintain the drawing-in maneuver (lower ab area pulled in toward your back) but because you are facing the ground with your body it challenges the deep ab muscles against gravity and requires smooth and coordinated movement between the arms and legs, making it great for your shoulders and butt muscles as well!

One of the best exercises you can do for your abs is called the drawing in technique.  This is simply where you pull your belly button to your spine, and squeeze your abs tight and hold it.  One of my favorite places to do this is while I am sitting at a stop light.  Try to hold your contraction through the whole light.  But don't stop there - you might as well squeeze that pillow you are sitting on as well called your glutes.  Before you do any active movement you should think about pulling that belly to your spine and squeezing that booty.  This is a not only a great way to exercise those areas but will also help ensure you don't get injured while being active.  So don't just sit at that stoplight ever again, make it an active light instead.

A great ab exercise you can do anywhere is the Prone-Iso-Abs exercise, also known as the Plank. To perform this exercise:

  1. Lie on your stomach with your feet together and forearms placed on the ground. 
  2. Clench your fists under your shoulders, draw-in your navel and contract your glutes. 
  3. Lift your body off of the floor and raise up until your body forms a straight line from head to toe. Hold the top position for the desired amount of time.

One thing I say to my clients is to "zip-it". This means tighten the abdominal area between your hip bones (lower abs) and then pull in the area just under your ribs causing your lower two ribs to pull in. Keep your shoulders back, your gluts tight and the top of your head pointing directly to the ceiling.

Maintain this posture throughout the day and you will have strong abdominals. 

Dr. Michael Roizen, MD
Internal Medicine
Here's an abdominal exercise you can (and should) do anywhere: Suck your belly button in tight and squeeze your butt in like you're trying to pull up a pair of too-tight jeans. Pretend the top of your head is being pulled by a string to the ceiling. Now hold that position. Besides putting you into proper posture, it's working your transverse abdominis muscle (your supportive girdle muscles). Do it on the elevator, waiting in line, at work, and anywhere you walk.
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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.