Planks are for the fit and unfit.

The plank exercise has become the gold standard for core workouts (abdominal muscle and muscles that surround the spine and the back). ACE (American Council on Exercise) conducted a study in which, of the 10 best abdominal exercises, the plank ranked number 10. You don't need any equipment, so it fits right into your budget.

Most of my clients and boot-camp enthusiasts incorporate planks in their workout. I realized that everyone can do a plank, from seniors to women who are expecting. You just have to find the best plank position for your fitness level. Planks strengthen your abdominal wall; thus, providing support for your back. Anyone experiencing back pain due to weak abdominal muscle should definitely consider planks as an exercise for your rehab program.

If you never performed a plank, start with holding the static plank position for 5 to 10 seconds and gradually build up to 45 seconds to a full minute. After you have conquered basic planks, move up to the next level or to a different plank for advanced exercisers. If your lower back aches while doing the plank, you are pushing yourself to hard; drop down to a lower level.

I tried putting the pictures on this thread but they say the pictures are too large. Here's the article.

http://thefrugalexerciser..com/2012/09/planks-for-everyone.html

Planks are evil

UmericanGirl: Planks are evil

I have my parents doing the beginner planks and they really feel them.