Who doesn't want firm, flat abs or even a six-pack? As a personal fitness trainer, I am often asked what is the best way to tone that tummy or slim down that bulging belly. Is it crunches? Sit ups maybe? Or what about hanging leg raises?

While there is nothing especially wrong with any of those exercises, my go to ab exercise is actually one that involves no movement at all - the plank.

The plank is an isometric exercise. All that means, in personal fitness trainer-speak, is that, although your muscles are working, they do not cause any movement to occur. In fact, planks are best thought of as an anti-movement exercise.

Your abs, in fact, like most of the muscles that make up your midsection, are seldom called upon to generate movement. Most of the time they are actually required to eliminate and prevent movement. For example, if you bend down and pick up a shopping bag, your oblique muscles in your waist must work to keep your spine vertical and stop you leaning over towards the load you are lifting. If you palpate (a fancy way of saying touch) your waist muscles when you do this, you'll feel that the side opposite the load you are lifting is tight - a sure sign they are working.

Lifting, pushing, pulling and carrying anything even slightly heavy means your abs will have to work isometrically to keep your spine properly aligned. The plank exercise helps develop this ability.
To perform a plank, using technique your personal fitness trainer would be proud of, follow these simple instructions below.

1.Lie on your front on the floor.
2.Rest your elbows and forearms on the floor and clasp your hands.
3.Make sure your upper arms are vertical by placing your elbows directly under your shoulders.
4.Lift your hips up and off the floor and create a straight line from your shoulders to your hips to your feet. Keep your neck long but relaxed.
5.Hold this position for as long as feels comfortable but stop if you feel your hips lifting or dropping or have any pain in your lower back. Shaking is normal by the way!
6.Keep breathing slowly and evenly - no holding your breath.
7.On completion, bend your legs and rest on your knees. This position is also a good beginner option if you aren't quite up to doing a full plank.

There are a number of variations you can do to make the plank even more effective and challenging...

Three point plank - lift a foot or elbow off the floor so you are only using three points of balance.
Rest your elbows on a stability ball - this really makes planks harder!

Perform pushup planks - rather than rest on your elbows, rest in the top position of a pushup.

Body saw planks - rock forwards and backwards on your toes and elbows in a sawing-type motion.

Side planks - same as regular planks but performed resting on one arm and the side of your feet. This variation hits the obliques more.

Hard style planks - tense every muscle in your body along with your abs but remember the personal fitness trainer's rule - don't hold your breath!

Planks require no equipment and yet are supremely effective. They are my favorite ab exercise so give them a go - I'm sure when you see how effective they are they will be one of your favorites too!