There's been a lot of talk about "the core" these days, thanks in part to the increasing popularity of yoga. But what is the core, why do we need to strengthen it and what are the best ways to work it? We asked fitness expert Kathleen Trotter to explain.

Q: What is the core?

A: There is a common misconception that the superficial "six pack" abdominals are what constitutes your "core." In truth, the core spans the entire torso region and ends above the knee. The muscles that make up the core include the superficial abdominals (rectus abdominals) as well as the deep abdominals (transverse abdominals), external and internal obliques, back muscles (for example the multifitus muscles) and the muscles of the hips (for example your glutes and inner thighs). A well functioning core is able to use all these muscles to effectively and efficiently carry out movement.

Q: Why do we need to strengthen it?

A: Strengthening the core is important for two main reasons. First, by strengthening the core one can help train one's body to move functionally as a system. Training one's body "as a system" means training the muscles of the body to work together to perform "functional movements." When the body performs these functional movements the core muscles link the muscles of the lower limbs with the muscles of the upper body. Examples of functional movements are everyday activities like picking up a heavy box, vacuuming, walking up stairs, running for the bus and/or sitting down or standing up from a chair. Second, since the deep abdominal muscles are part of the core, training the core can teach your body how to activate your deep abdominals to help stabilize the spine. Traditional abdominal training, like crunches, do not teach the deep abdominals to fire.

Q: What are the best ways to work the core?

A: The best way to work the core is through functional movements like squats and wood chops, lunges and rotations, planks, side planks and the bird dog (see below for descriptions). The worst exercise that people can do for core training is traditional crunches. Workouts that exclusively use crunches to train the mid section are not functional. Crunches can be bad for one's posture and can be hard on the spine.

How are traditional crunches bad for posture? As a society we spend eight or more hours a day sitting in a slumped over position. We spend hours with our shoulders hunched over and our upper back rounded (at a desk, in the car or on the coach). Going to the gym and doing a million crunches just promotes the same bad habits that sitting at a desk promotes.

How can crunches be bad for your back? Crunches do not work the deep stabilizing abdominal muscles that help prevent back pain. Bird dogs, planks and other pilates-based core exercises help train all layers of the abdominals. Therefore these exercises will help train the deep core muscles to work. The deep core muscles, when working properly, help stabilize one's spine.

Q: Can you suggest a good exercise for the core?

1. Front Plank
Place your forearms and toes on the mat. Your body should make a straight line and your elbows are directly under your shoulders. Try to engage your lower abdominals through the entire exercise (imagine putting on a tight pair of jeans). Hold for 10-30 seconds.

2. Side Plank
Start on your side, on a mat. Place your elbow directly under your shoulder. Stack your legs on top of one another and lift your hips off the ground. Do not let your shoulder go past the elbow. Think about using your bottom waist to hold your hips up.

3. The Bird Dog
Get onto all fours on a mat. Your hands should be placed directly below your shoulders and your knees should be hip distance apart. Without letting your hips wobble side to side, lift and straighten your right arm and left leg. Keep your pelvis level and engage your core. Continue for 10 reps and then switch and repeat on the opposite side.

4. Squat and Wood Chop
Make your feet parallel and hip distance apart. Hold a dumbbell with both hands. Bend at your ankles, knees and hips to sit your hips backwards like you were going to sit in a chair. As you make this squatting motion bring the dumbbells towards your left foot. As you stand up out of your squat (think about your glutes and your low abs) "chop" the weight on a diagonal towards your right shoulder. Try to feel your core link the power you get from your legs up into your arms. Repeat 10 times and then switch sides.