If you're still doing crunches and sit ups to achieve that six-pack, you could be wasting your time. Fitness experts say that it's important to strengthen your core muscles with a variety of isometric exercises that encourage deep muscle contractions in your abs and obliques, and these can help to improve your posture while giving you a killer workout. If you need to ramp up your ab routine, here are some essential exercises for a svelte midsection:

Oblique Twists
Oblique twists, also known as oblique crunches, are better than the standard crunch because they target the core muscles around your waist. You will need to perform these on both the right and left sides, and hold for a count of two to five seconds depending on your strength level. These exercises are among the easiest and most effective ways to achieve that streamlined midsection.

Renegade Rows
This routine involves lifting weights up off the floor while keeping the body in a push-up position with feet spread apart. The idea is to keep the hips as stable as possible while contracting the biceps and triceps during the lift. You'll feel the contraction in your midsection and sides, and a few sets of these each week can help you get those rock-solid abs you crave.

Planks
Plank routines, also known as hover exercises help build strength and stability in your core muscle groups and will also elongate your midsection. These can be performed on a workout mat with your forearms and toes on the floor. Your torso needs to be straight and rigid, and you will need to raise yourself up on your elbows slowly for a count of 10, 20 or 30 seconds depending on your strength level.

Medicine Ball Sit Ups
If you like doing sit ups but want to get better results, use a medicine ball. You can perform these crunches on a basic workout mat or yoga mat with or without shoes, and will need to hold at least a 5-pound ball between your legs and slowly lift it to your chest. The weight from the ball will encourage a deep contraction of your oblique and stomach muscles.

If you have ever done any Pilates before you probably know that the hundred is the fundamental exercise for Pilates. If you have never done Pilates or even heard about Pilates, then the hundred may be something new to you that you can incorporate into your workout routine.

The reason why the hundred is the fundamental exercise when it comes to Pilates is because it is one of the best exercises to work your core muscles and also improve your cardiovascular endurance. One thing that you have to remember when doing the hundred exercise and any Pilates moves is that you must remember to control your breathing and make an easy transition from one stage of the exercise to other. It should be one easy fluid movement.

Here is how to the hundred exercise:
1. Lie on your back on a carpeted surface or a padded exercise mat. Bend your knees with your feet flat on the ground as you place your hands behind your knees.
2. Use your abdominals to bring your head off the ground as you continue to curl your spine so that your shoulders are off the ground as well.
3. Extend your legs out as you also extend your arms straight out. The lower your legs are the harder the exercise, so only go as low as you can without lifting your lower back off the ground.
4. Pump your arms up and down in a small and controlled way as you take small breaths in and out. Be sure that you are not straining your back or neck as your abdominals should be tight and doing all the work.
5. Inhale for 5 counts and then exhale for 5 counts and that counts as one cycle. To fully complete the hundred, do 10 cycles, as you continue to pump your arms up and down.
6. When done with 100 counts, keep your upper body curved and off the ground as you bring your knees back into your chest. Lower your shoulders and head back to the ground as you slightly stretch your back.

If you are looking for something a little more challenging, then try lowering your legs so that they are only off the ground a few inches. This makes the hundred exercise much more difficult. If you need something that is a little easier if you have neck problems or are new to exercising, you can keep your knees bent with your feet on the floor and do the entire exercise that way.

The hundred will definitely provide you with a huge improvement in your core strength and will also improve your endurance when it comes to exercising.

When most people think about working their core muscles, they only think about exercises that mostly work your abdominals like crunches and sit ups. Your core muscles also consist of your lower back muscles. When you have strong lower back muscles, you can actually appear taller and thinner. The Superman exercise is a great way to strengthen your core muscles, including your lower back.

Here is how to do the Superman exercise:

1. Lie face down on your stomach on a carpeted surface or on an exercise mat. Extend your arms straight out in front of you on the floor with your palms on the floor. Your legs should be straight behind you.
2. While keeping them straight, lift your arms and legs up off the ground. Keep your head facing downwards. You should have a slight curve in your back.
3. Hold this position for at least 5-10 seconds making sure to breathe and keep your abdominals contracted.
4. Lower your arms and legs back to the ground. Repeat the exercise for the desired amount of reps. Aim for at least 8 reps at a time.

There is also an alternating version of the Superman exercise. In this version of the exercise you only lift one arm off the ground as you lift the opposite leg at the same time. Then, you alternate between the two sides to complete a set.

The Superman exercise is one of the best ways to strengthen the lower back. Not many people realize just how important a strong lower back is. Think about how many things you lift in a day. Without strong lower back muscles, you may not be able to lift heavy items such as your kids.

Also, as mentioned, your lower back muscles support your whole body as they make up your core. When you have weak lower back muscles you may find that you have bad posture or you have trouble doing everyday tasks such as sitting for an extended period of time. If you can strengthen your lower back muscles, you help tighten up your core, which can make you not only look taller, but much thinner as well.

The Superman exercise can be added to any type of strength training workout or even a yoga routine.

Strengthening your core muscles not only improves your posture, but it also makes it easier to carry out your most challenging workout routines. Dancers, gymnasts and other athletes do a number of exercises to strengthen and tone up their core muscles - the obliques, abdominals and lower back - because it helps them maintain balance and allows them to shift and control their body weight easily.

Strong core muscles will also reduce the risk of back pain and injury when you're working out. So how do you go about strengthening and toning that midriff? You'll need to start with a set of simple floor exercises that will train your muscles to move in a certain way.

You can begin with some simple oblique stretches and ab-lengthening exercises. Lie flat on your back with your toes pointed forward, then slowly raise each leg for a count of five, then lower it. Do the same with the other leg, and make sure you inhale on the lift and exhale as you lower the leg. Do the same movement with both legs to warm up the lower abdominal muscles, then slowly move your legs to the left or to the right to warm up the obliques.

Another great midriff-strengthening exercise is the standing twist. Here, you can hold two light weights at your side and lift your arms to shoulder length. The next step is to twist to the left, then the right in a fluid motion, all while keeping your abs tucked in. You won't feel a burn until you do several twists, but it's a simple and effective exercise for training your core muscles.

Finally, enhance the effects of your average floor-based crunch movement by focusing on keeping your shoulder blades off the floor, and not doing a complete sit up. Sit ups don't work if you have weak abdominal muscles. You need to focus on contracting the abdominals and obliques, then holding them in place for several seconds in an isolation move.

Don't think you have time to strengthen your core? Just grab your yoga mat and yoga gear during a commercial break and squeeze in some posture-enhancing stretches and crunches to tighten that midriff. Just a few quick spurts of intensive activity is all it really takes to help train those core muscles and start seeing results!