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.