Crunches are one of the most popular exercises to tighten and tone abdominal muscles. It is sometimes also referred to as a curl-up. Many people prefer to do crunches over actual sit ups since crunches put less of a strain on your back and are considered safer.

Here is how you can do a basic crunch:

1. Lie on your back on the floor with your knees bent and feet flat on the floor. If you are not on a carpeted surface, you may want to use an exercise mat.

2. Fold your arms behind your head and place them on your neck for support.

3. Lift your upper body, including your shoulders off the floor using only your abdominal muscles.

4. Lower your body back down to the floor, making sure to keep your abdominal muscles contracted and tight.

5. Return to starting position and repeat.

There are several variations of the crunch that you can do. There are ones that will work your obliques (side of your stomach) and other ones that will focus on either the upper or lower part of the stomach. Here are just some of the different variations of crunches that you can do.

Twisting crunch
This is done by lifting the shoulders to the opposite knee. It focuses on slimming the obliques.

Bicycle Crunch
This is another variation done to work your obliques. Instead of just lifting the shoulders towards the opposite knee you are alternating your knees in and out like riding a bicycle in the air.

Reverse Crunch
This exercise can be done different ways. You can extend your legs straight out and lift them straight up in the air. The other way is to bend your knees into your chest and pull them in and out towards your chest while your upper body is flat on the ground.

You can also make your crunches harder by adding weights to your exercise. Place a weight on your stomach or hold weights over your chest as you perform the exercise.

Crunches can be added into strength training workouts, interval workouts, or at the end of a cardiovascular workout. Crunches must be done properly for them to be effective. This means that you should go on quality not so quantity when it comes to doing crunches.