The abdominal muscles are extremely important in bodybuilding and general fitness. They are notoriously difficult to train because they are such a large complicated group of muscles. The ab crunch machine primarily works the upper abdominals as the following steps will show.
Place the pin on the weight setting that will allow you to do 15 repetitions with good form. This will require some trial and error before you find the correct setting.
Position yourself in the ab crunch machine. Many different types of ab crunch machines are available, but there is basically only 1 way you can sit in them. Place your feet behind the ankle rollers and hold the handles.
Inhale deeply and curl your upper body forward. Pull with your abdominal muscles without using your legs or arms and allow the machine to guide your movements towards your knees. It will become more difficult to prevent your arms and legs from becoming involved as you become fatigued.
Hold your position once you reach the maximum extension the machine will allow for 1 second as you contract your abdominals. Allow the weight to pull you back slowly as far as you can go without resting the weights. This completes 1 rep.
Perform a total of 15 reps. You will not be able to complete the 16th rep without cheating if you are using enough weight. This completes 1 set on an abdominal crunch machine. You should rest only 10 to 15 seconds between sets because the abdominal muscles have an extremely good blood supply.