When you twist your torso to the side, you activate the obliques. The obliques are part of the muscle group collectively called the abs, though sometimes they are grouped separately from the other two abdominal muscles. Regardless of how you categorize them, the obliques are muscles on the side of your stomach and are used when you do ab exercises that involve twisting to the side.
Alternating Oblique Sit-ups
The alternate oblique sit-ups exercise works the rectus abdominis, internal obliques and external obliques. The rectus abdominis is the muscle of the stomach that activates to bend your spine forward, which is the first part of the alternating oblique sit-ups. After the initial forward bend off the floor, twist your upper body from side to side to target the obliques. To perform alternating oblique sit-ups, lie face up on the floor. Put your feet flat on the floor with the knees bent. With your hands behind your head to support your neck, curl your head and shoulders toward the ceiling and then rotate to the left. Twist back to center and return to the floor. Repeat, but alternate sides so you twist to the right.
Weighted Seated Twist
The weighted seated twist is a simple exercise that involves twisting the spine from side to side in a seated position. Use a broom stick, body bar or a pair of dumbbells as weight to give your obliques something to move besides your upper body. To perform this exercise, sit up straight on an exercise bench with your feet flat on the floor. Hold a bar across your shoulders or place a dumbbell on top of each shoulder and hold them in place with your elbows bent. Keep your spine straight and then rotate to the left. Rotate back to the center, and then twist to the right.
Machine Lying Twist
The lying twist abdominal machine allows you to target your obliques by rotating your upper body from side to side while lying on the machine. To perform this exercise, lie face up on the machine. Rest your head and back against the pad and then grasp the handles of the machine with your palms facing inward. Bend your knees and hook your feet under the foot rests. Then, twist as far as you can to the right as is comfortable for your flexibility. Next, twist all the way to the left without stopping in the center. This completes one repetition.