The obliques are actually two muscles: internal obliques and external obliques. They are located on the sides of the abdominals (six-pack muscles) running from the hips to the rib cage. The internal obliques are located directly under the external obliques, and the muscle fibers travel perpendicular to each other.
Your obliques fire when you rotate, bend and flex your trunk.
The obliques have three primary actions:
Lateral flexion - bending the torso sideways
Rotation - twisting the torso
Flexion - rounding the spine, like during a Sit-Up
Also, the obliques contract to help create intra-abdominal pressure—what happens when you take a deep breath in and tighten your core before a heavy lift. They help brace your spine and allow you to handle heavy loads with a lower risk of injury.
Bird Dog Crunches
Start on all fours on a mat, place your hands flat on the ground directly beneath your shoulders, and your knees beneath your hips with a flat back position
Engage your core and stretch your right arm straight out from your shoulder, drawing your abs in towards your belly button while your left leg extends straight back from your hip, keeping both parallel to the floor throughout the “reach” portion of this movement. Make sure you are looking down at the mat to maintain a neutral spine.
Squeeze your right arm and left knee in towards your center. and hold for split second before starting the second rep. Squeeze your abs at the bottom of the movement.
A good goal is 10 reps without setting your right arm or left leg on the ground, then switch to the left arm/right leg combo. Try to complete 3-4 sets of 10 reps on each side with a 30 second rest between each set.
Beginners: Reach with only your arm at first. Then as you become more comfortable, reach with just your leg before you graduate to the full movement.