Oblique crunches are a common variation of abdominal crunches in which the elbows cross to the opposite knee rather than the knee on the same side. This change in direction toward the top of the crunch causes the oblique muscles to contract.
The oblique muscles run on either side of the torso parallel to the spine, framing the rectus abdominus. Exercising these muscles often involves flexion and extension from side to side, and can be achieved individually or in conjunction with other abdominal exercises. There are many variations of the oblique crunch, but each is based on the motion of contracting the side of the torso.
In one variation, the crunch motion is performed while the body lies on its side, supported by the elbow, allowing for full isolation of one group of obliques on one side. This can then be performed equally on the other side. Oblique crunches can also be performed at the top of the standard crunch: before returning back to the ground, the elbows touch one side of each knee repeatedly. This allows for the flexion of the abdominal muscles, rapid flexion and extension of the oblique muscles, then extension of the abdominal muscles back to the starting point, all in one repetition.