Lie on the back, with hands behind the head, thighs vertical, and knees bent:

• Inhale and raise the shoulders off the ground, bringing the knees and head toward each other by crunching, which means rounding the back and rolling the spine up.
• Exhale at the end of the movement.

This exercise mainly uses the rectus abdominis.
To work the obliques more intensely, bring the right elbow to the left knee, then the left elbow to the right knee alternately with each crunch.

Comment: Perform a crunch by rounding the back and rolling the spine up, bringing the pubis and sternum toward each through voluntary contraction.
Warning: Unlike other weightlifting movements, exercises for the abdominal core and especially those for the rectus abdominis absolutely must be worked with a rounded back (rolling up the spine).

When performing exercises that roll the spine up off the floor, as in crunches, you hold the spine differently than when performing squats, deadlifts, or other standing movements.