Hold a medicine ball with your arms are extended upward, perpendicular to the floor.
Your low back is pinned to the floor.
Take a deep breath out and imagine your abdominals curling your torso slightly off the ground.
You don't want a rigid, strict torso position. Your back should not be arched during the execution of the exercise.
From an anatomy and biomechanics perspective, the rectus abdominus flexes the trunk.
You cannot flex a muscle if it is tense.
Negate the urge to generate momentum upward using the weight. Your arms are merely a lever for your core.
At the top, your upper back is off the floor. Your neck is in a neutral position.
Take a breath out and uncurl from the top back to the starting position.
*Tips*
Keep your legs passive throughout the movement. Using your lower half with activate your hip flexors which will negate your abdominal musculature.
At first, your legs will want to elevate or "spray" out. Through practice and greater core stability, you will be able to execute the crunch without much hip flexor involvement.
