Sit on a Roman chair with your legs secured under the padded bars. Make sure to flex your feet.
Cross your arms over your chest.
Keeping your neck neutral, inhale as you lean back and lower your torso until your hips are almost fully extended.
Exhale as you flex your hips to raise your torso back to the starting position.
Repeat for the prescribed number of repetitions.
Comments and tips
The target muscle is the iliopsoas. Unless waist flexion occurs, the rectus abdominis and obliques will only act as stabilizers. However, the Roman chair sit-up still effectively overloads the rectus abdominis and oblique muscles isometrically, not dynamically.
The feet must be flexed in order to keep your legs secure under the padded bars.
Instead of crossing your arms over your chest, you can position your hands behind your head to make the roman chair sit-up more challenging. This position moves your center of gravity further away from your hips.
For the safety of your neck, keep it as neutral as possible during the exercise.