Lie supine (on your back) on a declined bench and hook your feet under the foot pads.
Either extend your arms straight up into the air, cross them over your chest, or place them behind your head.
Press your lower back against the bench.
Exhale as you slowly flex your waist and hips to raise your torso until it is upright.
Inhale as you slowly extend your hips and waist to lower your torso back to the starting position.
Comments and tips
Keep your lower back pressed against the bench.
Keep your neck neutral. There must be space between your chin and sternum.
Fully relax your waist before each repetition. This will ensure a full and dynamic contraction of your waist.
It is possible to raise your torso by only flexing your hips and not your waist (i.e. keeping your back straight). In this case, your rectus abdominis and obliques will act as stabilizers and your iliopsoas will become the target. What’s more, the exercise will become an isolation movement instead of a compound one.
If the decline sit-up and similar exercises that involve both hip and waist flexion hurt your lower back, it’s because your rectus abdominis is failing to counter the pull on your lower back by your iliopsoas, leading to excessive anterior pelvic tilt. Strengthen your rectus abdominis using the crunch and crunch variations (e.g. reverse crunch, vertical leg crunch, and bicycle crunch) for several weeks before trying the decline sit-up again.
Make the decline sit-up more difficult by holding a weight plate on your chest or behind your head.