Depending on whether or not you flex your waist, the decline sit-up can either target your iliopsoas or your rectus abdominis.
Targeting your iliopsoas, not your abs, the captain's chair leg raise is a hip flexion exercise. Your abs and obliques act as important stabilizers.
Work your iliopsoas and other hip flexors using the lying alternating leg raise, an isolation exercise. Your abs and obliques only act as stabilizers.
Use the weighted captain's chair leg and hip raise to strengthen your core and hip flexors. If you don't have a captain's chair, just hang from a bar.
Targeting your iliopsoas, not your rectus abdominis, the lying straight leg raise is an isolation pull exercise that activates all hip flexor muscles.
Great for core development, the captain's chair leg and hip raise is a compound hip-and-waist-flexion exercise that targets your rectus abdominis.
Great for inner-thigh development, the barbell sumo squat is a compound exercise that targets your quadriceps and, to a lesser extent, your gluteus maximus.
A compound leg and core exercise, the stability ball jackknife targets your iliopsoas (hip flexors). Your abs contract dynamically only with waist flexion.
An advanced core exercise, use the standing ab wheel rollout only after you have mastered the walkout, front plank, and wheel rollout on your knees.
Great for beginners, the bodyweight sumo squat is useful for learning the very basics of squat form (keeping your torso upright, knees out, and head up).
The dumbbell sumo squat (aka dumbbell plie squat) is great for learning squat form, especially how to keep your knees out and torso upright as you squat.
The barbell rollout is an great auxiliary core exercise that targets your iliopsoas (hip flexors), not your rectus abdominis (which acts as a stabilizer).