Hang from a pull-up bar with an overhand grip and your hands about shoulder-width apart. Flex your hips and bend your knees, holding them above your hips in front of your torso.
Now roll your torso upwards, bringing your knees toward your chin. Slowly reverse this motion without allowing your knees to become un-tucked from your body. Don't use momentum.
The way these exercises are usually performed – with your legs hanging down and flexing at your hips – is primarily a hip flexion exercise. Tuck your pelvis and you'll target the abs much better.
You're doing your mobility drills and foam-roller work, but you're still in pain. If that sounds like you, maybe you need "muscle activation training."
Within the realm of training for greater strength, muscle mass, and endurance lies an area of science that remains relatively untapped: Neuroscience. It's indeed the uncharted waters in the vast ocean of the science and practice of resistance training. That's because so little is known about how the nervous system actually works.