Iliopsoas weakness may result in a “sway back” posture or “flat back” posture.

Hanging Leg Lifts (Knee to Elbows)
Hang from a pull up bar. Lift both knees in towards the chest. It’s important to reach the knees to the chest area, or you can aim for your elbows like the knee to elbow movement.

Forearm stretches can feel great as well, and Pavel’s book Super Joints is a very good option. It could be as simple as going on your hands and knees and walking your hands to the outside and back until your fingers are pointing at your knees. From here, shift your weight to the rear while keeping the heels of your hands on the floor.