Sit into the shin box and open one hip at a time.
Keep your spine long and chest up. Rounding over loses the hip stretch.
The 90/90 shin box position parks one hip in external rotation and the other in internal rotation, two ranges most lifters never visit, and leaning toward the front leg drives a stretch into the back hip. The outer hip muscles get worked on both sides just holding the position tall. It earns its keep as a fix for hips that only ever squat and hinge in straight lines.
Sitting in 90/90 puts the front hip into external rotation and the back hip into internal rotation, and the lean sends the stretch into the back hip. Internal rotation is the range most people have quietly lost, which is why the rear hip usually complains first.
Sit on a cushion or low block, or place a hand on the floor behind you for support. Height under your hips instantly makes the position accessible, and you can lower it as your rotation improves.
Around 30 to 60 seconds per side, breathing steadily. Sit tall, lean until you feel the back hip stretch, and swap the leg positions to hit both hips in both roles.
