I was given this one for my sciatica to strengthen my back.
Lie flat on your back. Make sure you keep your back in contact with the floor. Bring your legs up, tucking your knees up to your chest.
You can use your hands to gently support your legs if you need to.
Slowly and gently, with your knees together, move your legs to one side, so the sides of your legs move towards the floor.
Still keep your back flat against the floor.
Slowly bring your legs back to the centre. Repeat but to the opposite side.
Start with 5 reps each side. Work upwards as you can.