To do the Dead Bug:
- Lie on your back with your back flat against the surface you are lying on
- Lift both hips / knees to 90 degrees and point both arms to the ceiling
- Slowly and in a controlled manor lower one leg towards the floor whilst straightening it and let the opposite arm fall behind your head
- Repeat on the other side.
This will hit your core and particularly strengthen your hip flexors.
If you ever experience pain doing this exercise, stop, and call your osteopath.