@isabelle7 - you are absolutely correct -no sit ups for sure!!! In fact he should train himself to avoid any trunk flexion postures. Therefore , for example, squatting rather than bending foward , when doing things , keeping his back in extension, straight alignment . One should be hinging at the hip to do tasks like that. And this idea needs to be used throughout all functional activities. The reason for this is that the pressure one puts on the anterior aspect of the vertebrae when you use flexion is what makes one vulnerable to compressing it , breaking it down causing a compression fracture. Good luck to you !!! Hope that helps your understanding.