Jack LaLanne, the godfather of fitness, offers exercises and breathing techniques for those with limited mobility and strength. LaLanne himself has undergone rehabilitation and understands how to provide easy-to-do exercises that build strength slowly. All of the exercises shown in the video can be done while lying down in bed.
https://www.youtube.com/watch?v=JTF9QCb33hw&list=PL0F70A56338995744