Can you exercise if you are at risk of a retinal detachment?

Depends on Risk. If you are at risk because you are nearsighted, then yes, you can exercise. If you are at risk because you have had multiple retinal tears, then i would be more cautious because movement of the vitreous gel could exert tractional force on the retina. Get examined by a retina specialist and ask for clearance to work out before proceeding. Hope this helps. Good luck.
No. Depends on how high risk. If you recently had a tear lasered and want to exercise right away, it's not advised until the laser scars. If you are a myope and have lattice degeneration, you can still exercise, but have to watch for any symptoms of a retinal problem, new onset floaters, flashes, shade over the vision.
Usually not. Certainly very vigorous exercise or hit to the eye may induce a retinal detachment at a place of weakness but generally, normal exercise should pose no problems. Your eye experiences very violent movements when you read!