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.