Zero/zero seats can eject sitting stationary on the ground. Zero altitute/zero velocity. Still, not all planes have them though. There is a maximum survivable ejection speed, but who really knows it. There have been plus mach ejections, but a load of broken bones came out of it. The last one I know about was an ANG guy over the water. He punched out as low altitude and plus mach. He broke both arms, both legs and generally got broke, but he survived.