Like pool-running, Alter-Gs reduce the weight bearing impact on the body while still allowing the athletes to do workouts that mimic the real thing. Alter-Gs are treadmills that have a closed container around the bottom. Runners get into a harness-like pair of shorts and zip into that container. Then the air pressure within that container is adjusted to lift the runner a bit as they run, altering" gravity.

I believe the athletes have access to a few underwater treadmills as well, which works on the same principle.