Astronauts exercise on the ISS treadmill to keep their muscles and bones healthy; on the ground, "flight analog" testers use similar equipment so medical scientists can measure cardiovascular fitness. How do they simulate weightlessness? By going vertical! Check out the camera angle, LOL!
This is probably the best video I have ever seen in terms of a side-by-side comparison of how the treadmill in space works, and how they gear equipment on the ground to simulate weightlessness.
If you would like to see a more in-depth view of the study, the NASA Edge guys interviewed the "guru for exercise counter-measures" at NASA Glenn, home of one of the many eZLS contraptions, or the Enhanced Zero-Gravity Locomotion Simulator... the one in Texas is identical, but with "floating" capability.
Co-host Blair Allen is a huge goofball, but highly entertaining... and he volunteered to don the space station harness and try out the eZLS treadmill... so if you are interested in participating in this study, you can watch him undergo all the protocols.
Their show also featured more updated footage on the ISS, Peggy Whitson during training, and a description of what "weightless jogging" feels like around the 5:30 and 6:40 marks. Great stuff!!