Studies were originally started by the European Space Agency. Much is actually understood about the physical effects this type of exercise and equipment can induce. What is very important however to understand is that 95% of machines on the market are not designed according to the performance specs of the machines used in studies. Therefore how therapeutic these other machine brands out there are is known to be less than 50% effective closer to 10-20%.
There are still several brands to choose from however where one can replicate results achieved in the studies.
Keep in mind that once you understand how these machines effect the body it is easy to understand why they are the most efficient form of exercise.