Hi Teratxu,
This is called "whole body vibration" these days. I think Juan in Chile is the expert. There is a similar story about a coachload of Parkies who were driven down a cobbled road, and all felt their symptoms improved by the shaking.
There is a company called Power Plate, who sell vibration machines, and I have been trying to persuade my local branch of the UK Parkinson's Society to buy one. They are expensive, and a machine for home use costs about $4,500. However, I found a German vibration machine costing only $250, and bought one. Unfortunately, the machine stopped working after a week, and the replacement machine lasted 3 weeks. However, the company gave me my money back in full.
These vibration machines work by stretching and contracting your muscles. It is like working out in the gym, but the machine does the work. About 10 minutes on the machine is equal to an hours workout in the gym. They also help you to slim.
Best wishes
Ron