Roger Federer Trainer Explain Clay Season

Roger Federer's fitness trainer fuels speculation about his clay season :
" Mystery continues to abound about Roger Federer playing the clay season and the words of his fitness trainer Pierre Paganini add to the speculation. Recently, interviewed by the New York Times, Paganini explained the difference between playing on hard and clay courts.

'The advantage when you play on clay for the joints is that there is less shock because there is the slide, and the disadvantage of [playing on] hard courts is that shock,' he said. 'But the advantage on hard courts is that the shock is brief.

In contrast, the disadvantage with the slide on clay is that there is a lot of vibration in the joints. We don’t see it from the outside, but to control this slide there is instability in the knee, the foot, the ankle. "