I am just starting the exercises in the book and wondering what size ball is required. Looking at the pictures the ball Eric uses seems to be level in height to his knee. The ball I have is quite a bit bigger. Should that matter?
Regarding the amount of air, does more air make it more or less difficult. The ball I am using needs inflating (I can't find the pump adaptor- DOH!) and I really struggle to even start some of the movements. Obviously some (maybe all) of that will be lack of core strength, but wondering if some of it is down to the deflated ball.
In many of the exercises, less air will make it more difficult and challenging. With more air, you add more stability which will make things a bit easier - across the board.