I used a ball when I had 1:1 Pilates lessons. I wasn't allowed to lie on ball until I had done a lot of work to strengthen my core area. A good exercise for this, in addition to being good for arms, is to put the ball chest height against a wall, stretched arms length away. Do slow upstanding pressups into the ball, holding core as tight as possible whilst doing so. Do x three lots of five reps. Key is doing them slowly, you'll be surprised how quick you gain and maintain cord strength.