TherabandDynair Senso 33cm Ball Cushion, Wobble Disc for Balance Training, Dynamic Sitting, & Physiotherapy, Great for Active Sitting & Correct Posture, Green

I love this. I started using an exercise ball at work to sit on a few years ago and it has really helped my back and neck (and I'm sure it has actually toned my abs). But at home there isn't enough space for one under my desk (re Covid and homeworking). My back slowly got bad again and I realised it was the chair. So I bought one of these, and it is pretty much the same as sitting on an exercise ball, but smaller. You do have to make sure that your chair goes down low enough to accommodate it though - its quite thick - but fortunately mine does.Just a note on using exercise balls for office work though - they do feel a bit uncomfortable at first - but that's just while your muscles get used to it. I met a lot of resistance from heath professionals, and health and safety at work, who tried to insist that I need a chair with lumbar support etc etc... but these really do make my back hurt. My current chair does have a back, but with this I don't ever sit back against it. A stool and one of these is perfect.