What is it? A large inflatable ball used for exercise. You may know it as a balance ball, birth ball, body ball, fitness ball, gym ball, stability ball, therapy ball or yoga ball.

How much does it cost? Shop around and you can find them for about a fiver. Alternatively, your gym will be packed to the gills with them.

What does it promise? Improved posture, better flexibility and tremendously increased core strength. Plus it might help with labour pains if you sit on one just before you give birth, although they might frown on that down at your local Fitness First.

What’s it actually like? The key thing about a Swiss ball is that it’s unstable. They’re so bouncy and huge that, when you sit on one, your core works overtime just to keep you upright. And there’s no way that anybody not raised in a circus could ever actually stand upright on one unaided. But this is absolutely by design. Putting your hands or feet on a Swiss ball during a plank exercise is brilliantly effective; as well as trying to remain rigid, you’ll be fighting the tiny movements of the ball. Same if you balance your feet on the ball and do press-ups. My favourite exercise, however, is the wall squat – you place the ball against the small of your back and lean against a wall, then squat down for as long as you can before rising. Looks easy, burns like hell.

Best and worst bit So many workouts can be enhanced with a Swiss ball – chances are there’s a wall chart of exercises in your gym – but you do tend to feel a little like a performing sea lion while you’re using one.

Is it worth it? Yes.