What is it?
A large, plastic macaroon you balance on while doing regular workouts such as press-ups, or core-stability Pilates-style exercises. You might, for instance, stand on it, balanced on one leg, and do 10 knee-bends of 30-45 degrees, then repeat three times for each leg. It's slightly awkward at first, but you get used to it.
Balancing on an unstable surface means that the ligaments and tendons around your joints have to work constantly to stay stable. As well as getting the usual benefits from a knee-bend, you strengthen the tissues around your knee, and tone the muscles in your abdomen and back. Balance trainers also improve proprioception - your sixth sense.
Proprioception is the unconscious sense by which you vary muscle contraction to maintain balance and stability when external factors are changing. It is particularly important to work on it after surgery or a serious injury, when your muscles must re-learn how to adjust to each other.
Anything I should worry about?
You can fall off, so have someone or something to hold on to at first. You can also over-extend your ligaments. Get tips from a fitness trainer when you first use one.
Where can I get one?
Most decent gyms have them. If you want one for your very own, they are not cheap, at around £120.
"They are excellent for rehabilitation," says exercise physiologist Nick Drake. "But I use one when fit. Balancing on one for press-ups strengthens shoulder muscles and tendons, so you get two workouts for the price of one."