The spiky balls, also made of a hard rubber, are amazing! They are about 9.5cm in diameter. These things can be used anywhere on the body from the neck to the feet (see the free video that you get with every purchase). I have a yellow one and have nicknamed it Lisa, as I feel everything in life is better with a Simpson’s reference. You can use these standing, by placing it between you and a wall or by putting it on the floor and lying on it, really there are no limits to the use of this thing. I personally use mine on my back, chest and glutes, and after a hard day, a cheeky roll on the feet will stop you from completely losing your mind!