Get two tennis balls and a sock
Dr. Cohen's Acuball
The Acuball is very hard and uncomfortable. With it positioned in the center of my spine there was little extension of vertebrae. Two tennis balls in a sock allow the contact points to be more lateral to the spine. This is more comfortable and generates more extension. Yes, the center of the Acuball can be heated, but with the nubbies that heat never reaches your back.