Rod Jenkins uses the Formaster in his classes to demonstrate the back muscles. The Astra trainer seems to be a newer version of the Formaster that 3 Rivers is selling. The muscles you should be using to draw your bow are not that obvious to most of us, and we have to learn to be aware of using them. Rod told us in one of his classes that he planned to use the Formaster himself to get ready for a tournament he planned to shoot in, so evidently back muscles are not like riding a bicycle, where you only have to learn one time, but instead are something you need continual practice to use correctly.