The Stick is offered in various sizes because muscles come in different sizes. Basically the models are listed by length in Long, Medium, Short and also Specialty sizes. Each of the Long, Medium and Short length categories feature a stiff, standard or flexible model. The more flexible the model the softer and more gentle it feels on your muscles. The stiff models are used on more dense body mass and when deep, firm release is desired or recommended. As an example, a weightlifter would choose the Big Stick while a distance runner would prefer the Marathon model. In the middle of these two extremes is the #1 best-seller, the Original Body Stick, and each of the others have their own purpose. All of these models are Phthalate-Free.