An air vest for supplying successive percussive forces to a patient during a therapy session. The air vest includes an air bladder and a belt for securing the bladder in place upon the patient. One or more stiffening elements are provided to promote uniform force distribution across the vest. The one or more extension panels are defined along upper or lower portions of the vest. Extension panels provide a spacing structure for separating a lower cover portion from an upper cover portion to improve the force transmission to the patient for a given air volume and pressure. A plurality of adjustable straps are provided for assisting in proper fitting of the vest to a patient.