Abstract: A method of treating a vessel in a human or animal body, including the steps of: positioning an implantable device against a portion of tubular or sac wall of the vessel, whereby a load applied to the vessel is borne by the vessel wall and also by the device to transfer energy to an energy storage means, the vessel being assisted when the energy storage means returns the stored energy to the device. Further disclosed is a treatment or assistance device for operating in or with a tubular or sac wall of a vessel in a human or animal body, the device including a changeable volume portion which is adapted to interact with the vessel to modify the vessel's volume; and an energy storage means functioning with the changeable volume portion whereby a decrease in the volume of said changeable volume portion creates an energy charge in the energy storage means, the energy charge being able to be subsequently released to cause the changeable volume portion to increase in volume.