The present invention relates to an implantable device for improving the pump function of the heart of a human patient by applying an external force on the heart muscle. The implantable device comprises at least one implantable pump device comprising: A fluid, A first reservoir having a first volume and at least one movable wall portion, for varying said first volume, and A second reservoir being in fluid connection with said first reservoir. Wherein said implantable pump device is adapted to allow free flow of fluid between said first reservoir and said second reservoir, and wherein said first reservoir, said second reservoir and said fluid connection forms a fully implantable closed pump device, and wherein said fully implantable closed pump device is adapted to transfer force from said first reservoir to said second reservoir.