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Abstract — Body powered voluntary (VC) closing hand prostheses allow users to proportionally control grasping devices in a very intuitive sense using the concept of extended physiological proprioception (EPP). The operating force required for voluntary closing devices are however so high that muscle fatigue becomes a problem. The goal of this paper is to introduce a system using pneumatic power that can increase the ratio of pinch force to operating force for prosthetic hands/hooks. This system is intended to be used in a hybrid body- and pneumatically- powered manner; the prehension phase is performed solely by the user while in the pinch phase the pinch force gets provided by both the user and a pneumatic piston. The piston linearly magnifies the force input of the user by using a Force Demand Valve (FDV). Using this system, a pinch force to operating force ratio of 3 is achieved (current prosthetic hands/hooks fall in the range of 0.11-0.45). Force control tests were carried out with 9 test subjects for both the system with pneumatic force magnification and without. These tests showed there is no large degradation in force control performance when the force magnification is engaged. Index Terms — upper limb prostheses, pneumatic, extended physiological proprioception, force demand valve I