1. A device to change the motion dynamics between a first member of the human skeletal system and a second member of the human skeletal system, comprising: a flowable substance, wherein this substance is a liquid, gas, or gel; one or more reservoirs, wherein: the one or more reservoirs contain the flowable substance; the one or more reservoirs are implanted into the human body; and movement of moving portions of the one or more reservoirs causes the flowable substance to flow; and an energy-transducing mechanism, wherein: this mechanism is implanted into the human body; and this mechanism transduces energy from a flow of the flowable substance into changes in the motion dynamics between the first member of the human skeletal system and the second member of the human skeletal system.