A motion sensor, and an implantable cardiac stimulator incorporating such a motion sensor, has a fluid-type housing containing a fluid that includes at least one type of anisotropic molecules, the anisotropic molecules exhibiting an anisotropic property having a state that changes dependent on motion. In one embodiment the signal allocation criteria is the length of an IIR filter. The cover provides unhindered access to fasteners for connecting the motor to an adjacent part, such as a pump, so that the cover may remain mounted on the endshield while the motor is attached to the pump.