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. The housing of the motion sensor is located externally on an animate subject, or is implanted in the animate subject, and includes externally accessible electrodes that detect a change in the state of the anisotropic property and emit an output signal representative of an activity level of the subject.