An arrow rest for supporting the shaft of an arrow includes a first support frame and a plurality of support members connected to the support frame for defining at least a portion of an opening with a central axis for receiving the shaft of an arrow. The system uses a conductive permanent magnet to generate a field H0. The skin engaging surface is generally non-flat with generally coplanar portions, and a recess being defined in the skin engaging surface which defines a void in or adjacent to the coplanar portions into which portions of a patient’s skin can be deformed into when the skin engaging surface is pressed against the patient’s skin. A signal outputted from the electric generator 4 and indicative of the number of revolutions of the wheel 13 is transmitted wireless by a transmitter 5.