A 2-dimensional conducting-paper analogue and a 3-dimensional field-theory model are used to investigate the influence of electrode spacing and size on the waveshape and frequency spectrum of the signal detected by bipolar surface electrodes, when an action potential propagates in a muscle fibre below the skin surfce. The spatial selectivity of bipolar electrodes, and its possible enhancement by linear filtering techniques, are then discussed. Finally, predicted e.m.g power spectra are compared with some practical measurements on biceps brachii muscle in normal subjects.