The surface myoelectric signal represents the superposition of action potentials from the motor units arranged within a given muscle. The electrical signal generated by each motor unit undergoes a space-time filtering that depends directly on its position relative to the electrodes on the surface of the skin. Therefore, mathematical models that represent the generation of myoelectric signals must take into account the positioning and distribution of motor units within the muscle cross-sectional area. The present study is aimed at investigating the influence of motor unit distribution on the generation of the surface myoelectric signal. A phenomenological mathematical model was used to represent motor unit action potentials recorded from a pair of electrodes in a differential bipolar configuration. Different muscle cross-sectional morphologies were tested to evaluate their influence on the magnitude of the myoelectric signal.