An example system for exciting a selected set of motor neurons of a patient comprises one or more signal generators configured for generating a first electrical signal and a second electrical signal; a first set of wires configured to provide the first electrical signal to a first set of electrodes, wherein the first set of electrodes are configured for being secured to a first set of positions on the patient; a second set of wires configured to provide the second electrical signal to a second set of electrodes, wherein the second set of electrodes are configured for being secured to a second set of positions on the patient; and a controller configured to control operation of the one or more signal generators. The first electrical signal is a periodic signal of a first frequency. The second electrical signal is a periodic signal of a second frequency. The first frequency and the second frequency differ by a non-zero frequency difference. The first signal is provided to the first electrodes and the second signal is provided to the second electrodes for exciting the set of motor neurons of the patient.