Abstract: A group of muscles, particularly the glutei, is stimulated electrically by the application of at least one pair of electrodes (18, 19; 20, 21) to the group and the supply of a symmetrically alternating voltage through the electrodes for equal periods of time (A) separated by intervals (B) of predetermined equal duration. In each period of time (A), the alternating voltage has a periodic behaviour with pulses of equal duration (PD) and alternating polarity symmetrical in adjacent half periods and separated by intervals (I) of equal duration. Each period (A) in which the alternating voltage is applied has a duration of between 0.5 and 1.5 s, and the intervals between the periods (A) are between 0.8 and 2 s long. The frequency of the alternating voltage is between 100 and 200 Hz, preferably 150 Hz. Each half period (T/2) in which the voltage is applied includes a pulse having a duration (PD) of between 40 and 60% of the half period.