A transmission unit for a vehicle equipped with propulsion pedals, comprising: a central shaft; a transmission output shaft coaxial with the central shaft; a planetary gear unit carried by the central shaft and configured for transmitting mechanical power from an input shaft of the planetary gear unit to the transmission output shaft; an electric motor unit; a housing carrying the central shaft, the transmission output shaft and the planetary gear unit; the electric motor unit is coaxial with the central shaft and comprises an output shaft fixedly coupled to the input shaft of the planetary gear unit, and the central shaft is coupled to the output shaft of the electric motor unit and the input shaft of the planetary gear unit by a freewheel.