the present invention relates to a transmission unit for a vehicle driven by muscle force , comprising an input shaft which can be connected to cranks on opposite sides for driving the vehicle , comprising a first partial transmission . the first partial transmission comprises a countershaft , wherein a plurality of driving gear wheels is mounted on the input shaft , and wherein a corresponding plurality of driven gear wheels of the first partial transmission is mounted on the partial transmission shaft . the driven gear wheels of the first partial transmission are designed as idler gears that can be connected to the countershaft by means of shifting means in a rotationally rigid manner . the countershaft forms an input shaft of a second partial transmission . a plurality of second drive wheels are mounted on the output shaft , wherein the second partial transmission comprises an output shaft , on which a corresponding plurality of second driven gear wheels are mounted .