An operative arm (1) of an earth moving machine, in particular of an excavator, has a bucket (4), which is rotatable about an axis (8) under the action of a jack (10). Between the rod (12) of the jack (10) and the bucket (4), a transmission (11) is interposed provided with a first and with a second lever (18, 19), which are mutually hinged about an intermediate axis (23) parallel to the axis (8) of rotation of the bucket (4). The first and second lever (18, 19) have respective mutually opposite ends (24, 25), hinged to the rod (12) of the jack (10) and, respectively, to the bucket (4). The transmission (11) is further provided with a lever with adjustable length (30) enabling to vary the torque transmitted by the jack (10) to the bucket (4) and hinged about the aforesaid intermediate axis (23) and the axis (8) of rotation of the bucket (4).