A drive unit comprising of a driving motor and of at least two flexible linking members, which twist on each other when a rotational shaft of the driving motor rotates and thus produce a pulling force on a motion element that is attached to or is a part of link of a legged robot's leg mechanism is disclosed. A control method to control the invented drive unit so that a passive, a passive-dynamic or an active walking modes and transition between the modes of a legged robot is achieved without any additional mechanical means is disclosed.