This paper reports on the use of a particular actuator in the field of legged robots. The proposed actuator, the Pleated Pneumatic Artificial Muscle, has some interesting characteristics which makes it suitable for machines which move by means of legs. An important issue is the actuator's adaptable passive behaviour which allows the stiffness of a joint that is actuated by two antagonistically coupled muscles, to be varied online. The natural frequency of the system can thus be changed in order to reduce control efforts and energy consumption. The idea of changing this natural frequency in combination with trajectory control will be implemented on a two-dimensional leg model. It will be shown that an appropriate choice of compliance can strongly reduce the amount of needed control activity and energy consumption while tracking a given trajectory.