Conducting polymers (CP) are a promising area in the field of micro actuators, and have potential applications in micro robotics. Their properties are modelled as having an electro-chemical active component and a passive viscoelastic component. Methods exist to model the passive component as a configuration of springs and dashpots and the electro-active effects as a strain generator. Typically, the strain is assumed to be proportional to the charge transferred, and the two components are assumed to be independent. We show that there is a significant interaction between the two components for polypyrrole actuators, by observing the dynamic elastic modulus whilst varying the electric potential. The elastic modulus was measured in-situ by applying a high frequency rectangular isotonic stress input and recording the corresponding strain output. Two separate potential control inputs, vs. a reference electrode, were used. In the first experiment, a triangular voltage signal with variable frequency was applied to the PPy helix tube actuator and in the second experiment; a step voltage signal was applied to the actuator. The value of total real modulus was calculated during both experiments to evaluate the effect of actuation on the mechanical properties of PPy actuator. The performance of the mentioned method was confirmed by comparing its results to that of a sinusoidal stress input during a temperature ramp through the glass transition of polyethylene terephthalate (PET). We show that polypyrrole actuators show a complex change in stiffness with contractile state, which mimic skeletal muscle . © 2006 Materials Research Society.