Prosody is a crucial aspect of the speech signal and its modelling is of great importance for various speech technologies. Intonation models based on physiology rely on an accurate model of muscle activation. Although most of them are based on the spring-damper-mass (SDM) muscle model, the more complex Hill type model offers a more accurate representation of muscle dynamics. In this paper we analyse and compare these two muscle models and discuss the benefits and disadvantages they bring. This research is a part of an on-going effort to develop an improved intonation model.