Currently, there is a gap in the literature regarding information on the passive mechanical properties of the various fibre and muscle types, as well as how passive muscle mechanical properties change with age and differ across species. As such, passive muscle stiffness (elastic modulus) of single muscle fibres were investigated in the current thesis. The purpose of this thesis was to: investigate what effect fibre type (type 1 versus type 2) had on passive elastic moduli; to compare passive elastic moduli amongst the rat hindlimb muscles (Tibialis Anterior (TA), Soleus (SOL), and Gastrocnemius (GAST)); to compare passive elastic moduli in young and old humans. Single muscle fibres were placed in a relaxing solution and repeatedly stretched to calculate the elastic modulus. Fibre types were determined via gel electrophoresis (SDS-PAGE). Type 1 fibres were stiffer then type 2 fibres; however when accounting for fibre diameter, this difference no longer existed. There was no difference in stiffness amongst the hindlimb muscles of the rat. “Young” single fibres were stiffer then “old” single fibres. Single fibres from humans were stiffer than single fibres from the rat hindlimb. Together these data improve the current understanding of passive muscle properties of the various fibre and muscle types, as well as how passive muscle properties change with age and differ across species.