A high resolution method for determining the complex stiffness of single muscle fibres is described. In this method the length of the fibre is oscillated sinusoidally, and the resulting force amplitude and phase shift are observed and interpreted in terms of chemo-mechanical energy transduction. In activated, fast skeletal muscles of rabbit (psoas), frog (semitendinosus) and crayfish (walking leg flexor), we resolved at least three exponential rate processes. We named these (A), (B), (C) in order of slow to fast. These processes should reflect ATP hydrolysis and concomitant energy transduction since they are absent in muscles that the relaxed, in rigor or fixed. The great similarities in the complex stiffness data from different muscles suggests that there is a common mechanism of chemo-mechanical energy transduction across a broad phylogenetic range.