The spike-triggered averaging technique, when used to obtain the twitch profile of a single motor unit, requires the maintenance by the subject of that unit's minimal rhythmic firing rate, which is typically between 8 and 12 pulses/sec. Literature on the rate-tension relationship of cat motor units shows that some twitches may start to fuse at stimulus rates as low as 2 pulses/sec. This fusion changes the mechanical characteristics of the observed apparent twitch from those of the unfused twitch. The present work has assessed the changes in contraction time, half-relaxation time, and twitch amplitude of slow versus fast twitch cat single motor units, when these units are demonstrating varying degrees of fusion of tension. As the stimulus rate applied to cat single motor units increased to those typically used during spike-triggered averaging, the values of all three twitch parameters decreased, with this decrease being more prominent for slow-contracting versus fast-contracting units.