When the PowerLab 26T and 15T were initially released, the built-in isolated stimulator was capable of a maximum repeat rate of 20Hz. Based on customer feedback, the design was later changed to allow a much faster repeat rate of 1000Hz. The change came into effect in March 2009. This means PowerLabs with serial numbers below 1356 (26T) and 0581 (15T) shipped with the old style stimulator. It is possible for ADInstruments to modify older units to include the more capable stimulator. Please contact your local Technical Support Representative for more information.