In rear impact tests using OEM seats, neck extension of the Hybrid III, defined as rearward head-toT1 relative rotation (HTTRR), has been shown to correlate to backsets that were related to whiplash injury. However, it is unknown whether rearward HTTRR in an ATD designed specifically for rear-impacts, e.g., the BioRID-II, also correlates to increasing backsets. As a first step towards developing appropriate neck extensionbased injury criteria for the BioRID-II, increased backsets are explored. A series of ATD sled tests was first conducted using the BioRID-II and Hybrid III with increased backsets in moderate-speed rear impacts. Next, another sled test series was conducted using mid-size male PMHS and both ATDs using a head restraint that was modified to provide an excessive backset intended to guarantee extension of the neck. In the first series, the rearward HTTRR from the Hybrid III increased in the tests with increased backsets. However, when the Hybrid III had rearward HTTRR, the BioRID-II exhibited forward HTTRR, and never exhibited rearward HTTRR throughout the entire event regardless of backset. In the second series, the Hybrid III exhibited rearward HTTRR much earlier in the event and ultimately with much higher magnitude than the BioRID-II or any of the PMHS.