Also referred to as XSSR, it represents the work rate (power or intensity) that is equivalent to generating a given amount of XSS in one hour.  It is used in workout designs to provide a way to target an intensity that will produce a given amount of strain.  For example, maintaining an XSSR of 200 for 15 minutes will generate 50 XSS points (200 / ( 60 / 15)).

XSSR based intervals can have variable power.  As MPA declines with fatigue, the work rate associated with a given XSSR also declines.  Similarly, during recovery, as MPA rises, XSSR based intervals also see a rising target power.  Xert Mobile and the Xert Workout Player for Connect IQ calculate power levels that are equivalent to XSSR values defined in the workout interactively using a complex algorithm.  These intervals can also be used in combination with Target MPA intervals which make these dynamic in both intensity and duration.