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 often have variable power if MPA decreases or recovers during the interval.  As MPA declines with fatigue, the work rate associated with a given XSSR also declines, resulting in a downward curving interval.  In contrast, as MPA rises during recovery, XSSR based intervals also see a rising target power, resulting in an upward curving interval.  The Xert EBC apps for Android & iOS and the Xert Workout Player for Garmin Connect IQ calculate power targets that are equivalent to XSSR values defined in Xert’s SMART workouts 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.

Bangarang is sweetspot workout that uses 4×8 XSSR intervals. This implements the principle of recovery under fatigue using variable power intervals to give a high dose of XSS without generating a lot of suffering.