This is a new performance metric introduced in Xert that provides a way to measure an athlete’s ability to endure discomfort.  It is determined via a Hardness Test.  There are 16 levels.  The basis for the tests is to use a similar workout pattern but increment total XSS for the workout, whereby Level 1 generates 50 XSS in one hour and Level 16 generates 200 XSS in one hour, with each level incrementing the total XSS by 10 XSS points.  Hardness tests are based entirely on XSSR intervals.

Hardness Tests add more strain without bringing the athlete to a point-of-failure (i.e. without having power reaching MPA).  Each workout gets harder and harder, in effect, without forcing the athlete to have to stop.  Stopping is voluntary.