Difficulty provides a measure of how much strain has accumulated over the course of an activity or workout.  Rather than using simple averages or normalized averages, Difficulty establishes the impact of repetition and constant, higher intensity efforts are to perform for the athlete based on their unique fitness signature.  It is determined as an exponentially weighted moving average of XSS Rate.  For example, single, exhaustive efforts are easier to perform then repeated, exhaustive efforts.  Difficulty captures and quantifies the effects of repeated and/or sustained hard efforts and is shown in activity charts and workouts. The highest value reached in an activity/workout determines the Difficulty Rating.  Reaching higher levels of difficulty corresponds with increasing athletic hardness and is a marker of elite athletic ability.

Consider this metric to improve your pacing and strategy during group rides, races and time-trials by looking to prevent increasing the difficulty unnecessarily. One can compare the trade-offs in improved results with the challenge of meeting the higher levels of difficulty that will need to be endured.