Many of Xert workouts incorporate “Smart” intervals.  These intervals can have either varying intensity, varying duration or both.  For example, let’s look at the Sweet-Spot Under Fatigue @82% Workout:

The “Intervals under Fatigue” start with an interval that’s 369W that lasts 101s.  Why 101s? Seems like an odd number to choose, right?  The duration of this interval is calculated by the system and not specified.  The duration depends on your specific fitness signature.  Here is the corresponding definition row in the interval table:

You’ll notice that the Power is 5 min MMP.  This means that the system will calculate your 5 minute power based upon your signature and use that as the target power.  This makes the interval the same for everyone and does not need to be fine tuned using %FTP as a coach would do for you.  Secondly, you’ll notice that the Duration is 70% Reserve MPA.  What this means is that the duration of the interval is calculated to be how long it will take for your MPA to reach 70% Reserve, i.e. 70% of what you have between TP and PP or TP + 70% of (PP-TP).  Using an Target MPA and Reserve also makes this interval work the same for you as it does for someone with a very different signature than you do.  This is how the 101s is calculated.

Now to make things even more interesting and valuable to your training, the 101s can change as you do the interval.  How? Since the value 101s is based on your signature as well as your fatigue, the workout players can interactively calculate how long you still need to go at your target power in order to reach 70% Reserve MPA.  So, if you decide to soft pedal a bit during the interval, the duration will go from 101s to perhaps 110s.  You’ll notice this as the interval timer slowing down.  Likewise if you are a bit eager and decide to push a lot harder than the target power, you’ll see the interval timer speed up indicating you’re going to reach the 70% Reserve MPA sooner as a result.  Note that the interval timers will only slow down to twice the duration and speed up to half the duration.  This variable duration SMART interval helps you get to the target MPA so that the next interval, which is a classic 20 minute 82% sweet-spot interval is done under the same fatigue independent of how you did the first interval, dependent on your specific fitness levels defined by your signature.  It works the same for everyone.  These variable duration intervals also make them well suited to outdoor or adhoc indoor training that doesn’t depend on using trainer control.  The flexibility of doing these workouts in a more natural environment make them both interesting and highly effective.