Also referred to as LTP, this represents a modeling artifact associated with the depletion of Endurance Energy. As endurance energy declines over the course of a long activity, slow-recovery / long-term fatigue takes hold and Threshold Power, High Intensity Energy and Peak Power decline.  The affect of long-term fatigue manifests in reduced MPA and MPA that declines more rapidly.  LTP represents the lowest possible value for Threshold Power when all Endurance Energy reserves have been depleted: