Threshold Power

Also used as an acronym TPSee Threshold Power.... in Xert and sometimes described using the more colloquial term FTPAlso referred to as Functional Threshold Power, it is regarded by most athletes as the power that can be sustained..., Threshold Power is a Fitness Signature parameter that represents the highest power that can be sustained without accumulating short-term fatigueFatigue represents a reduction in an athlete's ability to produce power.  It either manifests as a reduction in Maximal Power....  Threshold Power can generally be sustained for just under an hour by most athletes, assuming that the discomfort generated is tolerable by the athlete over the entire period.

Worthy of note is that Threshold Power is affected by work performed, with work performed at higher intensities having greater affect.  This must be true otherwise it could be sustainable at any time, even after an hour of high intensity effort.