Also used as the acronym HIE, this is the amount of energy or work capacity an athlete has above Threshold Power (TP). Analogous in many ways to the term W’ used in physiology research, which has also been referred to as Anaerobic Work Capacity (AWC) in the past, HIE is measured in kilojoules. The most notable difference between HIE and W’ is that HIE can never be completely depleted.
During exercise, HIE depletes when work is performed above TP and replenishes during rest or when work is performed below TP. At TP, HIE remains unchanged. For example, if your HIE is 20kJ, after performing 500 seconds at 10 watts above TP, your HIE would be reduced by 500*10 joules or 5kJ, i.e. 15kJ.
HIE is used together with TP and Peak Power (which together form your Fitness Signature) in Xert to calculate MPA, second-by-second. Note that mathematically-speaking, MPA limits how quickly HIE can be depleted.