The zone of optimal functioning (often abbreviated IZOF, for Individual Zone of Optimal Functioning) is the range of arousal and emotional intensity within which a specific athlete performs at their best. Developed by sport psychologist Yuri Hanin, it refines the inverted-U idea by emphasizing that the optimal level is highly individual.
Two athletes in the same sport may have very different optimal zones — one performs best calm and relaxed, another best highly fired up. The same athlete may even have different optimal zones for different tasks. The model also recognizes that emotions beyond simple arousal (including some that seem "negative," like a degree of anger or anxiety) can be part of an individual's optimal state.
The practical value is in self-knowledge: identifying your own optimal zone tells you whether, before a given performance, you need to bring yourself down or rev yourself up. Reaching and maintaining that zone is the aim of arousal regulation, and operating within it is closely associated with experiencing flow.