997cc047_ng_Zones___Guide___Wattbike_UK__Physiological_Adaptations
[Training Zone] Zone 4 [Purpose] Push threshold up [Race fitness] Improved sustainable race pace, useful during tapering or pre-competition periods: too much time in this zone can cause staleness [Physiological Adaptations] 
Improves carbohydrate metabolism, develops lactate threshold, changes some fast twitch muscle to slow-twitch
Heart Rate and Power Training Zones - Guide - Wattbike UK
Physiological Adaptations