Adrenaline

Certain Warrior skills are fueled by adrenaline rather than Energy. It is gradually built up each time the Warrior hits or is struck in combat, each such blow being added to the characters adrenaline level. Some skills increase the rate of adrenaline build up, while others expend it entirely.

Adrenaline is not actually gained in strikes, but in points. You gain 25 adrenaline points for succesfully attacking and by taking damage you gain 1 point of adrenaline for every 1% health lost from your maximum. Commonly 4 strike skills are charged in about 3 hits time, because you take some damage at the same time. See this table for approximate number of adrenaline points needed for skills.

1Sever Artery and Counter Blow are 4 strike skills, but they actually need about 100 adrenaline points.

To gain more adrenaline when taking damage there is Balthazar's Spirit.

Reference
Research done by Ensign and Silmor. Original GW Guru thread.