A citizen can increase his stats by training daily at the Workout Area and School.
The strength of a citizen is a sum of his abilities: S = POWER+AGILITY+ENDURANCE+INTELIGENCE
In international battlefields the damage done by a citizen on each hit is: DAMAGE_DONE = 10 × (1 + S/500) × (1 + R/5) where S is strength and R is the level of the citizen.
Strength also counts when you fight in Special Agent Fights.