In order to wall run, Alex and Heller shift their biomass to their limbs to grant them greatly augmented strength, speed, and agility, an effect giving them the ability to run along nearly any surface. Their speed and agility increased greatly while their movement abilities evolved. They were also capable of using their biomass-enhanced strength to perform a much slower, more controlled wall climb, which is useful for precision, accuracy and stealth when scaling buildings.
Ironically, despite being more stealth-like than wall-running, wall-climbing will still cause Mercer to be detected by the military, regardless if he is wall-running or not. However, Mercer, for without any known reason, will not be detected by the military if he is wall-running.