Trying to go beyond the classical model of man-machine systems on ergonomics, the aim of this paper is to argue the interest of the Activity Instrument Situation - A.I.S approach (Rabardel, 1995) in computerized work situations. The application of this model on a study of banking task (Payment Order - PO) shows a tripolar instrument interaction conflict. Analyzing the activity of bank clerks (n=13), the model brings a framework to explain the dysfunction's indicators
bank work assisted by computer; activity with instrument; problem solving