Cognitive impairments due to Alzheimer's disease lead to a diminution of performances in activities of daily living. The disease evolution is simulated thanks to the cognitive architecture ACT-R. In ACT-R, activities can be represented according to the retrieval of semantic units in declarative memory and the trigger of rules in procedural memory. The simulation of Alzheimer's disease decrease is simulated thanks to the variation of subsymbolic parameters. The model is applied to a cooking activity. Simulation of 100 hundred subjects shows results similar to those realised in a standardized assessment. Keywords: Alzheimer's Disease, Functional Performance Representation, Cognitive Modelling, Cognitive Architecture, ACT-R, Activity of Daily Living.