A system capable of analyzing image sequences of human motion is described. The system is structured as a ·feedback loop between high and low levels: predictions are made at the semantic level, and verifications are sought at the image level. The domain of human motion lends itself to a model-driven analysis, and the system includes a detailed model of the human body. All information extracted from the image is interpreted through a constraint network based on the structure of the human model. A constraint propagation operator is defined and its theoretical,properties developed. An implementation of this operator is described, and results of the analysis system for a short image sequence are presented.