This paper focuses on how to consider and model the safety issues of "unsafe" activities (Amalberti, 2013) in a systemic and dynamic approach of system analysis. Drawing on the case of the mountain guide profession, this paper adapts Rasmussen's model of migration (Rasmussen, 1997) for craft and self-employed activities conducted in settings which are risky by nature. After characterizing these activities and the practical and theoretical issues currently associated with them, we propose a first adaptation of the model applied to the mountain guide profession. Our results, based on the qualitative analysis of 17 interviews, are then discussed along three points. The first point develops the idea of a subspace of effective possibilities for a given guide as the intersection of the space of possibilities offered by the system, a space of truly accessible possibilities and a space of acceptable possibilities. The second point aims to show that these spaces must be understood in a dynamic way and are partly constructed over time through individual acts and choices. The third point finally addresses the possible diversity of spaces possibilities that are individually perceived as acceptable and effective in terms of safety and performance. In conclusion, this text discusses the main implications of our results on the application of the migration model as well as on the practical safety issues of the mountain guide profession.