Ergonomics characterizes ease in the use of tools thanks to their adaptation to users and their working conditions. Thus, it plays a very important role on the performance of users. Unfortunately, In West Africa, particularly in Senegal, the design of tools is artisanal and, therefore, does not take into account the future users. It is, therefore, important to make the design of these tools more formal by taking into account their performance in the early design stage. In this paper, a systemic modelling approach is proposed to take into account ergonomics at the early design stage of agricultural handle tools. Then, a performance indicator is proposed to evaluate the additional work induced by handle tools during agricultural work. To improve comfort during the use of agricultural handle tools and increase performance of users, the value of the additional work should be as low as possible.