Abstract: The invention relates to an aid, put into practice by computers and videos, for improving the actions of athletes, comprising the stages: obtaining (S1) from a system of filming an athlete's body, images of relative movements of parts of the body compared with a chosen reference frame which can advantageously be another part of the body; filing (T4) the movements in a database in at least two categories comprising, on the one hand, the movements which allowed an effective action to be performed (S5) and on the other hand, the movements which did not allow an effective action to be performed (S6); processing (T7, T9, S8) the contents of the database so as to define a limited set of parameters (a, ?, ?) essential for the performance of an effective action. which makes it possible to define (S10) training instructions for the athlete from just this restricted set of parameters.