The segment parameters (SP) consisting of inertia and position of the center of mass of each segment, of the human body are crucial data when one wants investigate motion dynamics. The segment parameters vary with time according to immobilization, physical training, rehabilitation, muscular diseases. This knowledge provides valuable information to support medical diagnosis and to quantify the effect of medical treatment, rehabilitation or training. However they are usually difficult to measure in-vivo for these kinds of applications and thus are not specifically used. In this paper we propose to apply a previously developed identification method in order to monitor the evolutions of those parameters over 5 months, during which the candidate followed a 16-week marathon training before running the 2009 Tokyo Marathon. The motion data is recorded on a weekly basis and the parameters are computed after each session. The obtained results are presented and the changes in body SP are discussed in the light of typical results occurring to the body fitness.