001621360 520__ $$9Grobid$$athe other adresses are given in marcillac et al {2004} We present a determination of the recent past star formation histories of a sample of Luminous Infrared Galaxies (LIRGs) selected on their mid infrared emission thanks to Spitzer and ISO. We used a Monte Carlo simulation of $2 10^5$ synthetic spectra to derive what happened in the latest Gyear. We derived a burst duration of about 0.1 Gyr which produce about 10 % of the stellar mass. Details results are given in marcillac et al (in prep) .