Synthetic spectra for a M1 and a M7 star (top and bottom spectrum respectively) as provided by Allard et al. (2012). For each spectral type, the dashed line shows the spectrum computed with log g provided by the 1 Myr old model of DM98 (3.63 and 3.33 respectively), while the solid line shows the same spectra with log g artificially increased by 0.16 dex (for the M7 star the two spectra overlap). The blue, green and red lines stand for the transmission curves of the 2MASS JHK_S filters respectively. We find that the M7 spectrum weakly depends on log g. On the contrary, for the M1 star the continuum spectrum in the HK_S bands decreases with increasing log g. No changes are seen in the J band.