  Context. M-type subdwarfs are metal-poor low-mass stars and probe for the old
populations in our Galaxy. Accurate knowledge of their atmospheric parameters
and especially the composition is essential for understanding the chemical
history of our Galaxy. Aims. The purpose of this work is to perform a detailed
study of M-subdwarf spectra covering the full wavelength range from the optical
to the near-infrared. It allows to do a more detailed analysis of the
atmospheric composition in order to determine the stellar parameters, and to
constrain the atmospheric models. The study will allow us to further understand
physical and chemical processes such as increasing condensation of gas into
dust, to point out the missing continuum opacities and see how the main band
features are reproduced by the models. The spectral resolution and the large
wavelength coverage used is a unique combination to constrain the process that
occur in cool atmosphere. Methods. We obtained medium-resolution (R =
5000-7000) spectra over the wavelength range 0.3-2.5 {\mu}m of ten M-type
subdwarfs with X-SHOOTER at VLT. These data constitute an unique atlas of
M-subdwarfs from optical to near-infrared. We performed spectral synthesis
analysis using a full grid of synthetic spectra computed from BT-Settl models
and obtained consistent stellar parameters such as effective temperature,
surface gravity and metallicity. Results. We show that state-of the-art
atmospheric models correctly represent the overall shape of their spectral
energy distribution, as well as atomic and molecular line profiles both in the
optical and near-infrared. We found that the actual fitted gravities of almost
all our sample are consistent with old objects, except for LHS 73 where it is
found surprisingly low.
