  The European collaboration ANTARES aims to operate a large neutrino telescope
in the Mediterranean Sea, 2400 m deep, 40 km from Toulon (France). Muon
neutrinos are detected through the muon produced in charged current
interactions in the medium surrounding the detector. The Cherenkov light
emitted by the muon is registered by a 3D photomultiplier array. Muon energy
can be inferred using 3 different methods based on the knowledge of the
features of muon energy losses.They result in an energy resolution of a factor
\~ 2 above 1 TeV. The ANTARES sensitivity to diffuse neutrino flux models is
obtained from an energy cut, rejecting most of the atmospheric neutrino
background which has a softer spectrum. Fake upgoing events from downgoing
atmospheric muons are rejected using dedicated variables. After 1 year of data
taking, the ANTARES sensitivity is E^2 dF/dE < 8 10^-8 GeV cm^-2 s^-1 sr^-1 for
a 10 string detector and an E^-2 diffuse flux spectrum.
