  The KASCADE-Grande detector is an air-shower array devoted to the study of
primary cosmic rays with very high-energies (E = 10^{16} - 10^{18} eV). The
instrument is composed of different particle detector systems suitable for the
detailed study of the properties of Extensive Air Showers (EAS) developed by
cosmic rays in the atmosphere. Among the EAS observables studied with the
detector, the charged number of particles, the muon content (at different
energy thresholds), and the number of electrons are found. By comparing the
measurements of these air-shower parameters with the expectations from MC
simulations, different hadronic interaction models can be tested at the
high-energy regime with the KASCADE-Grande experiment. In this work, the
results of a study on the evolution of the muon content of EAS with zenith
angle, performed with the KASCADE-Grande instrument, is presented. Measurements
are compared with predictions from MC simulations based on the QGSJET II,
QGSJET II-04, SIBYLL 2.1 and EPOS 1.99 hadronic interaction models. A mismatch
between experiment and simulations is observed. A similar problem is found for
the evolution of the lateral distribution function of muons in the atmosphere.
