  Within the framework of an isospin and momentum dependent transport model,
the emissions of isospin particles (nucleons and light clusters) squeezed out
in heavy-ion collisions are investigated as probes of the poorly known symmetry
energy at high baryon density. Two different mass splittings of neutrons and
protons in nuclear medium as $m_{n}^{\ast}>m_{p}^{\ast}$ and
$m_{n}^{\ast}<m_{p}^{\ast}$ are used in the model and their influence on the
isospin emission in heavy-ion collisions is discussed thoroughly. The
competition between the stiffness and the momentum dependence of the symmetry
potential on reaction dynamics are compared and systematically analyzed. It is
found that the difference of the neutron and proton directed flows and the
transverse momentum distribution of the neutron/proton ratio are sensitive to
the stiffness of the symmetry energy, which can not be changed with the
controversial effective mass splitting. The elliptic flows of free nucleons at
high transverse momentum within mid-rapidity emission are a promising
observable as distinguishing the nucleon effective mass splitting.
