  The dispersion of one hole in an extended $t$-$J$ model with additional
hopping terms to second and third nearest neighbours and a frustration term in
the exchange part has been investigated. Two methods, a Green's function
projection technique describing a magnetic polaron of minimal size and the
exact diagonalization of a $4*4$ lattice, have been applied, showing reasonable
agreement among each other. Using additional hopping integrals which are
characteristic for the CuO$_2$ plane in cuprates we find in the nonfrustrated
case an isotropic minimum of the dispersion at the point $(\pi/2,\pi/2)$ in
$k$-space in good coincidence with recent angle-resolved photoemission results
for the insulating compound Sr$_2$CuO$_2$Cl$_2$. Including frustration or
finite temperature which shall simulate the effect of doping, the dispersion is
drastically changed such that a flat region and an extended saddle point may be
observed between $(\pi/2,0)$ and $(\pi,0)$ in agreement with experimental
results for the optimally doped cuprates.
