  Using molecular dynamic simulation, we study the stretching of an adsorbed
homopolymer in a poor solvent with one end held at a distance $z_e$ from the
substrate. We measure the vertical force $f$ on the end of the chain as a
function of the extension $z_e$ and the substrate interaction energy $w$. The
force reaches a plateau value at large extensions. In the strong adsorption
limit, we show that the plateau value increase linearly in $w$ in good
agreement with a theoretical model. In the weak adsorption limit, a polymer
globule with a layered structure is formed and elastically deformed when
stretched. In both cases a simple theoretical model permits us to predict the
relation between the necessary force to fully detach the polymer and its
critical extension.
