  Tuning of active prestress e.g. through activity of molecular motors
constitutes a powerful cellular tool to adjust cellular stiffness through
nonlinear material properties. Understanding this tool is an important
prerequisite for our comprehension of cellular force response, cell shape
dynamics and tissue organisation. Experimental data obtained from
cell-mechanical measurements often show a simple linear dependence between
mechanical prestress and measured differential elastic moduli. While these
experimental findings could point to stress-induced structural changes in the
material, we propose here a surprisingly simple alternative explanation in a
theoretical study. We show how geometrical effects can give rise to increased
cellular force response of cells in the presence of active prestress. The
associated effective stress-stiffening is disconnected from actual
stress-induced changes of the elastic modulus and should therefore be regarded
as an apparent stiffening of the material. We argue that new approaches in
experimental design are necessary to separate this apparent stress-stiffening
due to geometrical effects from actual nonlinearities of the elastic modulus in
prestressed cellular material.
