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Microscopic origin of macroscopic contractility in actin-myosin active gel models

Actin filaments, crosslinkers and myosin molecular motors form contractile networks. For instance, the cell cortex is a thin network below the cell membrane ; contraction of the cell cortex allows cells to round up during cell division. Contractile actin-myosin networks are often represented at large scale by continuous theories such as active gel models. However, experimental perturbations are microscopic while parameters in continuous models are macroscopic, thus making those models hard to falsify experimentally. Here we use numerical simulations, in which we can access both microscopic and macroscopic quantities, to show that active gel models can indeed be applied to describe contractile actin. We predict that contractile stress should scale linearly with actin density, which is confirmed by numerical simulations. Moreover, we can accurately predict how the contractile stress depends on motor properties such as unloaded speed and stall force.