Loading History Determines the Velocity of

aracterized by Fv relationships, which describe velocity in a given biochemical system as a function of only the instantaneous applied force 9,10 . Multiple theoretical models posit such Fv relationships for growing actin networks 11--15 , but the predictions of these models vary widely because of different assumptions about the effects of filament branching and capping, network elasticity and network -load tethering. Fv relationships using reconstituted motility assays of Listeria and beads 17 have been reported where the visco-elasticity of the medium was varied to control the drag force. More recently, an Fv relationship for UCSF/UC Berkeley Joint Graduate Group in Bioengineering, University of California at Berkeley and San Francisco; Department of Bioengineering, University of California at Berkeley, Berkeley, CA 94720. Department of Biochemistry, Stanford University School of Medicine, Stanford, CA 94305. These authors contributed equally to this work. Corr