Cytoskeletal tension enables cells to adhere, spread, contract, and migrate. In adherent, non-muscle cells such as endothelial cells and fibroblasts, tension is generated by actomyosin stress fibers applying traction forces to cell-matrix adhesions. Tension extends stress fibers beyond their unloaded lengths and cells maintain a preferred level of fiber strain that depends on actomyosin activity (Lu 2008). Stretching the matrix upon which cells adhere perturbs the cell-matrix traction forces and cells respond by actively re-establishing the pre-existing level of force (Gavara 2006). Sudden large increases or decreases in strain result in rapid stress fiber disassembly and reassembly (Lu 2008), suggesting that perturbing fiber strain from a preferred level stimulates stress fiber turnover.