sentence: This effect can be due to the fact that whereas the retraction and protrusion rates are slower , the force required to overcome the friction of the substrate increases with the stickiness of the substrate , and thus the minimal length that filopodium can reach at the end of retraction phase increases , which consequently leads to longer filopodia on average.The treatment of adhesion in our model assumes that the traction force arises from uniform adhesion between the filopodium and the substrate .