Measurement of Single Platelet Contractile Forces by Atomic Force Microscopy. A fibrinogen-coated cantilever is pressed against an activated platelet just as it lands on a fibrinogen-coated surface. Contraction of the platelet pulls the cantilever down a measureable distance toward the surface. The stiffness of the cantilever, i.e., the force required to move it a given distance, is known. Thus, the force exerted by the platelet can be calculated.