Schematic illustration of force balance within a fiber (A) before and (B) after application of an external force, F. T: Tension before application of F. β: Angle from the direction perpendicular to the direction of the original filament. TL, TR: Tension of the filament during force application. θL, θR: Changes in the angle of the filaments before and after application of F. TL′, TR′: Element of the tension in the direction of the original filament. Subscripts L and R indicate the left and right parts of the filament, respectively, as determined by the force application point.