An article of apparel includes a padding system that dynamically changes configuration in response to an impending impact. The padding system may change from a state that has a high flexibility and offers low protection from impact to a state that has a low flexibility and offers increase protection from impact. The system may use a filament to constrict a plurality of padding elements together in order to increase the overall stiffness of the pad. The filament may be tightened and loosened by a spool. The spool may receive a signal regarding the impending impact from a sensor that is a part of the article of apparel, or a sensor that is separate from it.