Abstract: The invention relates to an impact core for insertion inside an outer shell of a helmet. The impact core comprises a shock-proof material and is substantially dome-shaped having cut outs to increase the flexibility of the impact core. The cut out makes the impact core deformable and as a result the impact core can be adjusted according to different head shapes and sizes. The impact core comprises a substantially star-shaped cut out at a top and a plurality of elongated edge cut outs near an edge of said impact core. Further, each of a plurality of arms of said star-shaped cut out extends downward between two neighboring edge cut outs at the edge.