The reinforcing element (34) has a cross-section with a flattened overall shape having an aspect ratio greater than or equal to 5 and extending in a main direction (Z1). It comprises a lateral edge (46) made of a polymeric composition comprising a thermoplastic polymer, the lateral edge (46) extending in a general direction substantially parallel to the main direction. The reinforcing element (34) is such that R=(Uc−U1)/Uc>11%, where Uc is the average value of the microhardness of the reinforcing element (34) measured in its central part, and U1 is the average value of the microhardness of the reinforcing element (34) measured at its lateral edge (36). The lateral edge (46) does not have any ridges.