1. A reinforcement element for reinforcing a structure of a door of a vehicle, said reinforcement element extending along a main direction and comprising:at least a main portion having a reinforcing cross-section extending in a plane perpendicular to the main direction,
wherein the reinforcing cross-section comprises seven adjacent branches, each extending in different directions, two successive branches forming a non-zero angle, at least two of said non-zero angles being of opposite signs,
said adjacent branches comprising a first branch, a second branch, a third branch, a fourth branch, a fifth branch, a sixth branch and a seventh branch, said first branch forming with said second branch a first non-zero angle, said second branch forming with said third branch a second non-zero angle, the third branch forming with the fourth branch a third non-zero angle, the fourth branch forming with the fifth branch a fourth non-zero angle, the fifth branch forming with the sixth branch a fifth non-zero angle and the sixth branch forming with the seventh branch a sixth non-zero angle,the first non-zero angle and the second non-zero angle being of a same first sign, said third and fourth non-zero angles being of a second sign opposite to the first sign, and the fifth and sixth non-zero angles being of the first sign,the first branch and the second branch being joined together by a first arcuate junction, the second branch and the third branch being joined together by a second arcuate junction, the radiuses of curvature of said first arcuate junction and said second arcuate junction being comprised between 5 mm and d/2, wherein d is a distance between said first branch and said third branch,the third branch and the fourth branch being joined together by a third arcuate junction, the fourth branch and the fifth branch being joined together by a fourth arcuate junction, the fifth branch and the sixth branch being joined together by a fifth arcuate junction, and the sixth branch and the seventh branch being joined together by a sixth arcuate junction,the first, second and third branches together forming a concave curve oriented towards a first direction, the third, fourth and fifth branches together forming a concave curve oriented towards a second direction opposite the first direction, the fifth, sixth and seventh branches together forming a concave curve oriented towards the first direction,
the reinforcing cross-section further comprising two legs joined respectively to the first branch and to the seventh branch and forming with the first branch and the seventh branch respectively non-zero angles,
the legs having a structure comprising martensite, bainite and optionally ferrite, the rest of the main portion having a structure consisting of martensite.