A method and device are disclosed for forming reinforcement element formed of fibers distributed along three dimensions, wherein, a continuous fiber is introduced in a substrate resting on a support formed of such fibers crossed in at least two directions, by stitching, from the free face of said substrate opposite said support by means of a needle driven with a reciprocal movement, a relative movement being in addition generated between said support and said needle so that said continuous fiber is formed inside said substrate of a succession of consecutive segments forming a zig-zag line.The invention is remarkable in that:said needle is curved and imposes on each segment of said continuous fiber a similar curved form; andthe reciprocal movement of said curved needle occurs while rotating about an axis perpendicular to the plane of said needle and disposed on the concave side thereof.