Lumbar support for a vehicle backrest

A lumbar support (1) for a vehicle backrest is provided with a rest member (12) having a front surface shaped to support an area (5) of a user's back. Behind the rest member (12) an elastic member (15) is provided, which has a middle portion (19) fixed to the rest member (12) and two opposing ends (17), which are designed to be connected to a backrest frame, the elastic member (15) being designed to maintain the rest member (12) in a rest position and to deform in response to a thrust of the user's back to backwardly release the rest member (12) between the rest position and a first position (D1) under normal use conditions, and then abruptly towards a second position (D2) when a thrust of a user's back exceeds a given force value (F1). The characteristic force/displacement curve of the elastic member (15) has a slope between zero and a given threshold (V) in the section between the first and the second positions (D1, D2), and the threshold (V) being equal to 0.5 [N/mm].