Spiral spring for a horological resonator mechanism provided with means for adjusting the stiffness

A spiral spring for a horological resonator mechanism, the spiral spring (1, 10) comprising a flexible strip (2) coiled on itself into several turns, the strip (2) having a predefined stiffness, the spiral spring (1, 10) including means for adjusting tensile/compressive forces acting on the spiral spring, wherein the adjustment means include a first elongate flexible element (5), and a second elongate flexible element (15) arranged in series with the strip (2), each elongate flexible element (5, 15) connecting the same end (4, 9) of said strip (2) to a fixed support (11, 14), so as to add an additional stiffness to the spiral spring, each elongate flexible element (5) having a stiffness higher than that of the strip (2), the adjustment means including prestressing means (6) for applying at least two different adjustable forces, the two different adjustable forces being of the tensile/compressive forces acting on the spiral spring and applied on the first elongate flexible element (5) so as to make the stiffness of the first elongate flexible element (5) vary according to the prestress level.