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 comprising:

• a flexible strip coiled on itself into several turns, the strip having a predefined stiffness;
• and
• means for adjusting stiffness of the spiral spring, the adjustment means including a elongate flexible element arranged in series with the strip, the elongate flexible element connecting one end of said strip to a fixed support, the elongate flexible element having a stiffness higher than that of the strip, the adjustment means including prestressing means to apply at least two different efforts on the elongate flexible element,
• wherein the prestressing means include a first lever attached to the end of the strip so as to allow adjusting a first effort, and a second lever also attached to the end of the strip so as to adjust a second effort independently of the first effort.