Ski boot having a rigid shell base surmounted with an upper including at least one front portion and one rear portion that are fastened, via at least a tightening system of the upper, on the lower part of the leg, and a flexion control device, both of amplitude and of force in the frontward direction located on the rear portion of the upper. The flexion control device includes an energy stirrup that is flexible and connected in a pivotable manner to the rear portion of the upper, this rear portion being pivotable on the shell base. The stirrup has at least one abutment adapted to cooperate with a complementary abutment obtained on the shell base when the upper is closed on the lower part of the leg of the skier in the skiing position.