The insert comprises a rigid base (2) with a U shaped section transverse to the boot longitudinal direction. A base part (20) is extended laterally upwards by two opposite side parts (21,22). Two side webs (3,4) are connected to the side parts by pivots (50,51). These pivots enable the webs to tip about an axis transverse to the boot longitudinal direction during bending of the leg bottom about the ankle. Stops (60,61) lock forward to rear tipping of the webs during leg rear support. A flexible rear support (7) connected to the webs provides rear support in the calf region. One of the webs has lower (30) and upper portions which are joined by a second pivot (80), above the ankle, which enables lateral inclination of the upper portion during leg bending.