A sport boot that allows for a reduced resistance plantar extension coupled with a controlled, resistive plantar flexion motion. The preferred embodiments provide a one-piece boot made of a flexible material, such as leather. A triangular portion of the boot material, from a point just posterior to the ankle opening to a larger area anterior of the ankle on both sides of the boot, is cut out. One or more piston assemblies are attached to the boot material on both sides of the cut away portion of the boot. The piston assemblies provide resistance to compression during flexion of the boot but offer no resistance to extension during plantar extension.