A method of controlling an external force applied to an animal to achieve as a target relation the relation between the external force and a motion variable varying with the motion of the animal. The method and apparatus includes providing a first voltage level using a voltage regulator, determining that a second voltage level is desired and initializing the voltage regulator to provide the second voltage level based on determining that the second voltage level is desired. The wedge plate is moved toward the base plate to cause the ramp of the base plate to bear on the ramp of the wedge plate to urge the base plate apart from the wedge plate creating a friction fit within the channel of the picture frame member thus securing the base plate relative to the frame. A medial foot portion 43 of the reinforcement device 4 is formed to have a higher hardness than a lateral foot portion 40 of the reinforcement device 4. or lower when measured by using a differential scanning calorimeter and a number average molecular weight of 100,000 or larger, and at least one of the plurality of layers other than the first solid electrolyte layer being formed by crosslinking a polymer solid electrolyte having a functional group that can be crosslinked. The retraction assembly releases the cords by pivoting either one of the shoulder stops toward the foot end of the frame.