AFO, or Ankle Foot Orthoses, is designed to be used in conjunction with Quest88 footplates. The purpose is to minimize foot drop, ie. the foot dropping to a point where pedaling is inefficient and difficult.
The AFO, also referred to sometimes as Calf Supports, control the relationship between foot and ankle by fixing this at 90 degrees. The foot can still move through the pedal axle, however, it creates a controlled pedal stroke, with all of the rider's energies going towards propelling the trike.
Quest 88 AFO supports are designed specifically for Quest 88 footplates and are available in different sizes.
Quest 88 AFO / Footplates should always be used in conjunction with a lap belt or other means of restraint at the hips or waist. It is not advisable to fasten the feet to pedals without first securing the upper body. If you do not have this function on your trike or bike, then you may wish to consider Toe Clips, allowing the feet to release more easily.