The AFO's effect on supporting the forefoot and preventing plantar flexion or "foot drop" during swing is straightforward. Less obvious is the AFO's role as a substitute for plantar flexor muscles. The plantar flexors must be active during midstance and terminal stance to counter the dorsiflexor moment that is produced by the ground reaction force. If the plantar flexors are weak, the ankle dorsiflexes too rapidly and, because the lower extremity is positioned in a closed chain, the knee flexes.
Midstance knee flexion threatens the person's stability. Someone with weak plantar flexors must compensate, or must wear an external device (an orthosis) that substitutes the force that the plantar flexors ordinarily provide.
When a person bears weight on an AFO that he or she wears inside the shoe, the AFO's rigid walls transfer the ground reaction force (black arrow) in the direction indicated by the red arrow. Thus, the AFO directs a force posteriorly against the anterior tibia during midstance and terminal stance, and prevents or controls tibial advancement. In this way, the AFO's force produces a plantar flexion moment that counters the ground reaction force's (GRF) tendency to dorsiflex the ankle.
The AFO's ability to redirect the ground reaction force, and thereby produce a plantar flexor moment, depends on its rigidity. An AFO that is flexible or articulated (hinged at the ankle) does not serve this purpose.
In addition to producing a force that pushes posteriorly on the tibia, in the direction of ankle plantar flexion, an AFO can influence the ground reaction force's effect on the knee.
The diagram illustrates an AFO that is aligned in five degrees of plantar flexion.
The diagram illustrates an AFO that is aligned in five degrees of dorsiflexion.