FHAs for subject 9821R. All FHA's are plotted relative to the tibial coordinate system. The talocrural (subtalar) FHA changes from deep red (blue) to yellow (green) as the ankle plantarflexes. The length of the FHA indicates the magnitude of the angular velocity with the center of the line being the closest point the tibial origin (A&C) and talar origin (B&D). Ankle angle for this subject ranged from 1.9° to 39.4°.