Answer: Plasticity
Plasticity or brain (neural) plasticity applies to injury. When we are injured, we use some movement patterns of our legs and arm to avoid strain on damaged tissues and to keep us going. But gradually, we are back to our original movement.
Amir gradually regained the use of his leg and arm due to the principle of plasticity (the capability of the central nervous system to let us adopt from its functional and structural organization to our current requirements).