The radius is 1 of 2 forearm bones, located on the thumb side when the hand is facing forward. Using the elbow crease a second point, explore how the radius moves rotationally and/or changes location. 

In the elbow locking reflex, which initiates in Stage 2 of the sympathetic response, the radius narrows within the arm "tube" and pulls up towards the humerus (the upper arm bone). In the context of movement, the radius can move in & out of what is considered a parasympathetic position, and every time it moves into a locked position, it triggers the fight-or-flight reflex system wide.