How do i know what i just did in front of the camera?,for example leg down and up

When i use the nuitrack sdk got a question,i need to know what I just did ,like leg down or up these actions.
In this method (onSkeletonUpdate(SkeletonData skeletonData)),i got Skeleton and Joints data,there also have some coordinates,i’m confused about those. So how to related to the actions?