Sentence:  It is the gait of the human who must <WSD>run</WSD> to live : arms dangling , legs barely swinging over the ground , head hung down and only occasionally swinging up to see the target , a loose motion that is just short of stumbling and yet is wonderfully graceful . 
Meaning of the word marked with <WSD> tags:  move fast by using one's feet, with one foot off the ground at any given time