Article: The 21 nocturnes are more structured, and of greater emotional depth, than those of Field (whom Chopin met in 1833). Many of the Chopin nocturnes have middle sections marked by agitated expression (and often making very difficult demands on the performer) which heightens their dramatic character.

Now answer this question: Which type of Chopin's compositons were difficult for perfomers due to their middle sections?
nocturnes