Box plots of the normalised force ratio between upper and lower lips. The top panel shows data across all notes for all participants; the bottom panel focuses on participants P22 to P31 across different pitch registers (loRan – low register: ≤B♭2; lomiRan – lower middle register: ≤B♭3 and >B♭2; himiRan – upper middle register: ≤F4 and >B♭3; and hiRan – high register: >F4). Positive values indicate greater upper-lip force, negative values indicate greater lower-lip force. Background colours denote force distribution types: red for upper-lip dominance, blue for lower-lip dominance, and yellow for balanced distribution.