Comparison of mouthpiece forces across all participants, ordered by level of proficiency (left: beginners; right: professional musicians). (A) Absolute mean mouthpiece force of the upper (lighter colour) and lower (darker colour) lips. (B) Corresponding normalised mean mouthpiece force, computed separately for each lip as $F_i/(p_{\mathrm{mp}}^{\mathrm{RMS}}{A}_{\mathrm{ref}})$ with i ∈ {UL, LL}. Each participant is represented by two boxes. Foreground boxes correspond to the easy excerpt, background boxes to the difficult excerpt. Vertical red lines separate proficiency groups.