3D ultrasonography reveals a twisted proximal aponeurosis within the biceps femoris long head

PURPOSE: This study aimed to characterize three-dimensional course and orientation of the proximal aponeurosis of the biceps femoris long head (BFlh) and examine whether local aponeurosis orientations vary with knee-joint angle and relate to fascicle architecture under passive conditions. METHODS: Three-dimensional ultrasonography of the passive BFlh was performed in eleven healthy adults with knee flexion of 0° and 30° and hip in neutral (0°). Aponeurosis orientation was quantified across the visible segmented portion relative to the most transverse proximal slice (0%) within each knee-angle condition and summarized within five longitudinal sections. Fascicle length (FL) was assessed using a two-segment approach and pennation angle (PA; local fascicle-aponeurosis angle) relative to proximal and distal aponeuroses. RESULTS: Proximal aponeurosis exhibited a twisted three-dimensional course with substantial inter-individual variability. Local orientation showed a significant section × knee angle interaction (p = 0.016), although overall aponeurosis orientation did not differ between knee angles. Fascicles were shorter at 30° (- 8.8% ± 3.4%). PA-distal was greater than PA-proximal (p < 0.001), with no knee-angle effect (p = 0.197). Knee-angle-dependent changes in aponeurosis orientation were not associated with changes in FL or PA. At 30°, however, aponeurosis orientation at 40-60% section was positively associated with FL (r = 0.879, 95% CI [0.558, 0.971], Holm-adjusted p = 0.005). CONCLUSION: BFlh proximal aponeurosis showed locally varying three-dimensional orientation that was largely maintained across passive knee flexion. Associations with fascicle architecture were limited and configuration-dependent, with no association between knee-angle-dependent changes in aponeurosis orientation and changes in FL or PA.