Validity and Reliability of Three-Dimensional Ultrasound for Measuring Fascicle Length in the Biceps Femoris Long Head: A Cadaver Study

Abstract The biceps femoris long head frequently experiences strain injury in athletes, pointing to the need for detailed assessment of its architecture in vivo. The present study evaluated the validity and intra-rater reliability of three distinct methods employing three-dimensional ultrasonography for estimating fascicle length from the mid-belly region of biceps femoris long head. Fascicle length measurements from three-dimensional ultrasonography were compared to those from anatomical dissection. Method 1 and 2 modeled fascicles being straight and running in a single longitudinal plane aligned with the longitudinal muscle axis and perpendicular to the proximal or distal aponeuroses, while Method 3 accounted for the possibility that fascicles run in multiple planes. Bland-Altman analysis demonstrated good agreement for all three-dimensional ultrasonography methods and dissection, with Method 3 exhibiting the lowest bias (2.5 mm) and narrowest limits of agreement (-8.5 to 13.6 mm). All methods demonstrated excellent intra-rater reliability (ICC > 0.9). These results indicate that three-dimensional ultrasonography is a valid and reliable method for assessing fascicle lengths in the mid-belly region of biceps femoris long head, and that taking fascicle curvature into account seems to yield better estimates of fascicle length.