Optimal path for measurement of biceps femoris long head architecture using synchronized ultrasound and...

The biceps femoris long head (BF) exhibits complex morphology, with fascicle orientation and aponeurotic curvature varying along its length. In vivo assessment of fascicle length (FL), pennation angle (PA), and muscle thickness (MT) is commonly performed with B-mode ultrasound (US), yet probe placement may confound interpretation of morphological variation. This study examined the effects of probe trajectory on BF architecture by synchronizing conventional B-mode US with two-dimensional kinematic tracking. Nineteen males were scanned in a side-lying position with the hip and knee flexed at 20°. B-mode US scans were performed along three paths: Central (center of the mediolateral width), 10 mm medial, and 10 mm lateral relative to Central. A high-speed camera tracked probe movement, synchronized with the US video. Measurements included FL, PA, and intermediate (IT) and full muscle thickness (MT) at four points along the muscle. Probe path significantly influenced FL and PA but not MT or IT. Medial and lateral scans produced longer fascicles and smaller PAs compared to the central path (p < 0.05). Differences in FL and PA exceeded 10 %, corresponding to ∼7-10 mm in FL and 2-3° in PA. These magnitudes equal or surpass values often attributed to training adaptations or injury-related remodeling. Probe trajectory can bias BF architectural estimates by amounts that rival physiologically meaningful changes, highlighting the need for standardized scanning protocols.