Linear Extrapolation and Extended Field-of-View Methods of Biceps Femoris Long Head Regional Architecture

This study compared extended field-of-view (EFOV) and traditional field-of-view (TFOV) methods for deriving fascicle length (FL), pennation angle (PA), and muscle thickness (MT) at proximal, mid-belly, and distal regions of the biceps femoris long head muscle.

Ultrasound images were obtained from 32 healthy participants (15 females and 17 males) using both EFOV and TFOV imaging techniques. FL was directly measured in EFOV images and derived from three separate linear extrapolation (LE) methods (termed equations A, B, and C) in TFOV images. PA and MT were also measured.

FL measured with EFOV was significantly shorter at the mid-belly and distal regions (P < 0.001) compared with equation B but was not different from equations A and C. EFOV FL was different from equations A and C (P < 0.001) but not from equation B (P = 0.18) in the proximal region. EFOV and LE FL measures had moderate to strong correlation in the proximal (r = 0.45-0.71), mid-belly (r = 0.59-0.91), and distal regions (r = 0.44-0.87). PA was different between imaging techniques in the proximal and distal regions (P < 0.001), whereas MT was different at the distal region only (P < 0.001).

Agreement between EFOV and TFOV FL measurements depended upon both the muscle region and LE equation used. These differences are likely due to architectural variations along the biceps femoris long head and inherent geometric assumptions in LE methods. Care should be used when choosing measurement methods for muscle architecture. Our findings suggest researchers and clinicians without access to EFOV should use equation A with caution, since it has the least geometric constraints among LE methods when estimating nonvisible portions of the fascicle.