The site-dependent changes in vastus lateralis shear modulus and fascicle length with hip joint angle manipulation.

In humans, transmission of muscle force laterally along muscle fibers, known as epimuscular myofascial force transmission (EMFT) has typically relied on a single indicator, either muscle stiffness or length (despite their nonlinear relationship) which may obscure intermuscular interactions. This study examined whether manipulating muscle length, through changes in hip-joint angle that passively alters rectus femoris (RF) length, induces site-dependent changes in the mechanical properties of the monoarticular vastus lateralis (VL), specifically, muscle stiffness and fascicle length. Eighteen young men sat with knee flexed at 90° under two hip-joint angles: 70° and 40° hip flexion. Shear modulus and fascicle length were evaluated in different regions of each muscle using shear-wave elastography and B-mode ultrasound. For RF, at rest, shear modulus and fascicle length were greater in 40° than in 70° hip flexion in the proximal and central regions (P ≤ 0.001). Fascicle shortening during twitch was smaller in 40° compared with 70° hip flexion (P = 0.017). For VL, at rest, shear modulus was lower in 40° than in 70° hip flexion in the proximal and central regions (P ≤ 0.041). Fascicle length was shorter in 40° than in 70° hip flexion in the proximal and central regions (P ≤ 0.001), but longer in the distal region (P ≤ 0.010). Fascicle shortening during twitch remained unchanged. Taken together, the results suggest that changes in RF length induced by hip-joint manipulation produce site-dependent mechanical responses in the VL, potentially implying the existence of EMFT in men.