Background Biceps femoris long head (BFlh) proximal aponeurosis (PA) size appears to be highly variable between individuals (Handsfield et al., 2010) and may not be proportional to muscle size and strength. Individuals with a relatively small aponeurosis ...

Meanwhile, all your hamstring muscles are active during knee flexion, the bending of your knee. Semimembranosus and semitendinosus insert at the tibia, while the biceps femoris insert at the head of the fibula. That placement means your lower leg internally rotates because of the semis, while it externally rotates because of the biceps femoris.