The movement of the biceps femoris is actually seen best when the thigh is flexed forward at 90 degrees and you bend the knee joint. The muscle movement occurs midthigh close to the knee joint. You will see the curve of the biceps muscle appear to move backwards as you bend the knee back and forwards to flatten out as you straighten the knee. The upper half of the back portion of the thigh is more covered with fat and should form a smooth curve with the buttocks when the thigh is flexed at 90 degrees as in the sitting position. When standing up, the curve of the border between the buttocks and back portion of the thigh becomes prominent because the buttock muscle is not stretched.