It's because the--
-quad contracts as U try to Flex.
U need 2 block it out of the equation. My trick was 2 have U sit on a couch cushion that U pull out so when U flex, U squeeze the cushion with Ur hams. Do this & U will see the quad doesn't contract. Another way is 2 sit on the couch and drag Ur heel toward U with hard downward force as if trying 2 make a trench. This also will stop the quad from contracting,