Jonah Rosner | Elite Running Scientist on Instagram: "Most runners are told to stretch their calves and Achilles to run better. The problem is that efficiency does not really come from flexibility. It comes from stiffness. Your calf muscle and Achilles tendon work together as one unit. When the Achilles is stiffer, it takes on more of that stretch. That allows the calf muscle fibers to work more isometrically (this is just a nerdy term we use to mean the muscle does not move much, but it makes u