Stand with one foot behind the other, both feet pointed forward, and press your hands into a wall in front of you for support. Keep your back leg straight and your back heel pressed flat into the floor as you lean your hips forward. A straight back knee is what puts the stretch into the larger calf muscle running up toward the back of your knee, rather than the deeper muscle underneath it. Keep your back foot pointed straight ahead instead of turned out to keep the line of pull honest. Lean only as far as your heel can stay grounded; a lifted heel means you've run out of range.