Do you believe that, as a runner, you should have a tight core? That the sine qua non of good running form is tightening your core?
I’d like to propose a different concept: The essence of good running form is directing force in the simplest possible line from foot through head. Since we have two legs and neither one is in the middle, but we have only one head and it is in the middle (more or less), the torso of the human body needs to make a very complex and beautiful movement to make it possible for force to flow from foot to head. Tightening your core to prevent this movement makes it impossible to get the right relationship between foot and head and thus you can’t use your legs right. You get tight hip flexors and quads, you overstride, you can’t use your strongest muscles – your glutes – properly, your shoulders rise, and running begins to feel unpleasant.
The cure for the wrong kind of core movement isn’t to stop all movement, it’s to learn the right kind of movement. Namely, the rhythmic counterrotation of pelvis and upper body that drives your legs and puts your weight in the right place for the legs to do something powerful and yet also easy and fluid.
Here’s one of my favorite YouTube clips to demonstrate.
Watch the white stripe down Haile Gebrselassie’s singlet and shorts when it goes into slow mo. (The first 50 seconds of the video are the relevant part; the remainder is something else entirely and not what I’m talking about here, nor does what I’m saying have anything to do with Pose Technique.)
Whether or not you choose to do Pilates, the movements you see in this video and more — after all, this is just a side view! — are the ones you need to cultivate. Find a great Pilates teacher who understands this (I know a few!) or find a different kind of cross-training.