You are right on here. The movement of the arms and shoulders can actually very effectively counterbalance the forces from the movement of the legs and hips. Although the legs are much stronger, the arms are mounted to the shoulders significantly wider than the legs are mounted to the hips, providing much higher torque per effort for the shoulders. Once shoulder rotational forces counter hip rotational forces in the running pattern, the core can virtually relax, greatly increasing efficiency.
Much depends on whether you are going for all-out speed/power vs efficiency-- you will cross or counter rotate in both, the amount of axial movement 'allowed' is what varies.
Problems start when on a physical, technical level the rotation occurs in just one or a few places, or hits mainly one or two discs in the lower back, and then other parts, like 5 or 10 adjacent vertebra rotate as a block.. and the rib cage is stiff because, I don't know, you had to sit still on an oddly shaped, reclined chair with bad design, hunched over a table with not enough light and STFU for 8 hours per day for 12+ years as a child, that's likely 20,000+ hours of bad training, and dad was an abusive alcoholic and mom co-dependent and most of your friends were smokers, and so some places in your spine get overused when attempting to run and do something for your fitness, well, it says it in that word, overused, or disproportionally loaded and challenged, minus enough time or opportunity for recovery, talk nutrition, you get the idea, it's really, really complex. So theory is one thing, but how to get there is another. Very, very lucky in life individuals, your running crush, add hard work and diligent training, some come out on top. We all can get better, and we all have the ability to improve, no matter the circumstances and hardship we experienced. If our spirit wasn't crushed, at least, we can recover, maybe even become better than we ever thought possible. Sorry for the convoluted writing. Great video btw! Keep moving!
This is little off topic but can spinal rotation/overall body rotation be crucial for achieving runner's high? I mean you never hear about cyclist's high.
