What comes after scapulae as engine is the power of crawling. Why do we need crawling? Because that let us learn how to support upper body weight by pelvis.
Pelvis functions as weight-bearing system. That means our bodies can’t resist against gravity if we had not crawled enough in baby-days. The power of pelvis generates the power of hip extension. Poor power of pelvis can’t generate it, but resorts to knee extension. To run fast, stay in balance, and even also prevent injuries, the power of pelvis is necessary. It’s not too late to redo crawling exercise. Restart your body in Naturalization.