We coordinate our movements according to our internal body map.
At some point in our lives we make up our minds where the legs ends and the back starts, which part we consider to be our neck, and so on. A lot of people have only a faint idea exactly where their hip-joints connects the legs to the pelvis. These people so happily bend their spines when asked to lean forward from their hips.
Most schools don't teach anything to increase body awareness, or to understand the body dynamics or anatomy involved in movement. Depending on the type and amount of physical activities, and the good or bad examples given, our body map gets more or less distorted. Of course, when we move a certain way, our body will distort to accommodate us through pure repetition.
We can re-align our body maps by experiencing the factual location of our joints, bones, muscles and other bodily tissues. The more we incorporate the experience of the connections of our factual structure into our internal body map, the easier movement will become over time.
If you'd like to learn more about body mapping, please - search and study!