If you know when an infant should smile, roll over, sit-up walk, and so you, you have a way to determine if functional development is on time or not. 3D viewing has been a advance in our ability to assess motor behavior over conventional 2D imaging, which only shows a cross sectional plane. We learn a lot about how the nervous system develops from the quality of individual movements and the combinations of complex movements into behaviors. This is Elf at 20 weeks GA . At this stage, movements are becoming smoother and more coordinated.
Control comes from being able to make a movement and to inhibit it.
When we study movements we also have to pay attention to what is not happening.