The atlanto-occipital joint (AO joint) is the joint at the bottom of your skull and the top of your spine. It is between your ears and behind your nose, in the center.
Your head rests lightly and easily on the top of your spine, if you don’t interfere with it.
You can interfere with the poise of your head on the top of your spine by over-tightening muscles in your neck.
When you over-tighten neck muscles, you prevent your head from moving freely, interfering with your natural coordination, making moving and balance more difficult.
The only joint between your arm and your body is between the medial end of your clavicle (collar bone) and your sternum (breast bone). Your clavicle and scapula are arm bones.
Your pelvis is part of your legs.
Your axial skeleton is your skull, spine, ribs, sternum and your hyoid bone (your tongue attaches to it). Everything else is arms or legs. In the picture, everything in purple is your axial skeleton; everything that is white is your appendicular skeleton (arms and legs).
It is easier to think of moving bones than to think of moving muscles. You don’t know what muscles to move, but you do know where you want to move your parts, e.g. hand, arm, leg, foot, etc.
When you start to take a step, the first joint that moves is your knee joint. Letting your knee go forward lets your foot come off the ground, thus letting you take the step.