When my pupils are struggling with the concept of restoring the relationship of the head, neck and back (or more simply the head and spine) they often castigate themselves for their weak 'core muscles'. I too believed that my problem was my flabby stomach and weak abdomen which was not helping my back muscles to hold up my back until my teacher, Tanya Shoop (http://www.artofposture.co.uk/) pointed out that toddlers have relatively undeveloped muscles and they not only have an upright posture, they have nice relaxed backs and upright posture because they have undeveloped muscles. They have not acquired the ability to hang their heads forward off their spines with a wad of tight upper back muscle. If a toddler's head moves too far off their spine in any direction they simply fall over.
If you watch the video embedded above of a toddler 'conducting' you can see how she is constantly moving with the music. but her back is never tightened or bent at the waist. Watch her eyes, she is watching the real conductor of the choir and her head and body follow her eyes. Her relaxed back allows her arms to be soft and expressive. Although she is wholly engaged in the moment, her head stays perfectly in balance with her movement.
In some ways this isn't fair on us adults because toddlers don't yet have fully curved spines, the proportion of head and body is different and they are less fearful of falling because they have less far to fall. But next time you are around some very young children have a look at their movement. Have they started to tense their muscles and movement? Can you see their backs and heads moving as a whole unit?
Below is another mini maestro. In this video you can see how his head, even though in constant motion, retains a poise, almost seeming still when his body is moving about underneath. You can also see more clearly how his legs bend when he lowers his head to allow his back to retain its length and width. The stability of his back allows for the enormous range of movement in his arms. If you watch to the end you will see him fall with abandon and seamlessly roll about the floor with joy.