Along these lines, I offer this essay which I wrote in regards to an early motor development program that I am helping develop. Please let me know if the topic is of interest. There a many more essays like this one in this feverish brain of mine. - KPM)
There is an ancient Greek fable about a dreaded creature called the Sphinx who used to waylay travelers with a riddle. Answer the riddle (as none did) and you could pass safely on your path. Fail to answer and you were torn apart by the ravenous beast. The riddle, made simple by the repetition of the years, was “What walks on four legs in the morning, two at midday and three at night?” The answer or course is a human – crawling in infancy, walking tall in midlife and using a cane in dotage.
What makes this riddle true is the remarkable changes that occur in the structure of the child to go from a fetal ball to a walking youth and nowhere are they more evident than at the hip joint. The hip is the only joint in the body to actually change its whole structure, positon and anatomy as it develops, and it is these changes that allow humans to eventually stand and walk on two feet in a way that no other creature can.
One of the major changes that the hip undergoes between birth and walking is the alignment and rotation. When first born, a child has hips that are very flat with respect to the pelvis (the technical term is retroverted). This allows the hip and legs to retract out of the way without undue stress as the child is delivered through the maternal pelvis and the birth canal. But it means that, at first, the child cannot bring the hips in front. We see that in the newborn alignment of the hips with the legs splayed out in what we call the frog position, bent up and rolled out. A newborn cannot (and should not) be able to bring the hips in to the midline and care should be made in the very early days of life not to pull the legs together to swaddle since it drives the hips into an unnatural position.
Over the course of the first few months of life, the hips begin to roll forward into what is called anteversion. Ultimately, this lets the child bring the hips straight in front and gives them the stability to start bearing weight and even to push the body forward into crawling. Ultimately the anteversion provides the child with a secure, forward directed center of balance that will allow upright stance and walking. Finally, as the child gets more secure and the balance and ambulation muscles develop fully, the posture becomes more upright and the hips assume their adult position.
As with all bone development, the hip realignment cannot happen in a vacuum. Bone shape follows function, so the bone will shape and position according how it is used. In children with neuromuscular disease who never walk or crawl, the hips stay retroverted. If the muscle forces don’t balance during early walking (as in some types of cerebral palsy) then the hips end up too anteverted by late childhood and adulthood.
Which brings us to Tummy Time. There was a trend through the 1980’s and early 90’s to avoid stomach lying as much as possible. Genuine fears about protecting the child’s airway led to this well-meaning but ultimately misguided conservatism. The prevailing feeling was that children ultimately roll over and crawl, so we will protect them until they figure out how to do those actions safely on their own. Added to the advent of car-seats and other important safety measures, we saw a generation of kids who were essentially told to figure it out on their own.
Many did, but we did see an increase in delayed walking and in some cases the loss of important milestones, like rolling, arm push up and other actions which help develop normal patterns of gait and balance. Although it is still early to tell, we seem to be seeing an increase in early joint problems in this protected generation.
We seem to have finally figured out that prone lying, or Tummy Time, is safe and vitally important to the development of upright function. It promotes the awareness of weight bearing and transfer. It leads directly to experimentation with arm and leg movements which facilitate crawl. And ultimately, it is from the prone position that humans learn to lift up and become upright walkers.
As for the all-important hip joint, which seems to govern all of propulsion, Tummy Time provides the ground and reactive forces necessary to start the process of realignment of the joint. The baby feels the ground below the pelvis and reacts with muscular contraction. The muscle function starts to reshape the hips. As the hips reshape, pulled inward and forward by the muscle function, they become stronger and better positioned to lift the body and to start forward propulsion. And ultimately, these functional changes lead to increased strength and balance which lift the child upward into stance and walking.
The Sphinx would be so proud!