The third episode of the Netflix Series ‘Babies’, is on Crawling. It has long been assumed that crawling is simply a stage that leads to walking, but in this episode, three scientists explore the possibility that crawling is more than just a stage. It could be foundational in understanding how we all move. In the first stage of life, babies are growing faster than they ever will, perhaps crawling has more to do with this growth and transformation than we have believed.

Is crawling just a step to walking?

The first section of this episode shares in the insights of Marianne Barbu-roth, Director Motor Development Group, at the University of Paris in France. While Marianne started out as a nuclear physicist, studying the movement of electrons and atoms, she later become a biologist who was interested in the movement of the body. For a long time people thought that crawling only emerged at 8 – 10 months and that it was a transitionary step to walking.

Marianne questioned whether could be more than just a stage and whether it is crucial to understanding the way that all humans move around. In the womb the feotus can move from one place to another if it wants – she compares it to being on the moon with zero gravity. When the baby is on earth, suddenly he is stuck with gravity holding him back. But in the hour after birth, when the baby has skin to skin contact with the mother, you will see the newborn moving slowly and doing the classic army crawl or wriggle up to the breast of the mother to start feeding. You can also see this same behaviour when you put a newborn in water and they start crawling in water. You then see this behaviour again when the baby is learning how to crawl at 8  - 10 months – so the question is – is there a link between a newborns crawling at birth and the baby crawling again 8 months later?

People believed that new born movements were just reflexes after birth and had nothing to do with a conscious movement controlled by the brain. So Marianne decided to do an experiment to see whether the crawling that newborns do is actually something that is controlled by its brain, rather than a reflexive movement. One of the most important systems that controls crawling at 7 – 8 months is vision, so they created an experiment that would give the illusion to the newborn that they are moving around, to see if they would move.

They used a projector to create black dots moving on a pattern that is completely white and iwht the dots moving in the same direction, it gives the impression of movement. Once the babies are held over this and see the dots moving, they think that the yare also moving and they move hteir legs and arms and start swimming in the air. This experiment showed that there was a primitive link between the brain and the spine, and the vision and motor activity which means that a baby’s movement is conscious – that the crawling pattern that we see is coming from the brain.

Next they decided to try the same experiment on land and see if babies would move if they put them on land. Because the baby’s head is very heavy, being a third of their body weight, it is difficult for them to support themselves due to gravity, so they constructed a tool which is a miniskateboard called the crawly-skate, which took them 1 – 2 years to create. When they put babies on the crawleyskate, they found that with their heavy heads supported, some babies made huge movements, even across the mattress once, or 2 or 3 times. It was remarkable what a newborn was capable of doing to propel themselves. For decades people believed htat we were born as bipeds, but this experiment suggests that perhaps we are still quadripedes, and the nervous system recognises that – with moving both arms and legs as aggregates – that maybe crawling is not just a transitory stage.

Do babies grow constantly or do they have growth spurts?

The next section of the documentary is with Michelle Lampl, Professor, Emory Centre for the Study of Human Health, Emory University, USA. Michelle started out in Anthropology with an interest in the interaction that babies had iwht their families. She was curious about how babies grew and started off doing an experiment with her neighbour who had a baby. She would measure the infant every day and noticed that some days, the baby didn’t grow at all, and other days it grew 2 centimeters. She was very surprised by the results because people used to think that babies grew a little every day very slowly – not that they had sudden bursts of growth. Doctors offices have growth charts that have the child’s size plotted and it shows the real measurements of 882 children measured at ages 1 week, 2 weeks, 4 weeks, 6 weeks, 8 weeks and then monthly until they reach 1 year of age. But when these figures were plotted on the growth chart they join the dots to make curves. The scientists didn’t actually know how they grew between the dots, but the curves just joint the dots which is misleading – there is no measurement between the two specific time points but it creates the visual image of growth occurring slowly and continuously through time.

Michelle decided to measure 30 babies to get a reasonable sample size, and also asked the babies to keep a record of their babies behaviour on a day to day basis. She started on 23rd august 1981 and visited 12 children and for each of them measured their head and length, and measured the babies once a week. She noticed that the babies were growing over the course of the week but she wasn’t sure if it was continuous growth, so she then measured the babies twice weekly and saw they grew in half a week as well. When she started measuring the babies daily, she noticed that babies would have days – sometimes 5 days – of no growth, and then suddenly in a 24 hour period, their growth would be half a centimeter or 1.65 centimeters in one day. The growth days were separated from one another by 2 – 28 days of no growth and then there would be growth spurts. From the parents notebooks on their behaviour she was able to identify a link between the times when they were having a growth spurt and changes in behaviour with sleeping patterns, tantrums and sometimes insatiable hunger.

When they found with this experiment is that growth does not happen in a curve, growth happens in jumps with separate times of no growth – there are two very distinct phases and the reason she could see this is because of the increased frequency in the measurements themselves. Unfortunately for Michelle, the Medical community was not ready for her results and while the media and papers were excited to report on it, the scientific community questioned the data because it was such a stretch of their imagination to accept that the growth charts were wrong. Michelle was not prepared for the hostile disbelief but asserted that this is consistent with how science progresses. Understanding what we know now about how a baby experiences its own growth and its body changing and adjusting to the space and what it can do – will help inform our thinking about the influences on early development and for a baby learning to crawl.

Do babies perceive what they can do and what they cannot do?

The final segment of the episode follows Dr. Karen Adolph, a Psychology Professor in New York city, NY USA. While Karen started off studying fine arts in college, she was always interested in movement and growth. She explains that in the real world, the ground is not perfectly flat and empty and uniform, but for the last 100 years and even today, people study infant crawling by encouraging babies to crawl in a linear way in a straight line on the ground. But in the real world, babies are crawling around and have to make decisions – they have to use perceptions to make decisions about their crawling movements – like should I crawl over the edge of the end or should I stop at the end?

She wanted to find a way to test how babies use perception to guide their crawling movements. If a baby really could perceive what they can do and what they cannot do then they should be able to do it in a ‘novel’ situation. She built an adjustable slop apparatus to test this to see if babies know what actions are possible and what actions are not by asking them to crawl over a walkway and then increasing the gradient of the slant by 6 degrees at a time. They found that each baby was very different but in general it takes 20 weeks of crawling experience before a baby can somewhat accurately predict what they are capable of when looking at a sleep slope. They are learning in this time how to move their body in an environment based on how they are dressed, the texture and gradient of the ground etc. Its good for babies to crawl on grass, sand or whatever because learning to crawl is not just the coordination of arms and legs in a pattern its learning to behave in a flexible way to get around in the world.

In another experiment she asked parents to keep a diary of every little skill their babies might display on the way to being able to crawl in order to identify if there is a ‘crawling switch’ that turns on when a baby cannot crawl one day, and the next day they can crawl. They found that crawling is incredibly variable and creative and every babies trajectory looks different from other babies. There is no orderly progression of motor milestones or steady march from one stage to the next - from lifting your head up and then all the way to walking. Babies acquire their skills in different orders and different skills are appearing and disappearing and reappearing. Crawling Is not mandatory – its not an obligatory stage that babies have to do before they begin walking.

Babies all develop in their own way so relax and enjoy their journey.