The reason babies kick in the womb has finally been explained as scientists believe it "wires up" the nervous system. And the phenomenon may be key to treating illnesses and even training robots.
The kick can carry a force of more than 10lbs and has mystified scientists for centuries. Now a model shows it helps the infant learn to control its body. The discovery has implications for medical conditions and developing more agile machines.
Lead author Professor Henrik Jorntell, of Lund University, Sweden, said: "The complex circuits are not pre-determined by genes but rather are reinforced by body movements."
It sheds fresh light on a host of disorders - from cramps and spasms to multiple sclerosis, spinal cord injury and motor neurone disease. The hundreds of neurons that control each muscle are synchronised in the foetus to create strong contractions that activate 'sensors'.
Prof Jorntell said: "These correlated patterns of activity can be used to wire up the spinal cord circuitry that coordinates them through reflexes. The brain can then use that circuitry to learn voluntary movements that are graceful and efficient."
Showing how the body learns and adapts builds on an evolutionary theory first put forward by legendary American psychologist James Mark Baldwin in 1896. It suggested a newborn animal with a potentially useful mutation of the musculoskeletal system must reproduce and propagate it. If the animal's nervous system were hardwired for the old body, it probably wouldn't survive.
Prof Jorntell said: "The new model outlines how neural circuits in the spinal cord can learn the mechanics of the new body from its early, spontaneous movements. It has implications for how to treat neuromuscular disorders and could also provide a simple way to design better controllers for robots."
The international team is now investigating how the brain learns to get itself wired to the spinal cord.
Prof Jorntell said: "It would be a step in building a complete nervous system capable of intelligent behaviour which can automatically separate notions of 'self' from 'world.'"
They also hope it will lead to a better understanding of developmental problems such as cerebral palsy and the difficulties of recovering from spinal cord injuries and strokes. To date, it has been difficult to get robots to perform movement tasks that humans do easily.
The spinal cord is more than just a cable connecting brain to muscles. It contains complex networks that generate everything from simple knee-jerk reflexes during medical exams to walking and other more advanced movements.
Prof Jorntell said: "The brain learns to use these spinal circuits to generate the graceful and efficient behaviours that we take for granted. Robots are often clumsy because they lack such circuits. The circuits of the spinal cord can't be applied because the mechanics are different from animals."
The computer simulations provide a way for any robot to recapitulate the stages of human development to spawn the equivalent of spinal circuits for its body. Most pregnant women begin to feel their baby move between 16 and 24 weeks.
The movements can be described as anything from a kick, flutter, swish or roll, which may change as the pregnancy progresses. The study is in the Journal of Neurophysiology.
