I was just thinking: You know how you are sometimes falling asleep–or are asleep–when your limbs suddenly jerk autonomically (by themselves)? At least, I experience that sometimes, and I assume that you do, too (if you’re attentive).
That limb jerk might have a purpose. Here’s what occurred to me:
When babies are in the womb, they often “kick,” though sometimes the kicks might actually be “punches.” In any event, they move their limbs in a jerky manner that causes their mothers some discomfort (while simultaneously providing delight to the mother, who can feel the child within her, and to others who place their hands on her stomach).
The reason that babies jerk their limbs in this way seems to be that they need to do it: It plays a useful role in their development. By moving their limbs around, the babies are ensuring that their joints work. Our joints–particularly at our shoulders, elbows, hips, and knees–are ball joints that could lock up if the bones grew in the wrong ways. Motion keeps the bones from growing in this way, and if the babies didn’t move in the womb then their bones might grow in ways that would cause their joints to lock up. But by moving them, they make sure that their joints remain fluid and flexible, so that when they emerge from the womb, they can move their arms and legs properly.
So here was my thought: Maybe the limb jerk that we experience when asleep or falling asleep is a survial of the prenatal limb jerk we all have. Maybe it’s a remnant of the reflex that causes our bones to grow right.
If any readers are doctors or others who have info on this, lemme know.