A newborn is a floppy creature whose muscles are flaccid. As a baby grows, he strengthens muscle tone, masters control of his limbs, and learns how to make his body move the way he wants it to. Something as seemingly straightforward as sitting involves a range of muscles and muscle control as well as balance, and a baby actually starts working toward sitting up several months before he is able to. In order to sit up unaided, the baby has to have strong back and neck muscles and a sense of balance in his torso, head, and neck.
If you hold an infant's hands while he is lying down and help him into a sitting position, you can see his eyes go wide with wonder at the new view. The first step toward this fascinating new way of seeing the world is strong neck muscles and a balanced head. Once the baby learns to hold his head up, he will start working on strengthening his upper body by arching his back and lifting his chest when lying on his stomach. Once the baby is strong enough to lift up his chest, sitting upright is close at hand. But though he may have the strength, he may not have the balance. Tripoding — or sitting on his tush with his arms straight out in front of him and his hands on the floor — is a way for a baby to sit up by himself before he has the balance to stay sitting unaided. Expect a tripoding baby to tumble often to one side or another (some propping with pillows will help keep him stable).
When a baby can finally sit by himself, he'll have much to do — he can stare at his hands as they move in front of him, play with toys, chew on teething rings, watch you putter more easily, and examine the world from his happy seat on the floor. Try to enjoy it while it lasts as crawling will be just around the corner.
Adapted with permission from "Why Babies Do That: Baffling Baby Behavior Explained," by Jennifer Margulis, published by Willow Creek Press. 2005 by Jennifer Margulis. All rights reserved.