One of my favorite things to do is watch babies and kids explore movement. Often when a child is first learning a new skill, or they are building up their strength and endurance you might see them using what I describe as an ‘all or nothing response.’ For example, when a child is working on sitting balance, parents will often talk about how they arch backward or they flop forward. When this happens they are trying to find just the right amount of muscle force and activity to keep their trunk upright. They just don’t know how to grade it. It’s sort of like the story of Goldilocks and the 3 Bears. One was always not quite enough, one was always too much, and one was just right. Well kids are trying to find their ‘just right’ for muscle force and activation by moving between ‘not enough’ and ‘too much.’
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Even as they start to find their ‘just right’ they may not have the strength for it. As you can see in the video above, the baby is able to hold their head and neck up and have their feet kick around or lie flat on the ground, but as they start to fatigue their head and neck will only come up when their feet come up and then everything drops down at the same time. They are using All or Nothing to be able to lift up their head.
You might notice this same thing in sitting. When the child is sitting on the floor they may be fully slumped over (almost like they are folded in half at their hips) and then they will try to sit up to come upright but turn on too many muscles (their trunk and their hips) and extend all the way back. This is how they are learning to grade their movements. With practice they will be able to find just the right amount so that they can sit up tall!
You may also see this when kids are learning how to go on their hands and knees, because they have to try to turn on their head, neck, and back extension muscles, but keep their hips and knees flexed. A lot of times their extension muscles win out and everything extends at once!
Hopefully this helped explain some of what you may be seeing while your child is growing and developing!