First of all what is a container you might ask? Well in the Child Development world it means anything that contains your baby (see picture). It can be a jumper, walker, activity center, swing, bouncer...you get the point.
Why do containers hinder growth? Well, most containers like activity centers, walkers, and jumpers provide a false sense of movement, security or independence but all the child has to do is sit or jump.
I am going to come at this post from both a parent and professional perspective because I think they are both important.
When I was a first time mom, even though I'd had 15 + years of experience with and education about children there was obviously a lot I didn't know. My son was in his exersaucer a lot. We lived in NYC and it was an easy way to keep him safe and in a small amount of space. Although he had an atypical crawl (which I know now how to correct!) for the most part his gross motor skills were on target. I thought nothing of it.
Fast forward to my second son, who had a severe gross motor delay. By the time we got help from Early Intervention at 9 months he had been in his container for long periods of time, as well - because I didn't know any different. As a parent I was surprised to learn that containers could hinder development because um...they're marketed and sold to PARENTS! On purpose! But once I realized that it wasn't my fault and likely not entirely the containers' fault either I moved on to learning about this more.
Why do containers hinder growth? Well, most containers like activity centers, walkers, and jumpers provide a false sense of movement, security or independence but all the child has to do is sit or jump. And that isn't the way bodies grow and develop. All the bouncing and jumping and fake walking can actually build muscle tone where we don't want it and give children the need to be upright, straight legs out and preferring to bounce. It can also cause the heel chords to shorten which makes kids walk on their toes and actually unable to walk flat footed. Yes, this is extreme but yes I've seen it many times.
By the time my 3rd son was born I had been working in Early Intervention for a few years and saw first hand the many children who were kept in containers a lot and did have delayed milestones. I am not saying that containers cause delayed milestones 100% but I am saying that they do not support proper development and growth.
I decided to do something different with my 3rd and besides a swing and bouncer when he was teeny he was never in a container. We got rid of them and I just put him on the floor all the time and OH MY GOD. That kid could pivot, roll, and move around his environment with so much ease from a much younger age than my other two, and I was so happy! But also so mad that I hadn't exposed my other two to this kind of tummy time/floor time. Alas, the past was in the past, right?