“My 6-month old daughter rocks on her belly and moves her arms all around. It looks like she is swimming on land! What is going on while she is playing like that?”
This swimming motion, common during the development of 5-6 month olds, is also known as pivot prone. The first time you might see something like this would be at about 3 months of age during a Landau reaction. This emerges at as a reflex/postural reaction, allowing the baby to use their back muscles to extend against gravity while held at the stomach. However, by 5-6 months of age, the baby has developed the strength and flexibility to play with it in a variety of ways during their tummy time. These new sensations and movement often keep interest in the activity and are a delight to watch. You might see a few seconds of swimming motion followed by a push into the floor or rocking back and forth. In these actions, the baby is strengthening their postural control system to balance flexion and extension in their core muscles. Simultaneously the gluteals are learning to becoming active and the hips are beginning to fully straighten. With practice, the thighs and arms begin to come off the ground simultaneously. Ideally, even though the back muscles are working, the abdominal muscles are still active and slowly lengthening in order to control the motion, and the core is fully engaged.
During pivot prone play, the baby is strengthening and discovering: