“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 during a Landau reaction. The Landau reaction emerges at approximately 3 months as a reflex/postural reaction, allowing the baby 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 while on the floor. These new sensations and movement keep interest in the activity. 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. The difference in this stage is that the gluteals are becoming active and the hips are fully elongated. With practice the thighs begin to come off the ground through the action of the gluteals. During pivot prone, there is eccentric action of the abdominals as the baby extends so there is also controlled motion through the range.
During pivot prone play, the baby is strengthening and discovering: