I recently updated the Tracking infant development in prone page and in the process reviewed the video clips of Will and Toesies spending time on their tummies.
Will, a typically developing infant shows the more-or-less typical progression of motor skills in prone. As a back-to-sleep infant he was not particularly happy lying in prone in the first couple of months but by 7 months had acquired all the expected skills and started to push up into prone kneeling. Toesies who was 6 weeks premature was not happy in prone and as a result did not spend much time in this position.
Reviewing the video clips I was again struck by the importance of prone lying for the development of hip extension with adduction. This pattern emerges over time in a typically developing infant and along with the bridging pattern seen in supine provides the repeated active stretching of the hip abductors and flexors needed for the development of full range of hip movement.
|The full term newborn's hips are flexed and abducted.|
|In the first 3-4 months the hips start to extend but are still abducted.|
|At 5 months Will has started to transfer weight laterally at the pelvis and this exposes the hips to a different set of stretching forces on the hip muscles.|
|At 6 months the development of extended arm support coupled with transfer of weight onto the pelvis and active extension of the hips, stretches the hip flexors.|
|At 7 months Will is actively extending the hips as he lifts the torso up off the supporting surface.|
Preterm infants and those with hypermobility do not have the physiological flexion contracture of full term infants. In supine they lie with their hips in abduction and may be less vigorous kickers. In my experience these infants tend to develop tightness in the hip abductor muscles which interferes with the development of the hip adduction and extension pattern seen in the typically developing infants from 6-7 months.
At 10 months Toesies has limited extension in the trunk and hips. The hips are widely abducted.