In last week's tutorial you saw Will in prone at 5 months 
I highlighted the following aspects of his postural control and movement in prone. 

At five months Will nas learned to roll from supine to prone and is at last willing to spend time in prone, and is very busy exploring how to maintain his balance as he starts to reach for toys.

• He is starting to take weight on his forearms as well as on the hands with the elbows extended. 
• He holds his head up with the face vertical. 
• As before, pushing up onto his forearms and hands is supported by extension of the trunk  which now includes extension of the thoracic and the lumbar spine.
• Mostly his hips are in some flexion, abduction with some knee flexion. From time to time you see him extending the hips and knees. 
• With the torso lifted up off the support surface and some weight being taken on the upper extremities, Will's COM has shifted towards the pelvis. 
• In order to maintain stability and balance as he explores looking around and reaching for toys, Will has to learn how best to realign the head, limbs and trunk to support his actions. 
• You will see will exploring different options for shifting his weight over the pelvis and UE's  to maintain his balance. As he does this his COM shifts laterally and in a cephalo-caudal direction. 

At 7 months two things stand out for me

Firstly the ability to transfer weight onto one UE  with effective postural responses in the trunk and pelvis to provide a stable platform for reaching with the contralateral hand take time to develop. In Will's case this only really present at 7 months. 

Secondly, the ability to support the head and upper torso on the UE, either on the forearms or on the hands with extended elbows, requires effective trunk extensor muscle action.