This is a great piece on infant physical development. When my first son was 3 months old, I had the wonderful opportunity to take an infant developmental movement class at a nearby yoga studio. My teacher taught me so many wonderful things that I would never have otherwise learned about supporting my son's physical development. Even as someone who is quite physically attuned, with my background as a dancer and my current yoga practice, I just had no idea how many of the things that I was doing were not helpful, and potentially detrimental, to my kid's budding physicality. This article explains some of what I learned in my class.
"When an infant is propped or placed in sitting and standing before they have developed adequate upper body strength to move their own body into sitting and standing, their spinal curves will be develop out of sequence. The result, which I have seen over the years in my work with infants, children, athletes and adults, can become organ, glandular and/or spinal challenges such as scoliosis and lordosis because the cervical and lumbar curves have become dominant.
When a newborn or an infant is “propped” in a sitting position or placed into equipment by a caregiver, they will either stiffen or flop over. When a young infant is consistently sat and stood up, they will usually extend their limbs and stiffen throughout their entire body (a fear response) in an effort to support the weight of their head. This stiffening is easily felt when holding them and will not only disrupt the integration of their arms and legs with their torso but also delay their ability to roll over."