The lumbrical muscles do not attach to the proximal phalanx directly, but rather connect the tendons of flexor digitorum profundus (below, ie volar) to the extensor tendons (above, ie dorsal) and produce flexion by pulling on the extensor tendons as they pass over the proximal phalanx.
By the time they are 12 months old, most babies can weigh three times what they weighed at birth and gain about an inch per month in length. The average infant at one year may be between 2630 inches long. Social and Emotional Development-Infants respond when you say their name. They begin to fear strangers.
It's different for each baby - some are faster, some are slower. You'll know when your youngin' can look around without the head jerks. It all depends some babies can hold head up at a month or two. My son was holding his hea up at 3 weeks no lie. i can only tell you what my daughter did. She was able to hold her head up by her self at 4 months, meaning i did not pick her up holding her head nor did i hold her head when holding her on my chest.