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.
My son was holding his head up really well at 8 weeks i was told by the Pediatrician that he was right on target! So at 8 weeks! If not just give him alot of tummy time thats what i did! this is normal - my baby can only just hold her head up on her own + she is 9 wks old.