Why do babies vibrate their legs?

There is a certain reflex that newborn babies have where their arms and legs move involuntarily. I can't remember the name of it, but it is a reflex.

newborns do not shake when they are cold, this function develops when they are older, and their nervous systems are more mature, as a rule, a baby needs 1 more covering than the normal adult. if the infant's skin feels cold to the touch, then yes they are cold, which can cause a healthy infant to become sick, they "vibrate" their legs as part as their immature nuero system, unless you are witnessing a infantile seizure, which you can tell by placing your hand on the shaking or vibration of the leg, and the shaking will not stop. in an actual seizure, this movement would be rythmythcal and would not be stopped by placing a hand over the extremity. most newborns are jittery at times, like when they stretch/yawn/move. this is normal function for a newborn/neonate . lots of parents see this movement and think their baby is cold. no. baby is not able to shake or shiver yet. also, you can take the infants temp. to see if they are cold. normal is 98.6 f. hope this helps. good day

I thing it because the have been curled up in you stomach so long that when the straighten the legs out the automatic vibrate until they get strong enough bones they will continual to vibrate until about 6month.

How old is your baby? If he/she is newborn, then its sometimes because of medicine you had during delivery.