The baby lying down legs will be upward, within three or four months of the consideration of physiological high muscle tone caused, greater than this age is the pathology of high muscle tone, the consideration of cerebral palsy, extravertebral lesions, and other reasons related to. 1. Physiological factors: the musculoskeletal system of babies just three or four months old is not yet well developed. It is a natural physiological phenomenon that the baby’s legs will tilt upward when lying on his/her back. 2. Pathological dystonia: If the baby’s age has exceeded four months, and there is still a situation where the legs will buckle up when lying down, it may be related to cerebral palsy, extravertebral lesions and other abnormalities in the development of the central nervous system, and the nerve control of muscle activity is abnormal, which in turn leads to the baby’s limbs moving abnormally. There may be other clinical reasons why a baby’s legs may buckle upward when lying on his or her back. It is recommended to take your baby to the pediatrics or orthopedics department of the hospital as soon as possible to clarify the cause.