When babies are born their bodies are sort of stuck in a flexed or fetal position having been in the womb that way for nine months. In the first few months, tummy time is critical to help babies stretch out the muscles on the front of their bodies and strengthen the neck and upper back. By the middle of the first year, babies begin to push up on their arms and come to hands and knees, strengthening their lower back, pelvis, hips and legs.