DEVELOPMENT OF GAIT BY AGE:
Birth to 9 months: During the first few months of life, several things are happening that lead to upright movement. First, body compostion is changing. On average, in the first 6 months of life, body fat increases from 12% to 25%. This increase in fat content makes the infant weaker for a period of time. In fact, some studies have suggested that larger infants with higher body fat percentage may achieve locomotor milestones later than their smaller friends. As they move towards their first birthday, fat content tends to drop while muscle mass increases and therefore, we see babies getting upright. Second, growth is happening more so in the arms and legs than in the head and trunk. This growth allows the baby to provide a greater resistance against gravity. Third, the baby is naturally exercising muscles that need to be strong for typical walking. On their backs, they are kicking which develops antigravity hip flexor strength. Hip flexors are the big, thick muscles in front of our hips that allow us to pick our leg up and move it forward during walking. On their tummies, they are working out their hip extensors or booty muscles. These muscles work on and off and sometimes with the hip flexors to coordinate smooth walking. Studies show that antigravity control of movement by these two muscle groups at the hip joint typically develops by 8-9 months of age. Therefore, the baby may not even be able to stand independently and the hip muscles already know how to control gravitational forces. So, if the baby is moving and growing typically at this point, they are gaining muscle mass, losing fat content, and developing antigravity movement and therefore, postural control.