Infant Locomotor Development
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There are obvious differences in adult locomotion for humans compared to the great apes, but there are additional differences related to the development of locomotion. Many mammals are able to locomote shortly after birth, and it is likely that most mammals are born with an innate locomotion circuit, but the time to reach mature, adult-like locomotion varies significantly.
Human infants display coordinated leg movements that seem to mimic walking movements at a very young age, but they do not reach typical, adult walking behavior until 6-7 years of age. Newborn infants supported stepping patterns do not match the plantigrade locomotion of adults. Their foot lands on the forepart directly under the body instead of on the heel in front of the body. In addition, the hip and knee joints are hyperflexed, and there is no knee-ankle coordination. From 6-12 months, the supported walking begins to look more similar to the plantigrade pattern, and this progression continues throughout early unsupported walking (10-18 months).
Chimpanzee infants begin locomoting primarily with their upper limbs, with torso-orthograde suspensory locomotion as the most common mode. At about 3 years of age, they begin to transition to more quadrupedal locomotion. By 5 years of age, they begin to locomote completely independently. During this stage, they decrease clinging and torso-orthograde suspensory locomotion and increase quadrupedal locomotion. Locomotion finally reached the adult form by adolescence (10-13 years), when the majority of movement became quadrupedal walking.
Gorillas begin crawling on mother locomotion by about 3-4 months of age. By 6 months, the frequency of locomotion has increased, but the infants are still never venturing away from the mother. At this time, the infants are already locomoting with some quadrupedal walking, however it is palmigrade rather than the adult knuckle-walking. Between 10-15 months, they are now primarily using quadrupedal locomotion, and it is often in the form of knuckle-walking. At this age, they may also be seen jumping and vertical climbing for the first time. By 4 years of age, gorillas are locomoting much like an adult, with knuckle-walking quadrupedalism as the most common form.
The three different developmental trajectories are appropriate considering the differences in adult locomotion. Chimpanzees, with the most diverse range of adult locomotion, take the longest time to reach the mature form. Humans are much quicker, but the only locomotion is bipedalism. Gorillas are the quickest to reach mature locomotion, and they are also the least active (in terms of locomotion) of the three species.
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