Crawling (human)
Crawling by definition is using ones knees to propel forward. It is the first gait learned by most humans. Human crawling has similar features to crawling in other primates as well as nonprimate quadropeds, suggesting similar mechanisms.
When crawls are used
Crawling is used mainly:
When a person cannot yet walk because of being an infant
When a person cannot walk due to disability, being wounded, sick, or drunk
In very low places (caves, under a table, in a mine, etc.). Sometimes underground miners need to crawl long distances during their work
When searching for something on the ground
To get down to the ground in gardening, for maintenance or other work-related purposes which require a good reach on the ground
For stealth (camouflage and quietness)
To lower the field of vision
As an exercise
As a token of submission
For fun or comical purposes
During a fire, crawling may be used because the air near the ground is easier to breathe
In infants
Infants sometimes "crawl" with their stomachs on the ground as early as 3 months, but this crawling is infrequent with the baby remaining stationary most of the time. True crawling with the stomach off the ground and the baby frequently on the move usually develops between 7 and 11 months of age and lasts for 3 to 6 months before the child switches to walking. Even after taking their first unaided steps, most babies still crawl part of the time for 2 or 3 months. While crawling, infants gradually practice standing, at first by using people or objects for support and, later, without support. Crawling babies are notorious for getting into trouble, so parents are often advised to childproof their house before a baby reaches crawling age.