Research backed by the American Academy of Pediatrics states that «with prolonged immobilization on a firm mattress or a flat bed (as in a stroller), the constant influence of gravity flattens the body surface against the mattress producing positional disorders and infants with decreased muscle tone (Short, 1996)». (boba.com)
Equally important is connecting with a parent and experiencing the sensory awareness of extra-uterine life and the influences of gravity which tummy time provides. (abcdoula.com)
Scientists initially thought the Great Wall of Geller and Huchra was impossibly large to have formed under the influence of gravity, but computer simulations later revealed it fell within theoretical limits. (sciencemag.org)