In all of these conditions, upright posture makes the person sick. Walking and running are actually easier than standing or sitting still. The underlying cause is a disruption of the autonomic nervous system. This system automatically regulates bodily functions, including heart rate and blood pressure. It also responds to postural or orthostatic stress.
Blood is a liquid.
It tries to flow downward. When you are standing it tends to run into your legs and feet. This deprives your upper body and, especially, your brain, of an adequate blood supply. The autonomic nervous system prevents this by regulating how the heart beats and by constricting blood vessels in the lower parts of the body, forcing blood upward. In people with COI, this system doesn't work right and blood tends to pool in the belly, legs, and feet.