An individual who underpronates also initially strikes the ground on the lateral side of the heel. As the individual transfers weight from the heel to the metatarsus, the foot will not roll far enough in a medial direction. The weight is distributed unevenly across the metatarsus, with excessive weight borne on the fifth metatarsal, towards the lateral side of the foot. In this stage of the gait, the knee will generally, but not always, track laterally of the hallux.
Like an overpronator, an underpronator does not absorb shock efficiently - but for the opposite reason. The underpronated foot is like a diving board that, instead of failing to spring someone in the air because it is too flimsy, it fails to do so because it is too rigid. There is virtually no give. An underpronator's arches or ankles don't experience much motion as they cycle through the gait.
An individual whose bone structure involves internal rotation at the hip, knee, or ankle will be more likely to underpronate than one whose bone structure has external rotation or central alignment. Usually - but not always - those who are bow-legged tend to underpronate.
An individual who underpronates tends to wear down their running shoes on the lateral (outside) side of the shoe towards the rear of the shoe in the heel area.