Is it possible for humans to run on hands and feet?

Humans can move about on hands and feet, but our pelvic structure and distribution of body weight are such that it would never be efficient for us. Bipedal animals essentially move by falling forward, then catching themselves with the opposite foot and doing it again. Given the relative lengths of our arms and legs, it is unlikely that we could learn to move really well on all fours, even if we had a great deal of practice, and we would never be able to actually run.