Though you might not be able to tell from their plump bellies and adorable waddles, penguins do, in fact, have knees! They have the same leg bones as most other bipedal vertebrates, but certain adaptations have made walking a bit harder for them.

Scientists have determined that walking costs much more energy for penguins compared ot their body weight than it does for other land animals, and have concluded that this is a result of various adaptations that increase their mobility under water. Indeed, penguins are exceptionally fast and maneuverable swimmers, and all of their prey (and predators) live underwater. Their knees are usually hidden by their feathers.

More info: neaq.org