We can't look at just the side-to-side movement, ... but in terms of the combined side-to-side and forward movement, penguins conserved up to 80 percent of the mechanical energy required for walking. This was quite high, because other measurements for humans and other birds have found that they recovered only up to 70 percent of the mechanical energy.  -   Tim Griffin     Quotes
We can't look at just the side-to-side movement, ... but in terms of the combined side-to-side and forward movement, penguins conserved up to 80 percent of the mechanical energy required for walking. This was quite high, because other measurements for humans and other birds have found that they recovered only up to 70 percent of the mechanical energy. Tim Griffin