Watching these Atlas robots, you can see how important it is for them to keep their knees bent for balance stability and agility.
See the full article by Boston Dynamics at https://blog.bostondynamics.com/atlas-leaps-bounds-and-backflips.
Thumbnail image by DARPA - Website / image, Public Domain, https://commons.wikimedia.org/w/index.php?curid=27287089