Hips
I'm fairly sure it all comes from the hips. It takes incredible hip flexor/quad flexibility to attain that sort of knee flexion and hip extension for such a prolonged period of time. Even during the marathon he was maintaining it.
Try standing up and raising your heel towards your butt. Don't cheat by bring your knee forward. Most people will barely get beyond 90 degrees of knee bend unless they reach back and grab their foot.
Google 'the couch stretch' by Kelly Starrett for a great hip opening stretch.