Using the contents of some tweets from Trent Stellingwerff and Steve Magness, Canadian Running has an interesting article on how stiffness leads to better running economy. Scientific studies have shown that stiff runners have better economy and the older you get the stiffer you get so that may be why Kipchoge is so good approaching 40.
“As you get older, your tendons get stiffer, I think this is partly why we’re seeing masters marathoners run so quickly. Think of Malindi Elmore, or even Eliud Kipchoge. They’re getting older but their running economy is getting better and they’re running faster.”