What is it exactly about flying a Flyboard across the English channel that made Zapata's thighs burn? CNN's French inventor makes 'beautiful' flight across Channel on hoverboard reports that:


  "French inventor Franky Zapata has successfully crossed the Channel on a jet-powered hoverboard for the first time, after a failed attempt last month.


Later in the article it says:


  The inventor said that he tried to "take pleasure in not thinking about the pain," even though "his thighs were burning."


What is it exactly about flying a Flyboard across the English channel that made Zapata's thighs burn? 20 minutes of standing shouldn't be that stressing, what is it exactly about flying this board that requires so much muscle activity that it would be painful?
He's using the angle of his legs to control the attitude of the platform (and as a result, the direction he's flying in). So he can't take a relaxed pose, he's standing with his knees slightly bent the whole time.