The Flash's Grant Gustin isn't actually running when his character uses super speed on the show. On an episode of Conan, he revealed that he only uses his arms to mimic the motion of running for those super speed shots.
During the episode, Conan O'Brien pointed out that the series never shows Gustin's feet in the super speed shots, prompting him to ask if Gustin is actually running in those scenes. "We used to," Gustin said. "You know, Season 1, we'd put me on a treadmill with a harness and then I realized, on the show, that it was always like a waster shot and that my legs weren't in frame, so they had me running for no reason whatsoever."
"So now they frame it out and I'm literally just moving like that," he added, demonstrating. "It's all about the arms." Fellow guest Dax Shepard also got in on the action, "giving chase" to Gustin as he showed off his super speed skills.
When O'Brien gave it a whirl, he commented that it "doesn't look good" -- but Gustin assured him that, "You look good! You look fast!"
Airing Tuesdays at 8 p.m. ET/PT on The CW, The Flash stars Grant Gustin as Barry Allen, Carlos Valdes as Cisco Ramon, Danielle Panabaker as Caitlin Snow, Candice Patton as Iris West, Jesse L. Martin as Joe West, and Tom Cavanagh as Harry Wells, with guest appearances from Keiynan Lonsdale as Wally West and Hartley Sawyer as the Elongated Man.