I think there's also some other physiological action that trigger defecation as a response during long duration high intensity exercise like marathons and bike races.

Edit: here

"It’s related to the fact that during periods of physical stress, the body shunts blood away from organs that are not necessarily critical at that moment,” Michael Dobson, D.O., a colon and rectal surgeon with Novant Health in Charlotte, North Carolina, tells Mental Floss.

https://www.mentalfloss.com/article/648541/why-marathon-runn...