"Gravity biking" is a popular extreme sport in the United States, involving riding specially-adapted pushbikes down steep roads.

An even more extreme version has become popular among young men in poor neighbourhoods of Medellín, Colombia, who risk their lives on the mountain city's vertiginous highways, usually without helmets or protective clothing.

Marlon has lost two friends to crashes and recently nearly died himself. He faces a dilemma: whether or not to continue following a passion he knows could kill him.

Video journalist: Megan Janetsky. Producers: Eleanor Layhe, Simon Maybin and Jorge Caraballo.