Remember last week when Kendrick Lamar performed a rather impromptu mobile concert on the back of a moving truck in Los Angeles? In case you didn't know, the event was filmed by Reebok for its new ad campaign "I Am", in which Lamar stars. The rapper sported a slick grey tracksuit and performed a couple of numbers to the delight of the crowd of runners who followed him. Watch the Reebok ad below.