LONDON — The United States and Jamaica made its final baton exchanges in the men’s 4 x 100 meter relay virtually even. The anchor legs gripped the sticks in full stride and dashed toward the line, a blur of red, yellow and green. The Olympic Stadium crowd unleashed a deafening roar in anticipation of a thrilling finish.

During any other era in track and field history, the US would have been running for gold. But this is the Usain Bolt era.