A grizzly sprints at 35 miles an hour. The fastest human who ever lived, Usain Bolt, peaked at 27.8. No person alive can outrun one, which is why every wildlife agency gives the same instruction: do not run.Running also flips a switch in the animal. Bears evolved to chase things that flee, so a sprinting human starts to look like prey. The thing that actually stops a charge is bear spray. Two researchers, Tom Smith and Stephen Herrero, went through every recorded Alaska bear encounter from 1985