In one possible future, Impulse and his friend Carol Bucklen have travelled to the 30th century before the early death of Impulse's mentor Max Mercury. In that time, Carol began researching the speed force to find a way to accelerate the human metabolism, improving health (and speed) for everyone on Earth. Early tests were partially successful, granting test subjects super-speed...and an unexpected degree of suggestibility. Earthgov President Thawne quickly appropriated the technique to create his own cadre of super-fast soldiers: the Hyperguard!
Unfortunately, long-term effects of the ray included complete physical and mental breakdown. Most of the Hyperguards went insane, but enough of them remained to do Thawne’s bidding... which included turning the hyper-ray on the entire population of Earth.
This future was averted by the present-day Impulse when he gained time-travel powers of his own and prevented Max Mercury's death, cutting off the timeline in which he and Carol would travel to the future.