Researchers in the US believe they have made a significant breakthrough in unravelling the mystery of why muscles become less sore after regular visits to the gym.

Scientists have long been baffled as to why post-workout muscle soreness gets easier after each gym visit – known as the ‘repeated bout effect.’ It was known that the immune system plays a part in helping muscles repair themselves but little was understood as to how this occurrs.

But now, exercise science researchers at Brigham Young University (BYU) in Utah have produced evidence that shows for the first time the surprising presence of very specific immune workers: T-cells. In the same way that regular training helps build muscle mass, it also appears to help build the immune system’s ability to stave off muscle soreness.

"You think of T-cells as responding to infections, not repairing muscles – but we found a significant accumulation of T-cells infiltrating damaged muscle fibers," said Robert Hyldahl, assistant professor of exercise science at BYU.

"Our study is the first to show T-cells present in human muscle in response to exercise-induced damage."

The research appears this month in Frontiers in Physiology and builds off past studies that implicate immune cells in muscle healing.

The presence of the T-cells suggests that muscles become more effective at recruiting immune cells following a second bout of exercise and that these cells may facilitate accelerated repair. In other words, the muscle seems to remember the damage from a workout and reacts similarly to when the immune system responds to antigens – toxins, bacteria or viruses.

Combating muscle soreness has long been an area of interest for exercise professionals, with past research having shown it can even cause people to put off gym visits.