Endurance involved 45 minutes of walking or running. The resistance group completed a circuit of eight machine-based exercises, such as back extensions, crunches, pulldowns, seated rowing, seated leg curls and extensions, seated chest presses and lying leg presses.

The interval training group, meanwhile, were asked to perform HIIT according to the “4×4” method: Individuals exerted themselves to their maximum four times for four-minute sessions, punctuated by three-minute segments of lower intensity exercise.

Of the total participants, 124 finished the study. The research team compared the telomere lengths of the volunteers before and after the six-month regime.

Related: Two minutes of high-intensity exercise provides same benefit as 30-minute moderate workout

The study published in the journal European Heart Journal showed HIIT and endurance training lengthened telomeres and boosted telomerase activity. But resistance training didn’t produce the same results.

Researchers were surprised to find there was a “clear difference between endurance training and resistance training with regard to telomere regulation.”

Ioakim Spyridopoulos, a professor of cardiology and cardiovascular gerontology at Newcastle University, in the U.K., told Newsweek: “The most surprising result is that aerobic exercise, but not resistance training, induced telomerase activity.”

He explained this not only leads to longer telomeres, which are “regarded as the clock of life,” but these forms of exercise were also found to have a beneficial effect on inflammation in the body.