Endurance and high-intensity interval training are the key to aging gracefully, according to new research.

A study published Tuesday in the European Heart Journal found that exercises such as running, swimming and biking are better for aging than strength training with weights.

Researchers analyzed the effects of three exercises – endurance training, high-intensity interval training and resistance training – on how cells in the human body age. They examined 124 individuals who had previously been inactive but were healthy overall and randomly assigned participants to one of the three methods of exercise.

The endurance training consisted of continuous running. High-intensity interval training included a warmup, four bursts of alternating intense and slow running and a cool-down. Resistance exercises entailed circuit training on eight machines working to strengthen abs, the back, arms and legs. Each person completed three 45-minute sessions per week. A fourth group of participants continued to lead an inactive life.

Researchers discovered that both endurance and high-intensity training slowed cellular aging and in some cases even reversed aging, but resistance training did not. This was observed by measuring the participants' telomeres – structures at the end of chromosomes in DNA that protect the chromosomes from deteriorating.

According to the study, telomere length is "one of the major determinants of the cells' capability to divide and function." As people grow older, the protective structure begins to deteriorate. When the telomere can no longer protect the chromosomal DNA, cells die.

Diminishing telomeres is regulated by several proteins, including an enzyme called telomerase that can counteract telomere shortening and even lengthen the protective structures.

Researchers analyzed the length of participants' telomeres and telomerase activity at the beginning and end of the study six months later. It was discovered that, compared to the control group and people who underwent resistance training, telomerase activity increased twofold to threefold, and the length of telomeres increased significantly in people who participated in endurance and high-intensity exercise.

Researchers hypothesize that endurance and high-intensity exercise benefit telomeres because they affect levels of nitric oxide in blood vessels, which contributes to changes in cells.

Co-author Dr. Christian Werner of Saarland University in Germany said in the press release that the study implies that resistance training should not be used in place of endurance training, rather it should be a complementary form of exercise. The results can also be used to guide exercise recommendations for individuals.

"The data identify telomerase activity and telomere length as sensitive ways to measure at cell level the effects of different forms of exercise," Werner said. "Using these measurements to guide training recommendations for individuals may improve both adherence to and efficacy of exercise training programs in preventing cardiovascular disease."