Aerobic activities like jogging and interval training can make our cells biologically younger, according to a noteworthy new experiment. Weight training may not have the same effect, the study found, raising interesting questions about how various types of exercise affect us at a microscopic level and whether the differences should perhaps influence how we choose to move.

There is mounting and rousing evidence that being physically active affects how we age, with older people who exercise typically being healthier, more fit, better muscled and less likely to develop a variety of diseases and disabilities than their sedentary peers. But precisely how, at an interior, molecular level, exercise might be keeping us youthful has not been altogether clear. Past studies have shown that exercise alters the workings of many genes, as well as the immune system, muscle-repair mechanisms and many other systems within the body.

Some researchers have speculated that the most pervasive anti-aging effects of exercise may occur at the tips of our chromosomes, which are capped with tiny bits of matter known as telomeres. Telomeres seem to protect our DNA from damage during cell division but, unfortunately, shorten and fray as a cell ages. At some point, they no longer safeguard our DNA, and the cell becomes frail and inactive or dies.

Many scientists believe that telomere length is a useful measure of a cell’s functional age.

But researchers also have found that telomeres are mutable. They can be lengthened or shortened by lifestyle, including exercise. A 2009 study, for instance, found that middle-aged competitive runners tended to have much longer telomeres than inactive people of the same age. Their telomeres were, in fact, almost as lengthy of those of healthy, young people. But that study was associational; it showed only that older people who ran also were people with extended telomeres, not that the exercise necessarily caused that desirable condition.