Running can help keep you young. That's the conclusion of a study that looked at the effects of running on people over 65.

It's not that walking doesn't have health benefits. Its physical benefits include helping to fend off heart disease, diabetes and weight gain. Walking also provides mental benefits such as helping to fend off depression.

But one benefit doesn't seem to come from walking, only from running.

The difference in energy use appears not to lie not in the mechanics of running, but runners' muscle cells.

The researchers found that older people who regularly ran for exercise used 7-10% less energy while walking than older people who regularly walked for exercise. But the surprise came when both were compared to younger people. Older runners were nearly as energy efficient while walking as some people in their 20s were. Older walkers were not, needing visibly more energy to do the same amount of walking.

Apparently, walking is no sweat when you're used to running.

The study looked at 30 volunteers, average age 69. Half were male, half female. All either walked or ran regularly for exercise, at least 90 minutes a week in the previous six months. They walked on a treadmill in the laboratory at three different speeds — about 1.5, 3 and 4 miles per hour. Measurements were made of their oxygen consumption and carbon dioxide production to see how much energy they were using up.

The frequent walkers were 7 to 10% less energy-efficient than the frequent runners — they needed more energy to accomplish the same amount of work.

When the researchers compared this result to findings from an earlier study they were surprised. In their prior study they had monitored energy usage while walking for sedentary seniors and sedentary people in their 20s and found that the seniors were using 14-23% more energy.

Combining the results from the two studies, they noted that older walkers and older sedentary people showed nearly identical energy usage and were clearly the least efficient walkers. The younger people were the most efficient, but older runners were right behind, almost as energy efficient as people in their 20s.

There weren't many physical or mechanical differences in the way older walkers and runners were walking on the treadmill, other than the fact that the runners were taking slightly shorter strides than the walkers.

The researchers suspect that the reason behind the difference in energy use lies not in mechanics but inside muscle cells. Cellular organelles called mitochondria are prime candidates. They're the cell's powerhouse, responsible for supplying most of a cell's energy needs. And no tissue needs as much energy as muscle tissue does.

Mitochondria are the focus of many aging studies because older people's mitochondria show many differences from those of younger people. The study appears in PLOS ONE.

In more good news for runners, there appears to be no link between running and increased knee arthritis, according to another study presented recently at the American College of Rheumatology's 2014 Annual Meeting. The authors reviewed data from more than 2,000 people, nearly 30% of whom had run extensively at some point in their life. Previous studies suggesting a link between runners and knee arthritis had focused on professional male runners.

Using both knee X-rays and knee pain assessments, the study found no indication that the runners were likelier to develop knee arthritis. In fact, the authors found a lower incidence of knee pain and knee arthritis among the runners, regardless of the age at which they had run.