Men who want to keep their bones strong may want to add running to their exercise routine, new research suggests.

In a study of 42 athletic men ages 19 to 45, researchers found that running seemed to have even bigger benefits for bone mass than strength training did. Both runners and weight trainers had greater bone density in the spine compared with road cyclists, but much of the benefit in weight trainers seemed to stem from their greater muscle mass.

In contrast, running appeared to build bone density regardless of the men's muscle mass.

"The results of the study confirm that both resistance training and high-impact endurance activities increase bone mineral density," senior researcher Pamela S. Hinton, an assistant professor at the University of Missouri in Columbia, said in a news release from the university.

"However," she added, "high-impact sports, like running, appear to have a greater beneficial effect."

Hinton and her colleagues report the findings in the Journal of Strength and Conditioning Research.

Bone is living tissue that reacts to exercise by becoming stronger.

Exercise that forces the body to work against gravity — like running, jumping and weight training — is most effective. In contrast, low- impact activities, such as cycling or swimming, put relatively little stress on the bones.

In this study, cyclists generally had the lowest bone density at all body sites measured. That sports-related difference did diminish once Hinton's team factored in the men's muscle mass; in general, as muscle mass — or body weight — increases, bone mass does as well.

However, even with muscle mass considered, weight trainers and runners still had greater bone density in the spine. And the effects of running appeared to be independent of muscle mass.

Hinton recommended that athletes involved in low-impact sports like swimming, cycling and rowing add weight training or high-impact activities to their workouts. It is key, she noted, to target muscles throughout the body.

"Exercise programs to increase bone strength should be designed using what is known about how bones respond to exercise," Hinton said. "Only the skeletal sites that experience increased stress from exercise will become stronger."