When my 2-year-old son asked my 66-year-old mother to go down a slide with him at the playground last fall, I swear I did not hear the request. What I heard was a scream, followed by my mom's voice asking Wes to go get help.

Diagnosis: a broken ankle, requiring surgery, four days in the hospital and three months of recovery.

As Americans live longer and stay healthier into their 60s and beyond, grandparents are under more pressure to keep up with the new kids in their lives. Even before we left the park for the hospital that day, my mom made her No. 1 concern clear: How would she play with her three grandchildren if she couldn't walk?

The oldest members of the baby boom generation are moving into their 60s, and many are grandparents themselves; others rely on their own aging parents to help with child care.

Like my mom, many of today's grandparents are already in good shape when their first grandkids are born. Many more find that they need to be in order to carry those babies and keep up with the toddlers in their lives.

Injuries are common but often preventable.

Maintaining a high level of fitness helps, said Dr. Robert Schoene, a professor at the University of California-San Diego's School of Medicine who has studied the effects of aging on exercise — and has regularly seen injured grandparents.

"People feel that once they get to a certain age they can't train any more," Schoene said. "But barring disease, people at almost any age should be able to do aerobic and strength training."

Sonja Hermann, a 67-year-old grandmother to 3-year-old Patrick, was put to the test when she spent last summer with her daughter's family in Bend, Ore., and nannied full-time.

"I lost weight," she said, "which is not hard to do running after a 2-year-old."

At the beginning of the summer, when she realized the stamina that would be required, she started a walking regimen, pushing Patrick in a jogging stroller up a hill to view an osprey nest.

"By the time I got to the top the first time, I was panting," she said. "I thought, if I don't have a heart attack, I'm going to make it up here every day."

By the end of the summer, she was walking up the hill easily, she had dropped a dress size and her back no longer hurt when she bent over.

When she returned home to Fayetteville, Pa., a stress test showed that her walking had helped, especially good news since heart attacks and strokes run in her family.

"I feel twice as young when I'm with Patrick as I do when I'm sitting at home watching my soap opera," she said. "Being around young people gives me more energy."

Hermann's sensible buildup is exactly the sort of training Schoene recommends.