Katarzyna Massie, Rita Nirola and Connie Winternitz attend a Bone Builders class at Potomac Community Methodist Church. (Sue Schick)

It doesn’t take much more than a chair, a set of dumbbells and an occasional ankle weight to make Pat Miller, 83, feel strong.

Twice a week, Miller heads to Potomac Community Methodist Church in Maryland, where she spends an hour doing resistance and weight-bearing exercises designed to strengthen her bones. Sometimes that means balancing on one leg while slowly lifting the other with weights strapped to her ankle, or lowering herself into a chair squat.

The aptly named Bone Builders class has become a staple in Miller’s fitness routine, one that for decades solely consisted of taking and teaching yoga classes. That all changed a few years back when Miller was diagnosed with osteopenia, a condition that softens the bone and puts her at greater risk for developing osteoporosis.

Her doctor recommended Miller integrate some resistance training into her workouts, and she did for a time, but she lost interest until a friend encouraged her to sign up for Bone Builders, a free class sponsored by the Montgomery County Department of Health and Human Services for ages 55 and up.

“What I notice is, I have more energy when I’m doing the classes on a regular basis,” Miller said. “It makes a difference. I haven’t had any fractures in years.”

Bone Builders is one of a number of fitness programs specifically designed to improve bone density and combat osteoporosis. The disease, which causes bones that have lost density or mass to become susceptible to breaking, affects the lives of an estimated 54 million Americans, according to the National Osteoporosis Foundation. It can occur in men and women at any age but is most common in older women, especially after menopause.

Any exercise in which your bones are working against gravity to support your body weight — jogging, walking, push-ups — can strengthen bones. Physical force applied to bones causes them to work to withstand the pressure, said Pete McCall, a certified trainer with the American Council on Exercise.

“People in their 70s and 80s can still add muscle and improve bone density through strength training,” he said. “It’s a pretty important component of staying fit during the aging process.”

There are a wide variety of programs working to prevent or reduce the risk of bone fractures. There are self-paced online programs, such as Strong Bones by Miriam Nelson, that guide users through exercises with animation, and programs that focus more on weight-bearing moves with an emphasis on balance, such as Matthew Taylor’s Safe Yoga for Bone Health webinar.

Many programs are run by state or city agencies, including Bone Builders and Project Healthy Bones, from the New Jersey Department of Human Services’ aging-services division. The Retired and Senior Volunteer Program, an offshoot of the federal Senior Corps, partners with government or community groups to run free Bone Builders classes throughout New England, the Midwest and the Mid-Atlantic.

Phyllis Elperin started out as a volunteer teacher with Montgomery County’s Bone Builders program six years ago before becoming the director. The county has offered the program for about a decade and now hosts 32 classes at churches, community centers, libraries and other locations in the area.

“The greatest challenge in these classes is watching, if you’re seeing people registering pain on their faces, getting to them and finding out are you okay,” Elperin said. “It is a senior population. You don’t want to take any chances.”

Each class is manned by two teachers to make sure everyone is doing the exercises safely. Elperin said there are typically 15 to 25 people in a class. You must register to attend. Slots fill up quickly, and there’s often a wait list.

Miller was on the wait list for a few weeks before she got into a class, a wait she said was well worth the time. Despite her years of yoga training, with all of its planks and chaturanga push-ups, Miller found the upper-body exercises in her Bone Builders class to be difficult. Biceps curls and overhead triceps presses initially threw her for a loop, but in time she got the hang of it.

“Phyllis will tell us, of course, that if you’re finding there’s not any difficulty, there not an effort being made, then you need to add weight,” Miller said. “I have added weight, but not as much as I could.”

Elperin said the routines are centered around slow, methodical movements. Each exercise is usually done for two sets of eight repetitions, using light to moderate weight, if any weight at all. Participants are encouraged to gradually increase the weight but not to overdo it. Elperin stressed that every move can be modified.

“Slower repetitions, longer periods of raising and lowering the weights, work the muscles in such a way that the bones are triggered,” Elperin said.

A recent study in the Journal of Sports Medicine and Physical Fitness found that low-weight, high-repetition resistance training is especially effective for increasing bone mineral density in adults. Participants in the study experienced up to 8 percent bone density increases in their arms, pelvis, spine and legs.

Paula Burnett of the University of Maine’s Center on Aging recommends seniors get clearance from their doctors before participating in any program. Observe the class before joining, she advised, and make sure the facilities are safe, meaning sturdy chairs and no slippery floors.

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For more information or to sign up for Montgomery County’s Bone Builders program, call 240-773-8268 or go to apm.activecommunities.com/montgomerycounty.