The three women grimace and groan as they lower into their 20th -- and final -- triceps dip.
Then, without pause, they pant through the wet grass at Boxford's Boy Scout Park to a set of metal bleachers with weeds poking through the slats.
Instructor Robin Beardsley gently urges them on, "If you're tired, take it easy," then counts their progress up the steps: "1 . . . 2 . . . 3 . . . 4 . . ."
Suddenly, a shriek -- the staccato, high-pitched kind that can be emitted only by toddlers -- causes her to stop.
She turns to a quartet of strollers parked close by, where the gurgling of small children greets her, and reaches for a container of bubbles.
Behind her, the women continue their exercises: Step up, kick left leg out, step down; step up, kick right leg out, step down.
Beardsley glances back at them. "I'm still looking," she calls with a chuckle as she leans to stroller level and blows a string of bubbles.
It's multitasking with a maternal twist. Mothers say stroller fitness classes allow them to break out of the diaper-feeding-nap routine, socialize with full-sentence-forming adults, and strive to regain their prepregnancy bodies -- without abandoning the little ones.
"I like her to be in sight at all times," 20-year-old Stephanie Cobin of Seabrook, N.H., said of 9-month-old daughter Hannah. "And it's a lot easier to have her out in the fresh air, interacting and having fun."
The mom-baby fitness model is sprouting nationwide: A slew of classes, including Stroller Strides, StrollerFit, Itsy Bitsy Yoga, and Baby Boot Camp , have emerged in the past five years. Locally, there are several new groups: Stroller Strides, which costs $15 a class, in Boxford, Melrose, Topsfield, and Wakefield; and Club Stroll, offered through the Manchester Athletic Club for $10 a class, or $5 for club members.
A survey of fitness providers by the IDEA Health & Fitness Association last year noted the trend: 41 percent of respondents reported a growth in parent-baby fitness classes, while 35 percent said the practice remained steady compared with years past.
Health and fitness professionals are encouraged by the uptick: Exercise, besides toning the body, they say, helps fight postpartum depression, and getting together with other adults creates a social network that combats loneliness.
"Isolation is a big factor with new moms," said Linda Anne L'Abbe, director of the North Shore Birth Center in Beverly and Gloucester.
"It's incredibly important to get out and have contact with people."
The challenge for many mothers is finding the time -- and the motivation -- to do so. Many mothers acknowledge that they're sometimes locked into the repetitive trap of diaper changes, feedings, and nap schedules, especially if they're staying home to care for their child.
"It's hard to make yourself a priority," said 33-year-old Darien Huaman of Salem, a mother of two who joined Stroller Strides to lose 10 pounds and firm up her belly.
"I love them," she said as she glanced over at her girls, Alexandra, 3, and Lucia, 1, playing "Ring Around the Rosie" nearby, "but they're all-consuming."
That's another reason mothers say they like the classes: They provide a chance to vent about the frantic side of motherhood, as well as query each other about teething, crawling, and the many other milestones.
Stroller Strides participants even arrange regular play dates for the toddlers and nights out for the moms.
"It becomes your support center," said Erika Andrews, the instructor of Manchester-by-the-Sea's Club Stroll, which began in May and has enrolled roughly 10 mothers with babies from infancy to age 3.
"You automatically have something in common. A bond very easily forms," she said.
As for the exercise, classes are held in parks or along public walkways, and improvisation is a shared theme. Examples: Benches often become dip machines, bleachers double as stair climbers, and backstops morph into circuit trainers.
And, yes, the action also incorporates babies and strollers. The carriages are used as resistance during lunges and brisk walking, and the little ones can be incorporated as weights for moms while they are crunching or lifting.
Tots are entertained in other creative ways, too, including exercises set to popular children's songs. "The Wheels on the Bus" was part of the soundtrack during the recent Friday morning Stroller Strides class in Boxford.
With Beardsley in the lead, the three participants sang as they mimed the verses with resistance bands. For the "round and round" parts, they gyrated their arms in circles; when it was time for "beep, beep, beep," they cranked out a set of behind-the-back tricep pulls.
The babies, seated in their strollers a few feet away, clapped and sang along in discordant harmony.
But the real favorite was a running exercise called "tickle toes." The women sprinted to various field marks, then ran back and tickled their babies' feet. The activity elicited squeals of laughter; Hannah, Cobin's daughter, pulled on her chubby toes, captivated, as if she was seeing them for the first time.
Throughout the one-hour session, the women also lunged while pushing strollers and wrapped resistance bands around backstops to do presses and flies. Whenever needed, they paused to wipe noses, divvy out bags of snacks, and provide entertainment with plastic pom-poms.
Quirky as the whole thing might sound, the participants say it's a great workout.
Thirty-year-old Kate Ruff, after being skeptical of the likely benefits of the workouts at first, said she feels sore after every Club Stroll class. "I'm expecting some muscle tone after this," said the mother of 10-month-old Charlie.
"I'm starting to get a bikini body back," said stay-at-home mom Cobin, wearing a blue T-shirt and cut-off shorts. A curious but quiet Hannah, in a white dress speckled with blue flowers, rested on her left hip.