Five miles into her treadmill workout, Maddy Stamey’s lungs start to ache. With every stride, the burn in her thighs intensifies and her determination ebbs.

“Give it up,” her mind is telling her.

Then he appears.

“If you don’t finish those 6 miles, I’m going to make you start all over,” he warns, flashing her a stern look.

If it had been anyone else, the foreboding words might have backfired, prodding her to quit. But when it’s your teenage son, who has become your workout buddy during the past few months, it inspires a mother for many reasons.

“I call my son my own personal trainer,” said Stamey, who has watched Matt, 15, build muscle and confidence since the Lakewood, Colo., family made exercise a group affair. “I’m really proud of him.”

The Stameys are among a growing number of families combining workouts and family outings in an age of skyrocketing obesity rates and time-strapped schedules, fitness experts say.

“I think that having a healthier lifestyle is important to everyone right now,” said Kathy Boyle, director of recreation for the Jewish Community Center Sports and Fitness Center in Denver. “And there’s so much going on in our lives that people are constantly looking for a way to bring the family together.”

Melding the two goals seems natural, said Boyle, whose center has responded by offering classes ranging from baby yoga to family tae kwon do.

The rewards are many. Family members tend to support each other when they work out together, leading to better results, fitness experts say. Parents say they like to watch their kids build confidence and hone sports performance. And exercising together is a healthy way to bond, foster a lifelong habit and bring a fitness focus into the home, they say.

The entire fitness industry — from health clubs and fitness studios to YMCAs and recreation centers — has rallied around the trend.

“We’re trying to create a situation where everyone can work out together, and it’s not just parents dropping off the kids,” said Gary Hultberg, recreation manager for the Englewood (Colo.) Recreation Center, which is undergoing a $13 million renovation.

When families work out together, they have greater success in reaching fitness goals, said Robert Raymond, owner of RR Personal Training in Littleton, Colo., who encourages all his families to exercise as a group. “They offer each other support and motivation,” Raymond said.

Eric Marine, a Lakewood father who joined a workout program with his son a year ago, is an example: He has dropped 20 pounds and decreased his body fat from 24.5 percent to 16 percent, something he isn’t sure he could have done himself.

“It’s great incentive,” said Marine, 45, who works out with son Nick, 16, at Fitness Evolution, a personal training studio in Lakewood that recently introduced a family program. “There’s a push for each of us to show up.”

Nick, like Matt Stamey, has become the lead motivator, not only for workouts but also for diet, Marine said.

“He’s really funny. He’s like the parent now. He’ll say, ‘Are you sure you really want to eat that?'” Marine said.

The Stamey family nearly always meets its four-times-a-week workout goal at the Carmody Recreation Center in Lakewood, Maddy Stamey said. She’s lost 23 pounds in six months. “I feel better when I exercise,” she said.