If not for the scars that go up and over his kneecaps, little would distinguish Mick Stephens from the other runners, bikers and walkers rounding Lake of the Isle as dawn breaks on a fall morning.

Stephens, a longtime marathoner, is among an increasing number of people who are testing the limits of artificial knees and hips by returning to competitive sports and sometimes to activities that make doctors wince.

For many in a generation hooked on running and recreational sports, artificial joints not only relieve arthritic pain and make daily life easier, they also allow some athletes to get back to doing what they love.

“The baby boom generation wants to maintain the same level of activity that they always have had, particularly the ones who have always been active. They don’t want to give it up,” said Dr. Patrick Horst, an orthopedic surgeon at the University of Minnesota and the Tria Orthopedic Center.

Running, however, is one of the few things he and other surgeons discourage for those with artificial joints.

With two artificial knees, Patty Conlin of St. Peter has heard those warnings. But the 67-year-old who ran 11 marathons before her surgeries, will line up with 13,000 other racers to run the Medtronic TC 10 Mile on Sunday.

“Life is too short,” she said. Running a 13-minute-mile pace, her knees feel just fine, she said. “Life is uncertain. Friends have died. I’m going to run. I love it.”

Dave Daubert, 72, a former 2:30 marathoner who ran more than 50 26.2-mile races before he replaced both knees, now race-walks everything from 10-kilometers to 100-mile ultramarathons. Triathlete Dave Heffernan, of Fridley, is biking, swimming and running after his hip replacement surgery earlier this year, with hopes of competing in a marathon early next year and eventually, another Ironman.

Stephens, who owns a construction business, runs 3 to 5 miles a few times a week around Minneapolis’ lakes, mainly to relieve stress. But as a lifetime entrant in Grandma’s Marathon, he finished two of the races after his first knee was replaced in 2011 and another after his other knee was replaced in 2013. He now runs at a slower pace, which some surgeons say may be less damaging on his knees.

Then there’s Dick Beardsley, a former elite marathoner. He was running up to 50 miles a week before an infection forced the replacement of one of his artificial knees earlier this year. He’s now back running five to six miles every day before the sun rises.

Since his first knee replacement surgery in 2009, Beardsley has fielded questions from others who want advice on running after knee replacement surgery.

“I never tell anyone to go ahead and run,” said Beardsley, who still has the lean build of a competitive marathoner and a quick cadence that makes it look as if he is floating across the ground. “I tell them there are risks and you have to make that choice. It works for me right now.”

‘Stop is a hard sell’

Surgeons replace more than a million hips and knees a year, making those surgeries among the most frequently performed procedures in U.S. hospitals, behind Caesarean sections and circumcisions.

“Baby boomers and people who’ve grown up in the era of joint replacement are less willing to put up with a level of disability that might have been an expectation two generations ago,” said Dr. Daniel Berry, orthopedic surgeon at Mayo Clinic.

Over the past decade, the materials used for knee and hip replacements have improved, making them more durable, while better sizing for knee implants mean they fit better.

But the implants are mechanical, with plastic and metal rubbing against one another, explained Mark Pagnano, orthopedic surgeon at Mayo Clinic. Abusing the implant could mean that it will loosen, wear out or break quicker than it should, requiring a revision.

“It’s a bigger operation, longer recovery, more likely to result in some kind of complication and is less likely to result in excellent pain relief,” Pagnano said.

As a result, most surgeons discourage running or jumping. But they acknowledge the advice is based on opinion, not scientific proof. Short-term studies indicate running on artificial joints might not be a problem for five to seven years, Pagnano said. But no studies have been done over a 10- or 20-year span to show long-term effects.

Die-hard athletes

Most people who replace a knee or a hip have already had to give up high-impact activities because of age and arthritis. So after surgery, most patients merely plan to bike, swim, hike, walk and ski, surgeons say.

But there are always a few who aren’t ready to give up running or competitive sports.

Horst, a surgeon who is also an avid runner, biker and skier, recalled when longtime triathlete Dave Heffernan walked into his office with severe hip arthritis. Horst explained that an artificial hip would improve Heffernan’s quality of life.

“He looked at me and said, ‘I retired a year ago, and my plan is to continue to do Ironmans. That’s what I want to do,’ ” Horst recalled. So he gave him the OK to bike and swim after surgery. It was up to his patient if he wanted to test running.

Heffernan, a 68-year-old Vietnam veteran, has completed dozens of marathons since he started running in his 30s and many triathlons over the last 16 years, including two Ironman races, which involved a 2.4-mile swim, 112-mile bike ride and 26.2-mile run.

Running is part of his identity, so he’s gambling he can continue with his new hip.

“The plastic parts might wear out faster, but I’m OK with that,” Heffernan said, hoping to get at least 10 years out of his artificial hip.

Few athletes, however, are testing the limits of artificial knees more than Beardsley.

When he replaced his right knee with an artificial one in 2009, his surgeon told him “Don’t run.” But the doctor also knew Beardsley wouldn’t heed that, so he advised him to run in good shoes and on softer surfaces.

Beardsley, who’s now a fishing guide and bed-and-breakfast owner in Bemidji, decided it was worth the risk. “When I ran on it, it was like I’m 15 years old. … The right knee is perfect.”

A year later, his left knee, which was mangled in a farming accident decades ago, had to be replaced. He returned to running 65 to 70 miles a week but the left knee never felt as good as the right one, requiring a revision to replace an ill-fitting component, Beardsley said.

When he returned to running, he limited himself to no more than 50 miles a week. “Everything worked out well,” he said. That is, until last spring when an infection from a likely cut on his left foot traveled to his knee and damaged the bone. After another surgery, Beardsley gradually returned to running and plans to run a half marathon in the next few months.

Time will tell whether he has made the right choice.

“If I have to have that left knee redone, that will be it [for running,]” he said. “I don’t want to be 75 years old and have [my wife] push me around in a wheelchair. I have my limits.”