They charge up and down the basketball court at your local gym, playing for league bragging rights. They slug it out for the upper rungs of town tennis ladders. They ride wheel-to-wheel, Spandex-clad, in bike races.
Many have salt-and-pepper hair, or a thickness around their middles.
They are hardcore baby-boomer athletes, the would-be Navy Seals of the neighborhood. And in this year when their generation's trailing edge turns 40 - and the Olympic Games showcase athletic prowess - some are pushing for a level of physical performance not historically associated with nonprofessional athletes, let alone folks bearing down on retirement.
Health experts hail the broad adult-fitness trend - hardly a new one - as a sign that more men and women simply reject the personal limitations often associated with aging.
But some also cite the emergence of an intensely competitive boomer component - one whose full-throttle approach can verge toward self-destructiveness.
"Anything that you're passionate about, I think, is good if it's not hurting someone else. But it can hurt you" if taken too far, says David Haber, associate director of the Institute for Wellness and Gerontology at Ball State University in Muncie, Ind.
Nudged by his wife a few years ago, Mr. Haber, now in his late 50s, traded competitive basketball for jogging and some weight lifting. He relished the cameraderie and the fitness aspect of basketball, he says. "What surprised me most was the passion for the competition."
That passion is not exclusive to men. Soccer moms have strapped on shinguards, for example, and begun forming leagues of their own.
Still, where many women and some men seem to understand the value of taking a holistic approach to health - eating well, adopting such low-impact activities as yoga and swimming - this subset seems to run largely on testosterone.
"It's mostly a guy thing," says Nicholas DiNubile, an orthopedic surgeon who works with the Philadelphia 76ers and the Pennsylvania Ballet. Middle-aged men tend to push for quick, visible results, he says, pumping up their upper bodies, neglecting to stretch, misusing equipment.
"At the gym, women will get a trainer to show them something, where guys will say 'I'll do it the way I did in high school,' which is probably [the wrong way]," says Dr. DiNubile, who is credited with having coined the term "boomeritus" to describe the aches he says that improper exercise can impose on mature frames.
A special 2000 study of injuries to boomer-age participants in a handful of popular sports showed a 33 percent increase over a seven-year period, says the Consumer Product Safety Commission.
However safely they manage their regimens today, younger boomers are committed to sports as a filler of leisure time, experts say. And as boomers head toward the home stretches of their careers, that collective commitment is likely to deepen.
As they gain self-directed work arrangements, and more free time, boomers are beginning to swell new waves of "leisure types," says Laurel Hayden, research director for the Leisure Trends Group in Boulder, Colo. "What happens with boomers," she says, "is we see more achievers and more adventurers" - and more ardent competitors.
Experts offer explanations that range from the predictable - staving off old age, flexing in the face of the active Gen Xers behind them - to the provocative.
Challenged in the workplace, boomer men may be looking for a way to pull ahead of women, says Jay Coakley, professor of sociology at the University of Colorado at Colorado Springs and author of "Sport in Society." Or, in an era of middle-class, middle-age parity, they may view physical toughness as a way to stand out.
"Men have found that buying toys like big SUVs are not really a means of distinguishing self," Mr. Coakley writes in an e-mail. "So they seek activities that enable them to distinguish themselves from each other ... by being physically talented or putting their bodies on the line."
The trend is not limited to contact sports. Masters-class (over 40) running races have seen increasingly fast times. The United States Tennis Association reports that among league players making it to the organization's national championships, the percentage of those in the 40 to 49 age group is the only one that increased between 2002 and last year.
Ball State's Haber sees the creep of competitiveness showing up in some surprising places.
"I just read where they're turning yoga into a competitive sport," he says, "and actually having contests comparing people in their ability to do yoga positions."
Some experts suggest that leading-edge boomers - already in high school when trailing-edge boomers were born - could have a tempering effect if they offer models of self-restraint.
Haber is ready to serve. "I don't really do sports anymore," he says with a laugh, because he has trouble shutting off the competitive switch.
"I don't have a good [control] on that ambition of showing other people that I can do really well," he says, "even though I'm almost 60."