Don't ask guys about the state of their knees. Because they'll tell you. And if their stories aren't full of minute descriptions of aches, creaks, lacerations, buckling, and "strange clicking sounds," they'll go on about visits to physical therapists, acupuncturists, rheumatologists, orthopedic surgeons, and outpatient clinics. Other body parts may preoccupy younger men, but by early middle age, it's the knees.

And with good reason. Knee complaints are nearly ubiquitous, even among those who are otherwise healthy and active. Hundreds of thousands of runners are sidelined every year by "runner's knee" – pain behind the kneecap, mostly from overuse – while about 80,000 people annually injure an anterior cruciate ligament (ACL), the tiny band of tissue that provides crucial knee stability. More than 14 million Americans suffer from arthritic knees, and some 700,000 will undergo knee-replacement surgery this year, which is double the rate of a decade ago, with the largest increase occurring among people age 64 or younger.

In other words, if your knees are imperfect, you're in fine company: Tiger Woods, Rafael Nadal, Michael Jordan, and Tom Brady all complain of rickety joints. But what can you do at this point in your life to ensure that your knees, whatever their current condition, become and remain as healthy as possible?

Run to prevent future problems
One of the big surprises of recent knee science is that regular and vigorous physical activity, including distance running, seems to be good for healthy knees, despite what well-meaning friends may have told you. A resonant 2011 review of earlier research found that regular physical activity, especially running or playing sports, affected people's knees negatively at first glance: Active people had more bone spurs usually considered an indication of knee dysfunction – than the couch-bound.

Yet those in the fit group rarely suffered from the joint-space narrowing that normally accompanies bone spurs – when your knees' cushioning cartilage wears away, and your bones begin closing in on one another, threatening to cause painful bone-on-bone arthritis. In fact, in most reviewed studies, runners and other active people have greater cartilage volume and are at lower risk of developing knee arthritis than their sedentary counterparts.

What the review's findings show, says Dr. Flavia Cicuttini, a professor at Monash University in Australia and study co-author, is that knees adjust to the forces created during activity, in part, it seems, by sprouting adaptive bone spurs. Overall, "physical activity is good for joints," she says, but there is a caveat: Vigorous physical activity, particularly running, will exacerbate damage if you already have a knee problem like a torn ACL or meniscus (the pillows of cartilage that cushion the thigh bone and shin bone) or if you regularly have pain or swelling in your knee, especially after exercise, which can be a symptom of early arthritis. "Activity on an already diseased joint is what is bad for the knee," says Cicuttini.

Cycle or swim to slow old injuries
If you have a history of serious knee injury, particularly an ACL tear, now would be a fine time to invent a time machine. "The best thing that you can possibly do for the health of your knees is to never injure them," says Joe Hart, an athletic trainer and assistant professor of kinesiology at the University of Virginia. Past knee injuries typically result in slow but continuous degeneration of the knee, he says, for which there is no certain cure (although stem-cell research is promising, as are some types of cartilage-replacement surgery). Just why past injuries reverberate into the future isn't completely clear, although scientists have found that cartilage often weakens or dies after injuries, even far from the site of the original tissue tear or strain. Injured knees also often become slightly misaligned or unstable, and cartilage begins to deteriorate. When cartilage thins or disappears, you have arthritis. "It's almost inevitable, unfortunately, in my experience, that signs of arthritis follow" serious knee injuries within a decade or so, Hart says.

To slow the onslaught, he advises those with a history of injury to avoid weight-bearing activities, such as running, as well as sports that involve side-cutting and sudden stops, including basketball and tennis. Instead, try cycling or swimming (but avoid breaststroke, which stresses knees). These activities can help keep knee cartilage healthy, says Hart, without causing the repetitive rubbing that contributes to arthritis. If you take up cycling, have your bike correctly fitted. Riding with your seat too high, low, forward, or back can cause imbalances and instability that can lead to knee pain over time. Ride in a gear that allows you to keep your rpms between 70 and 100 – pedaling slower with more resistance may cause knee pain.

Squat and lunge for lifelong health
Whatever your injury history, "leg-muscle strength is extremely important," Hart says. The muscles in the upper and lower leg stabilize the knee joint and absorb some of the stress that otherwise reverberates through cartilage. The requisite protective muscles include not only the big, obvious quadriceps, hamstring, and gluteus muscles, but also the seemingly insignificant psoas (which wraps around your abdomen to connect your spine and thigh bone) and other often-overlooked muscles like the hip flexors that attach to the hip and, indirectly, to the knee. Recent studies have found that weak hip stabilizers can cause runner's knee, for instance.

So squat and lunge, Hart says. You don't need equipment for these exercises and might, in fact, be better off avoiding weight machines. "It's good to work on balance as well as strength, which doesn't happen when you use machines," he says. You also want symmetrical muscle strength: Imbalances between legs put undue stress on one knee or the other. To check your symmetry, place two bathroom scales side by side. Stand with one leg on each and squat. The subsequent weight reading should be the same on each scale. If not, concentrate on building strength on your weaker side. Try the scale test, too, after any leg workout. "Fatigue creates its own imbalances" and adds to injury risk, says Hart. If you are asymmetrical when tired, work on building your endurance, as well as leg strength.

The time and sweat you expend on strengthening your knees return ineffable benefits. "Without healthy knees, you can't live a full, independent life," says Hart. You move less and more slowly. Your physical boundaries and capabilities shrink. "But if your knees are in good shape," he concludes, "life is better."

Related links:
The Way to Build a Better Knee