1. AGE
As you grow older, circulation and muscles both tend to get weaker. In your joints, lubricating synovial fluid decreases and dries up. Soft tissue becomes less elastic and more susceptible to micro-tears; cartilage gets brittle and begins to

Q&A: Should I take glucosamine?

Yes, if you're suffering pain from osteoarthritis or are at risk for it. Both glucosamine and chondroitin, each found naturally in cartilage, help make the tissue shock-resistant. A recent National Institutes of Health–sponsored study determined that these supplements can help people with arthritis symptoms. Recommended daily dosage: 1,500mg of glucosamine; 800mg to 1,200mg of chondroitin. Both are available over the counter.

flake away. The meniscuses degenerate and lack the capacity to rebuild—a problem exacerbated by weight gain, which increases joint stress. As early as your teens, deterioration of your articular cartilage—a precursor of osteoarthritis—can set in.

2. BIOMECHANICS
Bowed legs, flat feet, pronating ankles, knock-knees, recurring injuries—any number of things can throw off proper alignment of your joints, and in time even minor tracking deviations can result in major problems. The kinetic chain that links your feet to your hips is highly interdependent: If one part veers off, the entire system can break down.

3. MUSCLE IMBALANCE AND OVERUSE
Simply put, too much of one sport or exercise creates disproportionate musculature. Cyclists, for example, often develop quads that are far stronger than corresponding hamstrings. This may be great for hammering your pals on Saturday morning, but it can leave the knee susceptible to injury, because underdeveloped muscles can't counter the force of stronger ones. Devotees of high-impact activities, like running and most ball sports, also increase their chances of knee problems if they don't mix in low-impact alternatives such as swimming and cycling.