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There's a joke I heard when I first started studying arthritis: A 90 year-old man sees his doctor complaining of pain in his knee. His doctor asks him, "What do you expect? You're 90 years-old." The old man replies, "Yes, but my other knee is also 90 and it feels fine."

That pretty much sums up why "wear and tear" arthritis is not an accurate description of osteoarthritis (OA), or degenerative joint disease. Just because you live a long time, doesn't mean you'll inevitably wear out your joints.

Why Your Joints Aren't Like Tires

Degenerative joint disease is more common with aging, so it may seem logical to assume osteoarthritis is due to "wear and tear." It's also true that extreme or unusual stress on joints can cause damage that leads to osteoarthritis. But for most people, arthritis due to normal wear and tear is a myth. Your joints aren't like a car's tires or a light bulb that inevitably wears out over time with enough use.

Osteoarthritis occurs when the smooth, shiny cartilage that lines the joint deteriorates. In some cases, this can happen after an injury but for most people it's a result of aging and genetics. The most common locations for osteoarthritis are these:

  • Knees
  • Hips
  • Finger and toe joints
  • Upper and lower spine

One thing we know for sure about osteoarthritis — it's very common. An estimated 21 million people in the United States alone have OA. If you're fortunate enough to live to age 75 or older, chances are perhaps as high as 70% to 90% that you'll have OA in at least some joints. With these numbers staring at you, it's reasonable to wonder if there's something you can do to prevent it. Specifically, should you limit your activities so you don't "wear out" your joints?

Is Exercise the Culprit?

For most people, exercise is a good thing. But at what point does exercise do more harm to joints than good? Are running and jogging actually harmful for the knees or other weight-bearing joints? Does exercising too long or too intensely damage the joints?

Clearly, running stresses the weight-bearing joints. In fact, up to 5 to 7 times your weight is supported by your knees while jogging. Weight-bearing joints are subject to even higher stresses with jumping or suddenly starting and stopping over and over. Although the research is mixed, long-term runners are not clearly more likely to wear out their weight-bearing joints than people who are sedentary.

For example, a large study of runners published in 1998 found that over a nine-year period, members of a running club ages 50 and older had no higher incidence of OA than an otherwise similar group of non-runners. A more recent study came to a similar conclusion. Runners averaging 3.5 miles of roadwork each day had 25% less musculoskeletal pain than those averaging just 2 miles each week.

A little soreness might be worth suffering through if you enjoy running (or other physical activity) and want the cardiovascular benefits. But sudden, severe injury (a fracture or ligament tear) from repetitive use can cause cartilage damage that can lead to osteoarthritis even though it may take years for it to develop.

That's why experts recommend that you avoid sudden increases in how far or how fast you run so that you don't develop a stress fracture. Here's what happens. The repeated pounding of running causes a bone in the foot or leg to break because it can't repair itself as fast as it is injured. This is one example of "overtraining," or exercising so much that an injury follows.

So, if you are training for a special event, such as a marathon or tryouts for the lacrosse team, you probably need to dial your training up a notch or two. To avoid a stress fracture or other injuries related to overtraining, follow these two rules of thumb:

  • Don't increase your effort by more than 5% to 10% per week. For example, if you've been comfortable running 3 miles in 30 minutes (10-minute miles) five days a week, don't suddenly start running eight-minute miles. You might be able to do it, but you may pay for it.
  • Don't increase your distance and speed at the same time. Change one or the other, and do it gradually. If you notice pain or discomfort in one part of your body that doesn't promptly improve with rest, back off — you may be pushing things too fast.

When it comes to the question of joint-use and osteoarthritis, it's important to distinguish a sudden, severe injury like a fracture or ligament tear, from a repetitive-use injury that can occur from running, for example. A joint fracture may lead to osteoarthritis years later. So, for people who are not participating in highly competitive activities in which acute injuries are common, there is no clear connection between repetitive use and osteoarthritis.