"My knees are killing me!" is the cry of many long distance hikers, especially at the beginning of their hikes. These complaints are frequent and vivid to me.
The Problem
What is the problem? Although Appalachian Trail thru hikers will tell you about leg, muscle and foot problems from the frequent tough uphills, they will complain more bitterly about the steep downhills, which they feel cause knee pain and damage. And they are right!
The Cause
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How does knee injury happen? Most hikers attribute it correctly to downhills, although sometimes their walking style or other problems cause it. Contrary to the popular impression that the Appalachian Trail is half uphill and half downhill, I suspect that 1/3 is uphill, 1/3 is level and 1/3 is downhill. This still means about 700 miles of downhill. Some of this is steeply downhill, especially in Maine and New Hampshire.
There are two types of bad downhill technique that contribute to bad knees: 1) walking rapidly and hitting the ground too hard with the foot 2) rapidly stepping down or jumping down steep slopes. (Fast downhill travel can also lead to instability, loss of control, and falls.)
Knee injury is not always obvious at the time. Violent foot strikes and excessive, rapid, repeated flexion and twisting of the knee from going downhill too fast are major causes of knee injury. Hitting the ground hard with the foot punishes the knee joint, causing irritation of the joint surface (a mild form of arthritis) and too much joint fluid (bursitis, housemaid's knee). Repeatedly stepping down and jumping down also causes irritation and bursitis. Additionally it can cause pain around the joint due to the rapid and repeated extreme flexion of the knee, which stretches the joint capsule and the associated tendons.
These several types of pain in and around the knee joint are signs of overuse pathology, and are a warning to the hiker which should be heeded. In some cases, old sports injuries form the basis, but trail activity can damage previously normal knees
A hard day of downhills may cause acute pain, some tenderness, and possibly swelling. Resting at night will in most cases allow recovery. Reducing the daily mileage should also be considered. But if repeated episodes occur without adequate recuperation, the problem can become chronic and severe. If the hiker does not pay attention to the pain, there is the possibility of considerable and permanent damage.
