After thirty-five years of wearing a heavy backpack into remote areas, slipping and sliding on steep terrain in search of wonder and solitude, attempting to find another pristine high mountain lakeside campsite, I've noticed, more and more, my knees ache at the end of the day--and, sometimes, before the day has ended. Diagnosis: Worn out parts. Cause: Abuse and overuse. Cure: Well . . .
I have become another of the numberless victims of the human body's joint most vulnerable to wear and tear. No other of the skeletal system's 187 bone-to-bone connections causes as many chronic problems.
To take care of a knee, it is, perhaps, best to start with an understanding of its construction and function. If your car fails to run properly, you need some basic auto mechanics in order to decide 1) is it OK to keep driving it? 2) are there things I can do to make it work better? and 3) does it need a car doctor? Same goes for the human "machine."

BODY MECHANICS
Knees are directly comprised of three bones: the femur (thigh), the tibia (which we touch when we say "shin"), and the patella (kneecap). Another bone, the fibula, attaches behind the tibia, near the knee, but has no specific influence on the joint.
Femurs and tibias articulate, or move against each other, when legs are in motion. The articulating surfaces of the femur and tibia are semi-flat and, to ensure a secure fit, each knee is padded with two C-shaped pieces of cartilage, one on the outer half of joint space, the other on the inner half. Called the medial (inside) meniscus and the lateral (outside) meniscus, they also absorb some of the shock of movement. Placed strategically in the knee, for additional padding, are fluid-filled sacks, called bursae, at points of the greatest friction.
The knee is held together by ligaments--four of them. They're attached at the points of highest stress. Connecting the femur to the tibia are the medial and lateral collateral ligaments, on the inside and outside of the knee. They provide stability for side-to-side motion. For back-to-front and front-to-back stability, there are the cruciate (crossed) ligaments. They run through the joint space, between the two menisci. Both cruciate ligaments attach on their upper end to the femur, and on their lower end to the tibia, and they're named for where they attach to the tibia. The anterior (front) cruciate ligament (ACL)
attaches to the femur at the back of the knee and to the tibia in front, thus preventing the knee from sliding too far forward. The posterior (rear) cruciate ligament (PCL) attaches to the femur at the front of the knee and the tibia at the rear, thus preventing the knee from sliding too far backwards. Ligaments are made of connective tissue in which there is very, very little elasticity.
When in motion, the great muscles of the leg provide additional support to the knee. The quadriceps (thigh) muscles are a group of four muscles. They taper down into one tendon that crosses the knee and attaches to the top of the tibia. The patella lives in the middle of this tendon. Three muscles in the back of the leg, the hamstrings, also help support the knee. One attaches to the outside of the knee and the other two to the inside. The calf muscle (gastrocnemius) attaches in two places to the back of the femur and, finally, a long thin muscle runs from the groin to the inside of the knee adding a touch more of support.
In addition, a long tough tendon, called the ilio-tibial band, runs from your gluteals (the muscles of your hindquarters) down the thigh, across the knee, attaching to the outside of the tibia. This band, too, gives a bit of support.
You'd think, with all that support, the knee would last longer. Unfortunately, the fittings are only moderately snug, the demands put on the joint are great, and it is highly susceptible to damage.
From a trauma point of view, any force applied to the knee can partially or totally severe a ligament (a sprain), a nasty injury. If the force is applied to the outside of the knee, the medial collateral ligament and anterior cruciate ligament may be involved, as well as the medial cartilage. If the force is applied to the inside of the knee, the lateral collateral could be torn and lateral cartilage may be ruptured. Twisting forces may significantly damage the cruciate ligaments.
But the most common source of chronic knee pain is not injury but overuse of the muscles that support the knee. When the muscles are stressed too much, they tear (a strain) and create a great deal of discomfort. They most often strain near their attachment to the knee. Tendonitis, an inflammation of the tendons, has the same mechanism of injury. Muscle strains and tendonitis are commonly mistaken by the patient as a torn ligament or cartilage. This mistake is very common when the ilio-tibial band is involved. Since the band is required for uphill motion, it is often abused when hikers are unused to going uphill, or increase their uphill activity, especially if they're wearing a pack. The problem, called ilio-tibial band syndrome, causes pain primarily where the band attaches to the outside of the knee, simulating a torn collateral ligament.
General knee pain may have other causes including patellar compression syndrome, a problem created by too much pressure on the back of the kneecap by too much walking, especially downhill. A dull ache, constant and nagging, is the common complaint. Or, perhaps, the kneecap doesn't run quite correctly in its track. The additional side-to-side motion of the kneecap puts additional stress on its inner surface which eventually causes pain for up to several hours after use. If the pain becomes chronic, never going away, the condition may be chondromalacia of the patella. Chondromalacia refers to a disintegration of the cartilage under the kneecap, probably caused by a chemical change stimulated by past injury or overuse. The cartilage becomes frayed and eroded. Interestingly, the cartilage can't hurt since it has no nerve endings. So the pain must come from inflamed tissue around the cartilage.

ASSESSING THE DAMAGE
1. First, you need to assess the extent of the damage. Did the pain start as the result of trauma (a forceful blow or twist) or overuse? If it was trauma, was there a direct blow to the knee? Which way was the knee forced to move? Did it twist? Was the foot planted when the force struck? Did the hurt hiker hear any sounds, such as a popping noise? If it was overuse, has the sufferer ever had this kind of knee pain before? Does it hurt all the time or just when he or she moves? In both cases it is beneficial to ask, Have you ever had pain like this before? If the pain came on suddenly from trauma, especially if it made funny noises, and if it hurts most of the time, the patient needs to see a doctor.
2. Visually inspect the damage. Take a look at the knees. Do both knees look the same? Damaged knees may show swelling, discoloration, or some other obvious deformity such as a kneecap in the wrong place. The more a knee swells, and the more discolored it is, and the funnier it looks, the more it needs a doctor.
3. Palpate the damage. Touch the hurt knee with your fingers, probing gently. Do you find specific points of pain? Are the painful places over ligaments or tendons? Does it hurt when you push down on the kneecap, or wiggle it side-to-side? Is there pain along the line where the tibia and femur meet? The more specific pain in the knee is, the more likely there has been damage that needs repair.
4. Check range of motion of the knee. Can the patient flex and extend the knee through its full range of motion? Or does it lock up or get too painful to move past a certain point? Knees with a loss of range of motion should be taken to a doctor.
5. Check laxity of the knee. Each of the four ligaments holding the tibia to the femur can be individually assessed. These tests should be done with the patient sitting down and the leg relaxed. If the patient is unable to tolerate these checks, the knee needs a doctor.
The medial collateral ligament, the one on the inside of the leg, can be checked by holding the ankle, with the knee slightly bent, and pushing from the outside of the knee in. If it's loose or painful, stop pushing.
The lateral collateral ligament, on the outside of the leg, can be checked in the exact opposite way, pushing from the inside of the knee out. Again, looseness or pain is a sign to stop pushing.
The anterior cruciate ligament, one of two "crossed" ligaments inside the knee joint, can be checked by bending the knee slightly and pulling out on the tibia while pushing back on the femur. Watch for pain and looseness.
Posterior cruciate damage, which happens in only about one percent of all knee injuries, can be checked simply by lifting the relaxed leg by the ankle and letting the knee sag.
6. Test for function. This simple test should not be done until at least one hour after pain starts. If the patient can stand and walk, do halfway deep knee bends, and jump up and down on each knee individually, it's fine to keep hiking on those knees.