JUMPER'S KNEE
Tendon problems are caused by too much physical stress on a tendon. The body is designed to adapt to physical stress, but recovery abilities have limits. In particular, tendons and ligaments have relatively slow adaptation processes, which is why tendinitis is a common problem. The suffix "itis" means inflammation, so tendinitis simply means tendon inflammation. Tennis elbow is a common example of tendinitis. The repeated violent arm action in tennis places stress on the tendons of the forearm and triceps muscles. Too much of this results in an overworked tendon that becomes inflamed and causes pain.
Jumper's knee is a term given to tendinitis in the quadriceps tendon, which connects the quadriceps muscle group to the front of the tibia (shin bone). The knee cap (patella) is embedded within the quadriceps tendon, so it is also referred to as the patellar tendon. Jumper's knee is commonly developed by basketball players. The sprinting, stopping, cutting, jumping, landing, cutting, and shuffling in basketball require a lot of work from the quadriceps muscles and put stress on the knee joint. With two-hour practices and games all week long, it is easy for the quadriceps tendon of an in-season basketball player to get overworked, resulting in inflammation and pain. It's important to understand that inflammation is not the actual problem. The problem is that physical stress has caused a lack of structural strength in the tendon. At a microscopic level, the tendon matrix is not properly knit together. There is typically not much visible swelling associated with what most people call tendinitis. I prefer the term tendinosis. The suffix "-osis" just means "a condition." If you have jumper's knee, your tendon is in an unhealthy condition. Developing strong healthy tendons requires finding the right amount of stress that will stimulate positive adaptation rather than break the tendon down.

Jumper's knee is a problem I ran into my sophomore year of high school. I had added 8 inches to my vertical in the two months before the season, meaning that all my movements suddenly had significantly more force behind them. So looking back, it makes sense that I developed the tendinosis at that time. In my particular case, the pain flared up badly enough to sideline me for a couple of days during the season and then lasted for several months after.
I went through a process to overcome my jumper's knee, guided by an athletic trainer at my school. My first responsibility was to rest (reduce physical stress) and ice as much as possible. This meant no extra work at the end of practice and taking off days completely off. I iced whenever I got a chance, especially after any activity. The method the trainer preferred for icing was to rub a block of ice directly on the tendon. I also wore a knee strap to help get me through the rest of the season. The strap can take some pressure off the tendon to prevent further injury, but it has no rehabilitative effects in itself.
In order to actually rehab the patellar tendon, I did two exercises. (Both are shown in a video below in the knee valgus section.) The first was basic body squats. The goal of these is to put a light, controlled stress on the quadriceps tendon to provide stimulus for strengthening. Correct form (feet pointed out, knees wide) is critical. I actually did not discover that until years later. Body squats should cause minimal pain. The second exercise I did was seated leg raises with the hip externally rotated (foot pointed out). What the leg raises do is place the responsibility of holding up the lower leg entirely on the inner quad muscle, the vastus medialis, which tends to be weak. That weakness is one of the most common causes of problems related to the patella. These exercises are very easy and do not demand a lot of recovery. I used to do numerous sets of 10 each day. I just made a habit of it. I was able to limit the pain enough to make it through a few months of my season. I was able to manage that first bout with tendinosis, but it took a long time to fix the problem. Since then I have dealt with it again and again and learned much more.

There are a few common causes of jumper's knee. Identifying your particular cause(s) is important, because it will change the rehab process.
Poor Flexibility
Tight muscles experience more stress than loose muscles. The same can be said of the tendons attached to those muscles. Poor quadriceps flexibility increases the amount of stress on the quadriceps tendon. The solution is simple: stretch the quad. Most people are familiar with a basic quad stretch. Pull the heel to the butt. You should be able to touch your heel to your butt. Most people can do that. However, that stretch neglects one of the quad muscles, the rectus femoris. Most people are not aware of this muscle, so it's tight on almost everyone. Loosening it will help prevent or cure jumper's knee. Watch the video below.

An inflexible quad muscle will directly contribute to quad tendon pain. Also poor all-around flexibility can indirectly cause problems by causing poor biomechanics. For example, the hips are the most powerful joint in the body. Athletic movement should be largely driven by the hips, AKA hip-dominant. A person with tight hips and hamstrings may end up not getting much power from the hips. The consequence is knee-dominant movement which increases the risk for things like tendinosis. Knee dominant athletes can be identified by their posterior pelvic tilt and poorly developed glute muscles. In other words, they have a flat butt. So it is very important to have good all-around flexibility to allow proper mechanics and prevent all sorts of problems including jumper's knee.

Knee Valgus
This is another mechanical problem. Knee valgus is a term for inward bowing of the knees. It is a very common cause of patellar tendinosis. During knee extension, the quadriceps should pull the patellar tendon in a straight line. But if the knees bow inward, the tendon is slightly bent and twisted. That increases the physical stress on the tendon, and the real problem is it creates stress in multiple directions. The structure of tendons is designed to provide tensile strength in one straight line. Tension along other lines causes damage. So knee valgus places bad stress on the patellar tendon, not just too much stress. It also creates improper tracking of the patella through the femoral groove, which can lead to another problem called patello-femoral syndrome. In addition valgus increases stress on the medial collateral ligament and the lateral meniscus of the knee. There are athletes who have knee valgus but have not experienced any noticeable damage or pain yet, but it still needs to be corrected, because it negatively impacts athleticism. During any athletic movement, joint action that does not contribute to that movement is indicative of loss of energy. In the case of a 2-leg jump, knee valgus reduces the amount of force pushed into the ground and thus decreases the speed and possibly the height of the jump. This is a problem that needs to be fixed.
Biomechanical problems always have a variety of possible causes. Often times the source of the problem is muscles that are tight, weak, or poorly activated. Thus the general solution is stretching and proper lifting that develop all-around flexibility and strength. An athlete may correct a lot of issues with basic stretching and strengthening. However knee valgus is an issue that tends to show up even in very good athletes as a result of commonly developed muscular issues. One possible cause of valgus that is specifically linked to jumper's knee is an imbalance in the quadriceps muscles where the inner muscle, the vastus medialis, is weaker than the outer muscle, the vastus lateralis. (This was mentioned above.) The medialis must be strengthened to correct that imbalance. That muscle is targeted by performing knee extension with the hip externally rotated (feet pointing out). Watch the video below.
 
I have to be honest here. Although the quad imbalance is definitely linked to jumper's knee, not many people cite it as a cause of knee valgus. That is more of a personal theory of mine, but I do have a good explanation for it. The more widespread theory is that valgus is the result of weakness in muscles of the hip, particularly the hip abductors which move the thighs away from the midline of the body. Weakness in the external rotators may also play a role. Regardless of which muscle weakness causes your knee valgus, perfecting your squat technique with the feet turned out and the knees wide is a key part of the solution. That squat style addresses both the hip and the quad problems. There are also exercises that specifically target the hip abductors. Watch the video below.


Too Much Stress
This problem is simple. Even with proper mechanics and flexibility, you can still just wear your tendons out by doing too much activity that puts physical stress on them. The tricky part is figuring out how much stress will promote recovery and strengthening of the tendon and how much is too much. Recovery ability varies greatly with age, health, diet, genetics, etc. Jumping, plyometrics, and squats are activities that involve high knee stress. For a teenager, a large volume of those activities may strengthen tendons. For that same teenager 10 years later those activities may cause tendon degeneration. Those activities are great for increasing the strength of healthy tendons, but once someone develops tendinosis they will only make the problem worse. So there are various factors to consider when deciding what will help your tendons grow stronger. To some extent, you have to figure things out for yourself. However, generally speaking, if you have a tendon condition, getting healthy will require reducing high-stress activity and engaging in regular low-stress, rehab-type activity. If you have a serious condition, you may be limited to things like seated leg raises and stretching to start. As you gain some strength you can include things like body squats and resisted backwards walking. Then continue to progress to light weight squats and some of the other exercises discussed below. This process will vary a lot from person to person, so I cannot lay out a detailed plan. Again, you will have to figure things out for yourself.
In the long run, completely eliminating and preventing patellar tendinosis will require good flexibility, proper biomechanics, and a strong quadriceps muscle-tendon complex. If you can squat twice your body weight, your quads  have to be really strong, and you are most likely not going to develop any tendon problems. In the case of healthy tendons, simply follow a good squat program to continue increasing strength. For unhealthy tendons, the same activities that strengthen healthy tendons are now the activity that cause the most pain. How are you supposed to fix your knees by increasing your squat when squatting is the thing that kills your knees? Facing this situation has forced me to come up with exercises that target quad tendon strength but still allow recovery in an unhealthy tendon.

I have a few rules for effective quad tendon strengthening during rehab.
1. Avoid deep knee flexion if it hurts. This one took me a long time to realize. You wouldn't think that a body squat could be placing too much stress on the quad tendons. Yet I've been through long stretches where I could warm up and jump 40 inches with less pain than doing a body squat. In a deep squat, a lot of muscle tension is required to extend the knee, much more than is required in a position closer to standing. Thus, a body squat may actually be an exercise that you have to work up to if your condition is bad enough, embarrassing as that may be.
2. Limit high-force eccentric muscle contraction. Muscles can generate the highest tension during eccentric (lengthening) contractions. In running and jumping movements, peak muscle/tendon tension occurs while force is being absorbed during the eccentric phase. Many strength training exercises like squats involve a muscle tension peak at the bottom of the movement where the weight is stopped and turned around. Modifying exercises to eliminate those moments of high tension is important.
3. Use long sets and go for a burn. I have two reasons for this. First, the stimulus needs to provoke structural rebuilding. That is done with high volume. In typical sports training, high intensity is often the goal. This promotes neural proficiency and limits increase in muscle size. But for rehab, the focus needs to be different. Growth of the damaged tissue needs to be stimulated. Second, go for a good burn in the quad muscles, because lactic acid (the substance that causes the burn) provokes the release of an enzyme that stimulates collagen synthesis in tendons.

With those guidelines in mind, I have used the following exercises successfully in my battle against jumper's knee.
Reverse Sled Drags. This exercise features minimal knee flexion and almost no eccentric muscle action because each step is small and slow. I like to do the sled drags with my feet pointed out to focus on the vastus medialis. I do 3-5 sets that are long enough to give me a really good burn in my quads. The length of the set will vary with the difficulty of moving the sled. Use 1 minute rest periods.

Box Half Squats. This is a modification to regular squats that accommodates unhealthy quad tendons. Sitting on the box eliminates the muscle tension peak at the bottom of the movement, and obviously the half squat gets rid of deep knee flexion. I do 2-3 sets of 15 or more reps.
Car Push. This is not a joke. Pushing a car provides a great structural strengthening stimulus to the quadriceps muscles. It has almost no eccentric muscle action and also allows you to control how much knee flexion is involved. I like to push my car up a hill to make it more challenging. I do 3-5 sets of 15-30 seconds. It's exhausting.
Hill Sprints. Sprinting on flat ground involves a high-speed, high-force eccentric phase. But in an uphill sprint, because each step is up to a higher elevation that phase is greatly reduced. I use a hill in a local park that takes 6-7 seconds to sprint up. I make 10 trips up the hill using the walk down as my only rest, so it's like a conditioning session. Fewer reps of a longer sprint would probably be better. Maybe 5 sprints of 15 seconds. Doing that at the end of an easier workout or on an off day has done wonders for my jumper's knee.
Stair Bounding. I live in Wisconsin, so I've had to find an indoor version of the hill sprints for the cold weather months. Stair bounding has been a satisfactory replacement. I use a 20-step flight of stairs and go up it in 5 bounds, alternating which foot I start with. I make 20 trips, again resting only to return to the bottom. Quick tip: you may find that walking down a hill or down stairs hurts your jumper's knee, because it involves the exact eccentric action in the quad muscle that you want to avoid. Walking down backward will eliminate that problem.
As stated earlier, it can be tricky to find the appropriate amount of physical stress that will promote repair in the quad tendons. It will likely require some trial and error. Use pain as your guide. This is not the case with all rehab, but with tendinosis, if an exercise hurts it's probably putting more stress on the tendon than you want. Try out some of these exercises and find a couple that allow you to get some good work in on your quads without hurting your knees. I like to do one of these exercises 3 times each week with a day of rest between.

So here's a list in no particular order of what you need to do to get rid of jumper's knee.
1. Get strong and flexible, especially in the quads.
2. Fix mechanical issues (knee valgus, knee-dominant movement) by mastering the squat movement.
3. Reduce high-stress activities.
4. Perform regular rehab activities that are suited to the current condition of your tendon.
5. Use the universal rehab tool, rest.

I hope this helps.