First off, please visit the general knee pain page for some info!

What is Patellar Tendonitis?

First, why do runners get patellar tendonitis?

The patellar tendon is a short but very wide tendon that runs from your patella (kneecap) to the top of your tibia. The reason you have a kneecap in the first place is to generate a bigger mechanical advantage at the knee—this allows your quadriceps to create strong forces at the knee, which are important in any sport with running or jumping elements. However, the result of this is that the patellar tendon has to absorb a lot of this loading, and as a result, it’s prone to injury in runners and jumpers; one study found that patella tendonitis accounts for just under 5% of all running injuries.1

Unlike many common running ailments, patellar tendonitis is somewhat more common in men than in women. Patellar tendonitis usually begins with a stiff feeling in the patellar tendon, especially when running downhill or descending stairs. Like most tendon injuries, it may go away once you get warmed up, but as the injury worsens, it will remain painful for the duration of your workout.

It is also important to distinguish patellar tendonitis from patellofemoral pain syndrome: patellar tendonitis does not hurt along the top or the side of the kneecap but typically under it, and isn’t usually sensitive to the touch.

Treatments and Fixes for Patellar Tendonitis

If squats hurt: Decrease the load. You can achieve this by decreasing the weight you squat, decrease the range of movement you use, decrease the number of repetitions you do, increase the rest intervals in-between sets, change your technique by getting your bum back more and loading more through the hips than the knees. Do you find it hurts more running in shoes with a bigger ‘drop’: If so, you may find that switching to a more minimal running shoe, or even barefoot, is enough to offload the knee and switch the load more to the foot and ankle. If running, in general, is irritating your patellar tendon:. Try these running re-education cues. They all generally shift load away from the knee.

1. Increase cadence. Increase how many steps you take in a minute. Aim for 5%-10% increase and assess how it feels
2. Improve posture. Work on running up tall. This will prevent the foot landing excessively in front of you (over striding) as you try to catch a forward positioned centre of mass.
3. Increase heel lift. Something like the piston cue will help to get an increase in heel lift at toe off leading to a more circular movement of the foot, better knee drive and making it easier to land under your centre of mass.

John Davis has a nice exercise outline

Patellar tendonitis can be a tricky injury to bounce back from, particularly if it’s become a chronic problem.  Fortunately, new avenues for treatment have opened up in the past decade or so.  The gold standard of conservative treatment right now seems to be Alfredson’s eccentric decline squat protocol.  It’s summarized in the points below:

• The basic protocol is three sets of fifteen one-legged squats, twice a day.
• The squats are done on a 25° decline (most calf stretching boards will do just fine).
• Starting from a standing position, squat down on the affected side to 60° of knee flexion (see picture above).
• Use the unaffected leg to return to return to the starting position.  If both legs are affected, return to the starting position using both legs, assisting with your arms (on a railing or similar) if possible.  Of course, if you have patellar tendonitis in both legs, you should do 3×15 squats twice a day on each leg. 
• Exercise into tendon pain, but stop if the pain becomes debilitating.  Once you can complete the three sets of squats with little or no pain, add weight with a loaded backpack.  
• In most studies, the protocol is carried out every day for 12 weeks.  It is not a bad idea to keep doing this exercise beyond 12 weeks as maintenance. 
• Most studies mandate 8 weeks of no sporting activity.  It’s important to note, however, that the subjects in these studies usually have quite severe cases, and often participate in very high-impact sports like basketball and volleyball.  Your own plan for returning to running is something you’ll have to work out yourself, possibly with the help of your doctor or physical therapist.