I do three knee strengthening exercises regularly and throw in some others irregularly. Here are the most common three I do:
1) Static Wall Sit - feet appropriate distance from wall, I "sit" with my back against the wall so that my legs are bent about 90 degrees at the knees, and my knee joint does not extend forward beyond the ball of my feet.
|Static Wall Sit|
2) Quarter Squat - with two legs planted about hip width apart, or balanced on one leg with hips level and lower abdominal muscles engaged, I squat slightly by dipping my butt slowly and directly toward the floor (not leaning foward, but keeping back straight, and the hips stable). I dip/squat only as far as I can go while keeping my foot/feet planted flat and stable on the floor, hold this position for 10 seconds, then slowly straighten the leg.
|One-Legged Quarter Squat|
3) Leg Extension - I use a super heavy resistant tube for this. I pick a kitchen chair, and put the handles of the tube over the back legs of the chair, and pull the center of the tube forward through the two front legs of the chair. Sitting on the chair normally, I find the center of the tube and twist it once and insert my foot as shown in the image below. Then, I perform a set of leg extension exercises by straightening my leg. This is good for the latter half of the leg extension motion, which is the most important for strengthening the medial quadriceps muscles. I do them slowly, and hold the extended position for a few seconds on each repetition.
|Leg Extension (resistance tube)|
Leg extensions can be performed by sitting flat on the floor with your leg out in front of you with a pillow under the knee. Press the back of your knee down into the pillow forcefully several times, holding the extended position, and then relaxing. Concentrate on using just the muscles of the leg to do the work.
Rather than concentrating all of this into one single workout, I would typically do the wall sit exercises three to four times throughout the day, holding the sitting position for one minute each time. If you can't do one minute, start with what you can do and work up over time.
Quarter squats and leg extensions I often do all during one single workout period.
For me, these are the three most beneficial exercises to do for the knee when I have Runner's Knee. issues. By "Runner's Knee" here, I mean true Runner's Knee, or patellofemoral pain syndrome, which occurs as pain around and behind the knee cap. If you are having pain along the outside of the knee joint you are likely not experiencing true runner's knee, but rather an Lateral Collateral Ligament (LCL) strain/tear or more likely iliotibial band syndrome (ITBS), and in addition to exercises like these, there are some other things you might consider, which I have summarized in posts on my other blog. If on the other hand, you are experiencing pain to the inside of the kneecap, you may be experiencing a cartilage or meniscus injury and/or a strain or tear of the medial collateral ligament (MCL).
If you are not absolutely sure your condition is true "Runner's Knee," I would suggest you see a physician specializing in sports medicine and or orthopedics who is experienced with knee injuries for an assessment before you start doing these or any other exercises.