My right knee crackles everytime I bend it from a lying position, like when I do Ab crunches. It does urge me to crackle it on purpose at times even while standing. I have trouble applying pressure on that knee while cycling; the side of the knee hurts accompanied by the crackle sound. The knee also hurts when I stand on one leg without locking the knee. At times, the whole leg hurts and there is a strong urge to keep popping the knee. this problem has been there for almost 8-10 years now. I was an inactive and overweight person when it started but now I have been regularly for the past 2.5 years and almost at a right weight now. Still the problem persists. Left knee is fine.
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Quadriceps strengthening exercises-
Quad Clenches: Lie flat on your back or sit upright on a chair with leg kept horizontally on another surface. Now, tighten the muscle on the front of the thigh by pushing your knee down. You should feel your thigh muscles clench, Hold for 3 secs. Repeat 10 times twice a day.
Short Arcs: Lie flat on your back or sit upright with your leg placed horizontally on a flat surface like a chair or bed. Place a rolled up towel under the knee. Pull your toes towards you and clench you thigh muscles. Slowly lift your foot up off the bed until your knee is straight (keep your knee resting on the towel). Hold for 3 secs and slowly lower them on the chair. Repeat 10 times twice a day.
Straight Leg Raise: Lie flat on your back. One leg and knee will be straight and other leg should be bent. Pull your toes towards you and tighten/clench the muscle on the front of the thigh, locking your knee straight. Lift your foot up in the air, about 6 inches off the bed. Hold for 3 secs and slowly lower the leg. The knee must remain straight the whole time you are doing this exercise.
Towel Hamstring Stretch
Lie on the back, supporting the thigh with the hand or with a towel wrapped around it..
Slowly straighten the knee until a stretch is felt in the back of the thigh, trying to get the bottom of the foot to face the ceiling, one leg at a time.
Hold the position initially for 10 seconds, and gradually work up to 20 to 30 seconds.
Seated Hamstring Stretch
While sitting at the edge of a chair, straighten one leg in front of the body with the heel on the floor.
Then, sit up straight and try pushing the navel towards the thigh without leaning the trunk of the body forwards.
Hold this stretch for 30 seconds
Repeat 3 times for each leg.
Looks like some old injury or strain at knee joint has caused these kind of arthritic changes in the joint. Go for a thorough assessment like xray etc and consult the orthopaedic doctor for correct diagnosis
Possible diagnosis with this limited information seems to be Osteoarthritis although you are too young for this. Your Knee needs to be examined by a Orthopaedician or a Sports Medicine Specialist to reach the diagnosis.