Knee is a complex joint. An injury or ruptured anterior cruciate ligament (ACL) can cause a popping sound when you move your knees. ACL runs from the outer side of your thigh bone to the inside of anterior of shin bone. There are two cruciate ligaments which cross over each other and work together with other ligaments to stabilize the back and front knee movements.
Sometimes a popping sound can be harmless and may be caused when the tendons (fibers which connect muscles and bones) change new position when the joint moves under stress. Talk to a specialist if you have severe knee popping or cracking sound for correct diagnosis. If the sound is caused due to damage in your knee ligaments then regular exercise and good fitness level may help prevent further damage. Exercise will not only make your muscles stronger but also make them stable to support your joint. To prevent any muscle injury always start an physical activity with a 10 minutes warm up, then gradually increase the intensity and end it with a 10 minutes cool down.
answered by Dr S