According to WebMD, cracking noises from the knees are caused by uneven spots in the cartilage between bones, ligaments tightening as the joint moves or the joint lining moving over bones. In any of these cases, knee cracking is no cause for concern unless it is associated with pain and swelling, according to WebMD.
According to Live Science, the most common cause for creaking and cracking knees is tendons and ligaments popping into place as the knees is bent and extended. In addition to just bending, tendons and ligaments may make noises when simply walking as well, according to Everyday Mysteries.
If popping and cracking sounds are accompanied by pain and swelling, arthritis may be the cause, according to Live Science. Caused by the loss of smooth cartilage and the roughness of the joint surfaces rubbing together, arthritis causes the knees joints to become stiff and sore, according to Everyday Mysteries. Arthritis typically affects middle-aged people, according to WebMD.
Another possible cause for popping and cracking of knee joints is a meniscus tear, according to WebMD. The meniscus, which cushions the knee and acts as a shock absorber, is typically torn in a traumatic injury among younger people, but as the body ages it can be more easily torn during everyday activity, according to WebMD.