It sounds like you are generally a very active person. Joint problems can be very aggravating for active people and especially for runners. As you indicated, problems with the soft tissue supporting structures of the knee joint are among the most common reasons for a 'locking knee.' However, it is actually problems with the meniscus of the knee that are most likely to cause this kind of complaint. Your knee is supported by lateral and medial ligaments that run along the outside and inside edge of the joint, as well as the famous anterior cruciate ligament (ACL) and posterior cruciate ligament which help attach the bones of the upper and lower leg together as they form the knee joint. Problems with these ligaments can lead to pain and instability--the knee can become almost wobbly since it is missing key supports. However, the meniscus of the knee is like a cushion that separates the femur and the tibia as they come together to form the joint. When the meniscus tears, it can become locked in the 'hinge' of the knee joint as it bends, often leading to the locking sensation you describe. The best thing to do is to see your primary care physician
right away. He or she can perform some simple physical exam maneuvers to determine if you have symptoms of a meniscus tear. You may need further imaging (likely starting with Xrays but possibly also an MRI
) as well as an orthopedic referral to help identify the problem and the best means of treatment. Best of luck--hopefully you'll be running again without problems very soon!