In addition, recent cadaver studies have confirmed the presence of a sixth muscle, the tensor vastus intermedius.  While the muscle has variable presentations, it consistently originates at the proximal femur, runs between the vastus lateralis and vastus intermedius muscles, and inserts distally at the medial aspect of the patellar base.  Historically considered a part of the vastus lateralis , the tensor vastus intermedius muscle is innervated by an independent branch of the femoral nerve and its tendinous belly can be separated from the vasti lateralis and intermedius muscles in most cases. 
The good news is that knee popping can be semi-permanently fixed in a week or two. You should do two things to fix knee popping. First you need to strengthen the weak vastus medialis so that it can hold the knee cap in its proper place as your knee bends. To do this, you will simply perform an isometric contraction of the quadriceps muscle group. Sit on the ground with your legs out in front of you and something to support your back. Rotate your legs so your toes are pointing toward twelve o’clock as you look at them. Flex the quadriceps on the knee that pops. You should see muscles on the top of your thigh move around. Reach down and poke your vastus medialis to better innervate your vastus medialis before each contraction. This will give you a stronger contraction. Flex the quadriceps and hold for 10 seconds and then relax for 10 seconds. Repeat 10 times 1-2 times per day for several days or as much as 2-3 weeks. This exercise works the vastus medialis because the vastus medialis works the hardest during the first 10 and last 10 degrees of knee flexion (. when your knee is almost straight and when your foot is almost touching your butt). An exercise called the standing terminal knee extension is an alternative to the one described here.
Second, you should strengthen your hamstrings. The hamstrings are weaker than they should be in anyone who sits down for long periods every day. Physiologically speaking, given a sedentary lifestyle, women are more prone to have weaker-than-optimal hamstrings than men. Strong hamstrings will help with knee popping because strong hamstrings help to ensure that your femurs are not rotated too far out or in and are an important part of knee health. The one of the fastest and easiest ways to strengthen your hamstrings is to do an exercise described on , the supine bridge . As an added benefit, this exercise will strengthen your gluteal muscles (butt) as well. Perform two sets of 20-40 repetitions every other day until your knee popping resolves.
That sums it up: knee popping can best be fixed by isometric contractions of the quadriceps and concentric exercise of the hamstrings. Viola, your knee popping is gone. Unfortunately for many of you, you will have to do these exercises periodically for the rest of your life if and when your knee popping returns.
There are also 2 recognized causes of muscle imbalance. The first is a biomechanical cause from repeated movements in one direction or sustained postures. The biomechanical causes of muscle imblance have been popularized by Kendall and Sahrmann. The second cause is a neuromuscular imbalance due to the predisposition of certain muscle groups to be either tight or weak. The neuromuscular approach was popularized by Janda, and is based on movement patterns that evolve from birth. Dr. Janda noted that the ‘tonic’ group of muscles are prone to tightness and the ‘phasic’ group is prone to weakness: