The VMO muscle. It’s the teardrop shaped muscle on inside of knee. Sadly, when I heard about this muscle about a year ago, I couldn’t find it on myself…mine were non existent! 
The VMO has an inverse correlation to knee pain. In general, the bigger the VMO, the less knee pain we’ll have.
VMO is the most fast-twitch, and first to contract of the quads.
It is trained most effectively when knees go over toes, OR when knee is fully bent. How ironic that most of us totally AVOID those two positions.
The VMO also helps to efficiently track the patella (kneecap), and therefore helps guide and seat the patella properly. Many of us have weird shifting and floating of the kneecap. This, along with dislocations can feel cruddy and helpless!
You can see that my the VMO on my right knee (the one with 3 surgeries), is smaller than my left. I have work to do there, in effort to become more structurally balanced.
A question to consider especially for an athlete would be, what do we think the cause would be if we built up upper quads, and added mass/weight in the upper body, before or without addressing the VMO?
I actually don’t really care about studies. I care more about what’s working for you or me today. But I know it matters to some so I’ll include a few below that I’ve read. To me, the science matters, but real life application matters more. For example, the third link – study on patellar maltracking concludes that rest, a knee brace, and anti-inflammatory drugs (these break down cartilage) are commonly used non surgical treatments. AND that there are about 100 styles of surgery to TRY to correct this, resulting in NO gold standard amongst medical professionals. Good lord. Fortunately, it does mention range of motion and strengthening of the VMO.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8752958/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4609425/
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6570735/…
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