Question
I think my problem started a few months ago with a strain or something similar.

I was going fast and hard up a lot of stairs and then suddenly I (probably) slightly strained my right quad, I think so, because I felt sharp pain in the outer lateral side of right knee for a second. this repeated on another stair a few mins later. pain for one sec again. after this, a bit later, I noticed that I couldn't perform some specific movements with my right quad: I couldn't stretch it fully, and for a strengthening exercise where I'm sitting on the ground, I couldn't tell it to push my knee into the ground/towel below knee. walking was no problem though. I had no pain in the muscles. this gradually got better over a few days so it got to the point where I could stretch it okay, but couldn't do full extension with the right knee, only about 90%, and couldn't do some specific movements that required active full extension of knee. this stayed the same over the months - I did manage to make it a bit better by practicing the problematic movements but it would never become perfect, if I tried knee extension too hard I would feel a dull pain in the knee or knee just wouldn't go to full extension. and I noticed that when I sit down and then try to contract this part of my quadriceps on the outer thigh, I couldn't do it very well. I mean, if I tried to contract it, it would either do nothing or would contract a bit but at the same time my nearby gluteal muscle would move a bit along with it, etc. (but my left quad's just fine, as a comparison.)

now, this didn't really bother me, because I didn't really need 100-110% knee extension for my daily life or for sports - I'm a runner mostly (I run regularly). but a few days ago, something happened because I noticed, my right quad could contract normally, just out of the blue (maybe it was to do with some hard sprints I tried that afternoon? I don't know...)!

but soon after, I started having pain on the side of my right hip and below it (I suppose tensor fascia lata and iliotibial band below it) when walking and sometimes even in the right groin (groin: never while walking, only while sitting or standing and performing some specific movements), and most bothering was that I couldn't run far without hip flexor pain on my right side. hip muscles gradually got somewhat used to the extra load, but still had pain often. also the right sacroiliac joint (or near it for sure) would bother me a lot while sitting. this SI pain would really drive me crazy, not a strong pain, just a dull pain, but somehow REALLY a lot of discomfort... I never had lower back pain in my life before this.

in the end, I figured out that when my right outer quadriceps muscle (vastus lateralis??) started "working", my right gluteus medius got strongly inhibited (maybe not fully, but very strongly inhibited), and hip muscles were overworking themselves trying hard to compensate for the lost gluteus medius functions.

then I somehow managed to reactivate the gluteus medius, I think when I was standing, with knees in full extension (note right quad was working quite well for knee extension at this point), then I tried to contract the gluteus medius on the right side and I succeeded but next moment I couldn't keep the right knee fully extended, just about 95%. but suddenly I was able to walk without hip pains and do full abduction lying on my side, etc. etc.

so now the lateral right quad is again somewhat inhibited as I can't contract it much when I try, it's only a very little contraction that I can force and my gluteus medius will move along with it. but, gluteus medius has come back. all my hip pain disappeared except the SI joint pain...that is maybe getting better now, but still lingering around... also, when sitting, I sometimes have some new pain over the area where gluteus medius inserts and where lateral quad origin may be (judging from some pics online). that worries me a bit. (this has been only 2 days since I got the muscle reactivated, though.)

my problem is how can I make both muscles work? or how could someone else help me with that? because, I think, either one is inhibited and the other one works or the other way around. I'm not sure if this will do me good long term. I'm worried the quad will again win over the gluteus medius and then the nightmare would restart... or maybe some other long term issue could be caused by the inactive right later quadriceps muscle or by this muscle "competition" that I like to call it. or maybe something's going on where the insertion/origin of these muscles are (where I maybe got pain/discomfort lately). these are the things that worry me so I would like to fix this so I can be like before the quad strain happened.

many thanks for your help

Answer
This is not something I can diagnose over the Internet. I will tell you that if your pain has subsided it doesn't matter. You're body naturally will adapt to the pain to kill or reduce it. Your nerves in your lower back control ALL muscles in your legs. I would first look at the spine(chiropractor) then after a series of SI joint adjustments 6-12, over 4-6 weeks, after that if your still having muscle inhibition then do physical therapy to isolate muscles. Also tell your chiropractor about your knee pain. You may have had a misaligned bone in the knee joint and you compensated and developed SI pain due to re aligning your pelvis (because your knee was hurting). If you can find a chiropractor with additional sports injury training then he will be able to fix you up (both knee,SI joint, lumbar spine too).
Ps- stretch your IT band , glutes, and quads really good.