That’s what it feels like, some soreness in a lateral band right at the base of the quadriceps, top of kneecap. I’ve had this periodically for well over a year, usually hurts if I’m carrying 25 lbs of cat litter up a flight of stairs, or something similar. Never bothered me on the bike, till this fall when I started increasing my weekly mileage. That increase was a result of a new and more comfortable saddles, better Assos shorts as well as successfully dealing with a butt area rash. So now I could ride 4-5 days in a row and now the knee is bothering me. Debating if my saddle is too low, maybe too much bend in the knee is causing some inflammation. I’ve been at the same seat height -71cm from center of crank, for a decade or more. Thinking maybe raise the seat to 72 or so. I’m 5’9” on a 56 ETT bikes, long torso, shorter legs. Cant remember my inseam.

I'm the same height and have about a 30" inseam. I found I really benefit from 165mm crankarms. Lower gearing might help as well.

[sarc]Post some photos of the rash, and we can give more fine-tuned recommendations. [/sarc]

For us to pull a reasonable rabbit out of the hat, there really isn't much usable info.
One of my favorite SNL clips is the 'Sports Announcers Recap' clip, where they gives 'Scores'... you know... 5-2, 2-0, 6-1 ....

.... Quadriceps group usually refers to at least 4 muscles , 3 Vastus and a Rectus... All attach at the knee, Rectus Fem & Vastus Medialis being the one most medial.
Things to consider, besides seat height
Cleat Rotation
saddle fore-aft
gear selection - as in too large gear
extending the days riding could surface an issue not apparent before...
a one-time tweak can be exacerbated into long term condition, if not given an opportunity to 'heal'...
so many things which can contribute to the issue...
things like cleat rotation can be checked...
anytime I do a small 'tweak', I try to think back to when I noticed it occurring, for any possible immediate causes...
check to make sure that any setting I have is not, so how, completely different than what has been my settings range for many years.
if it's only nagging issue and not yet major.... I'll do similar rides, but at a much lower gear and keep the spin in my 'comfort' range...
I'll do these easy rides for at least a week or 2 - better to put yourself in 'hold' than cause more injury which could take you off the bike for a lengthy bit of time...
Ride On
Yuri

Thank you, good info to ponder.

SB

Usually if my quadriceps hurt, it's because I'm pushing too big a gear for too long. Might also be that your butt is too far back from the vertical axis of the BB.

It could also be from a saddle too low. But it'd probably have to be almost two inches too low.

I had switched my gravel bike road wheel gearing from a 12-25 to an 11-34 and have been riding that bike more, it has bigger jumps between gears, so might be I’m pushing more. I usually ride a 11-30 12 spd bike, it’s in for repairs. I just switched back to the 12-25, and raised the saddle a bit, will test ride tomorrow. My saddle height has been the same for many years. New saddles though, might be a slight position change. Or I might just be old and having body parts failing.

Finally went and looked this up. My inseam is 84cm. Most online calculators say a saddle height of 74cm. Possible I’ve just been riding too short for a long time and only now am discovering problems, will adjust and do a test ride tomorrow

Proportion sounds about right... I'm 88+ a scoosch, 45/11 shoe with cleat set about as far forward as it can go, and set road at 78+ from ctr to BB - 172 crank - 4-5 mm lower for 'gravel'.
might also check your saddle fore/aft... if you were 'low', a quite forward position at low height at constant/long ride times puts heavy strain on the knee.
A 'forward' seat position is ok, but works best with a slightly higher extension height...
.If you're a few centimeters low, you might consider adjusting in steps... giving a chance to allow the body to readjust over some rides...
Ride On
Yuri

If changing fit doesn't work, could be a tendon sheath or the tendon itself. If it's the sheath, ibuprofen will make the pain go away for a few hours. If that doesn't work, it's probably the tendon, though that seems unlikely, however the usual fix for that is heavy knee extension work, weight at the 4-5 rep limit. Over the years, I've kept various monsters at bay by doing sets of full depth squats with a 10-12 rep max once a week. So far, so good.

One of my mottos has been: don't try to fix pain with bike fit, fix it with bike fitness. Not to say that bike fit isn't important, but in my view it's more important for power and efficiency than anything else. That said, I usually start a fit with heel on pedal, then adjust for pull-back force at the bottom of the stroke. I never measure to get an initial fit, too many variables.

People can tolerate quite a bit of too-high saddle placement because they can point their toes if needed. So there is no downside to just moving the saddle up and giving it a try. But it sounds like your saddle has been too low for quite some time.

Pain where you describe should be thought of as a connective tissue strain - so be careful. You don't want to further damage a tendon or whatever in the process. And these types of strains can take a very long time to heal on their own.