Why would biking give me a knee effusion?

I, when possible, bikes to and from school. I attended a skills workshop today, the start of which was an osteoarthritis talk. I was a volunteer/victim of a knee exam, and the physiotherapist commented on the “bulge sign” evident when he milked my knee–something that was obvious to all of my colleagues. Later in the day, when I went to show a classmate, it was gone. Then, when I got home (after biking), I checked again, and sure enough, it was back!

So. Why? Assume I have a working knowledge of anatomy and physiology.

Well broadly speaking, any knee problems from biking are probably due to a bad seat position (too low or too high) and, if you use clipless pedals, poorly setup cleats on your shoes. I can’t help with the specifics of your knee problem though.

Yes. If you are into biking and have a quality bike find a bike shop that has expertise in fitting bicycles. Even if it costs to go through the fitting process, in the long run it is much cheaper than an ongoing knee problem. You don’t have to buy a custom made bike. Unless your bike it totally a wrong fit some modifications can be done to make it more comfortable.

With clipless pedals sometimes just moving the cleat 1/2 inch will eliminate a knee problem.

Bike fit is very important, if you are a casual rider you may never realize what a proper fit is. It’s ultimately very subjective to what works for you, but this is a good starting point:

Seat height and fore/aft position are the most common adjustment, followed by stem length and handlebar height. The latter may require changing the stem, but modern stems on threadless forks are very easy to change. If you have an old quill type stem, it’s a bit harder but the fit guidelines are the same.

Once you know the right position for you (and it will take a few rides) you’ll find that very small adjustments (about 1/8") make a big difference. The guideline at that link will get you pretty close to what works for most riders.

Cyclist, cycling instructor and Kinesiology student chiming in to say that I agree with the previous posters that it is likely due to bike set up! If you have a local bike shop (not Target!), they will be able to help set you up properly too.

A question - does your knee hurt, and did it hurt when it was touched?

Along with setup as the others said, pedaling in to high a gear(low cadence) will over-stress the knee.

90-120rpm is the recommended range for most people.

I’d never noticed it before today (probably because I was standing all day prior to biking home, unlike usual), but it ached. Wasn’t tender, though.