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Recently, on a beginner MTB lecture, I learned that the pedal axle should in no part of the stroke be horizontally in front of the knee. Not observing this is unhealthy for the knees.

Also, here (btw great video) they explain a similar thing. For a "correct fit", the knee should line up with the pedal axle.

Currently I am riding my little sister's mtb. The seat is high enough, but even being slided maximum to the front, I fail the above test miserably (the seat tube is quite angled). A couple of weeks of short rides, and my knees already hurt constantly (maybe placebo effect?).

Why would having the seat too far back be unhealthy for the knees?

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1 – Batman May 16 '14 at 13:33
I know placebo effects can be strong, but constant knee pain sounds real, and it can get a lot worse over time. Be careful, don't mess up your knees. I don't know if it's this aspect of the fit or not, but it sure sounds like your body is telling you something is wrong. – Jefromi May 16 '14 at 14:34
Well, one wonders how recumbent cyclists survive. But knee pain suggests a problem. Likely it's in part due to unbalanced muscles and gaining strength too fast. There are leg stretches you can do to help this a lot. – Daniel R Hicks May 16 '14 at 14:55
The main thing is that the knee should be ALMOST (but not totally) straight at the bottom of the stroke. – Daniel R Hicks May 16 '14 at 14:56
@DanielRHicks, indeed I am passing the 'heels on the pedals and the legs are straight' test. Then I ride with the front of the foot over the axle (not the arch) and my legs are just a little bent. – Vorac May 16 '14 at 15:39

This principle, "Knee Over Pedal Spindle" or KOPS is archaic and wrong. It assumes ideal morphology.

I am about 6ft/185cm tall. For my height, I have comparatively short legs. I have large feet, size EU48/UK13/US14. KOPS gives me inappropriate knee and ankle angles.

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But KOPS is still a good starting point if you have no idea if you have such morphological aberrations. – arne May 16 '14 at 12:58

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