But that last sentence is a huge add-on - a good fitter is not cheap, and fitting yourself properly is not simple. It's a bit like talking about an odd car then adding on "...especially if you're an auto mechanic".

It's not a motion that's used almost anywhere else that your leg moves while affixed to exactly the same point over and over again. Usually the leg moves around about how and where it hits as you use it. Like in running, walking, etc. When you're attached to the pedal in exactly the same spot all the time, what could otherwise be a minor problem in leg motion that your body would work out by using the leg slightly differently over time, turns into a exactly repeated motion that turns into a problem. What I mean is - a minor problem can turn into a bigger problem because of the exacting repetition.

Riding clipless is not natural. But in now way does that mean that it's inherently "bad". Surgery isn't natural either, but it's a huge benefit to people with diseases to people who it helps move around better. Likewise, if your body layout is good it can be fine. Or if someone who knows what they're doing sets things up, the posters above certainly suggest that it could even fix a problem if your "natural" motion is somehow screwed up.

But on average, minor problems that your body would adjust to by shifting your foot position around a bit as you ride can turn into noticeable pain on clipless because your foot is always locked into the same position. If your position is good with your body layout - no problem. If it's not, it amplifies what would otherwise be a problem so small it was unnoticeable.

That's my understanding, at least.