Kelly also was likely an athlete whose body allowed him the ability to adapt to different bike dimensions. Six jours may well be the same and is talking from that perspective having come from the upper echelons of European bike racing.
But there are many people who take up cycling with inherent minor disabilities that become significant if repetitive action occurs and things aren't perfect in regards to fit.
Homey is dead right when he points out there are many, many, many questions posted on bicycle forums all over the world complaining of pain somewhere, and that usually can be traced back to one of several things, the most obvious being bike fit, followed by lack of core strength, then inappropriate style or technique, and bike short fit.
Yes, we can all jump on a bike that doesn't fit and cruise maybe 30 miles without any deliterious effect and have great fun. But put high musculo-skeletal stress into the equation, and the ordinary rider will have problems.
I've had a few injuries over the years while randonneuring, ranging from ankle and achilles tendon through to ITB and patella, and have generally been able to solve them. Significantly, they all related to fit. My worst case was adjusting my saddle according to a fit calculated by a site on the web, and I ended up with fearful achilles tendon issues that had me hobbling around for weeks. I went back to the original seat height on that bike, and never had further AT problems as a result of riding it.
Maybe I could have short-cut the whole process by having a professional bike fit, but they were non-existent in the city from which I hail. So I had to learn myself the hard way. And as I pointed out earlier, my method seems to work for me because it was confirmed during the fit for the tandem.