So true! I have seen some of my friennds who are muslims with fair knees too!!! So that possibly means that darkening is not caused due to praying! And it is so embarassing for the author to mention dirty prayer knees! So what about the non-muslims? Do they have dirty “non-prayer” knees ???

“The pigmentation is a reflection of the type of skin around the joints and the fact that there is constant friction between these thickened areas and the environment.” R. Karamanoukian, MD, FACS

&

“The skin in these areas are always in motion because they are over some of the most utilized joints in the body.” M. S. Green, MD

Source: http://www.realself.com/question/procedures-for-dark-elbows-and-knees

These are answers from real doctors who give real reasons as to why skin on certain areas of your body can become dark.
Yes, we pray five times a day and therefore get down on our knees often; however, this is not the reason for your dark knees. Please do not foolishly blame something like this on prayers as if they were a burden. My little sister is only 3 years old and is already showing signs of darkened knees. I also have non-Muslim friends who have this condition.

If I wasn’t muslim, the first thing I would have thought would be, “wow! those poor girls have to dirty their pretty little knees everyday just to bow down to their own creator five times a day! Oh! what a waste!”

Also, you mentioned that praying can cause dark foreheads. I have never in my entire life seen a muslim, male or female, with dark foreheads. So I have no idea where you got that one. 🙂

Sincerely,
Lobna A.