Skin Hardening

What is causing my skin to darken and harden?  It happens above both my knees where I cross my legs and on my ankle where I cross my leg.

[Dr_Aparna]

Hi,
Hyperkeratosis is a thickening of the skin that occurs in people  with brown skin. It is due to excessive accumulation of keratin in the outer layers of the skin. Thickening of the skin protects it against rubbing, pressure and irritation. The majority of the forms of hyperkeratosis are painless. Although hyperkeratosis may occur anywhere on the skin, there are several areas that are typically involved.
Elbows and knees are common sites for thickening of the skin. Hyperpigmentation accompanies this type of hyperkeratosis which is due to repeated leaning on the elbows and kneeling on the knees.
Most treatments are aimed at softening the keratin and removing the thickened skin. This may be accomplished with topical creams that contain as the active ingredient salicylic acid, alpha-hydroxy acid (lactic or glycolic acid), urea, or tretinoin. In brown skin, discoloration often accompanies the hyperkeratosis and skin lightening creams may also be used.
ref:http://www.brownskin.net/hyperkeratosis.html