you seem to have described a condition called frictional melanosis due to the continuous friction or rubbing of these areas like your knees on the ground. The skin over the area has been under constant stress and healed with dark areas and dryness. I would advise you NOT to scrub the areas more with a rough scrub/pumice stone as you would be adding to the skin stress. Instead use a good mild moisturizing soap such as Moisturex or other syndet based soaps/glycerine soap and moisturize well over the area after cleansing/bath. apply moisturizer or cream (moisturex cream etc with glycerin) before and after you work. you can use the lightening agent in the night. It seems a similar cause has affected your ankles either due to squatting on the floor or possibly footwear(anything putting constant pressure on the area). If the skin is very thick and hard you can start with a salicylic acid 3% or 6% cream for 15-20 days after soaking and then continue plain moisturizer. It will take a while to get better. The only way to completely prevent it would be to avoid the cause.